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__NOTOC__
__NOTOC__
Welcome to the Georgia Tech Wiki for Introductory Physics. This resource was created so that students can contribute and curate content to help those with limited or no access to a textbook.  When reading this website, please correct any errors you may come across. If you read something that isn't clear, please consider revising it for future students!
= '''Georgia Tech Student Wiki for Introductory Physics.''' =
 
This resource was created so that students can contribute and curate content to help those with limited or no access to a textbook.  When reading this website, please correct any errors you may come across. If you read something that isn't clear, please consider revising it for future students!


Looking to make a contribution?
Looking to make a contribution?
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* A wiki written for students by a physics expert [http://p3server.pa.msu.edu/coursewiki/doku.php?id=183_notes MSU Physics Wiki]
* A wiki written for students by a physics expert [http://p3server.pa.msu.edu/coursewiki/doku.php?id=183_notes MSU Physics Wiki]
* A wiki book on modern physics [https://en.wikibooks.org/wiki/Modern_Physics Modern Physics Wiki]
* A wiki book on modern physics [https://en.wikibooks.org/wiki/Modern_Physics Modern Physics Wiki]
* A collection of 26 volumes of lecture notes by Prof. Wheeler of Reed College [https://rdc.reed.edu/c/wheeler/home/]
* The MIT open courseware for intro physics [http://ocw.mit.edu/resources/res-8-002-a-wikitextbook-for-introductory-mechanics-fall-2009/index.htm MITOCW Wiki]
* The MIT open courseware for intro physics [http://ocw.mit.edu/resources/res-8-002-a-wikitextbook-for-introductory-mechanics-fall-2009/index.htm MITOCW Wiki]
* An online concept map of intro physics [http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html HyperPhysics]
* An online concept map of intro physics [http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html HyperPhysics]
* Interactive physics simulations [https://phet.colorado.edu/en/simulations/category/physics PhET]
* Interactive physics simulations [https://phet.colorado.edu/en/simulations/category/physics PhET]
* OpenStax algebra based intro physics textbook [https://openstaxcollege.org/textbooks/college-physics College Physics]
* OpenStax intro physics textbooks: [https://openstax.org/details/books/university-physics-volume-1  Vol1], [https://openstax.org/details/books/university-physics-volume-2  Vol2], [https://openstax.org/details/books/university-physics-volume-3  Vol3]
* The Open Source Physics project is a collection of online physics resources [http://www.opensourcephysics.org/ OSP]
* The Open Source Physics project is a collection of online physics resources [http://www.opensourcephysics.org/ OSP]
* A resource guide compiled by the [http://www.aapt.org/ AAPT] for educators [http://www.compadre.org/ ComPADRE]
* A resource guide compiled by the [http://www.aapt.org/ AAPT] for educators [http://www.compadre.org/ ComPADRE]
* The Feynman lectures on physics are free to read [http://www.feynmanlectures.caltech.edu/ Feynman]
* Final Study Guide for Modern Physics II created by a lab TA [https://docs.google.com/document/d/1_6GktDPq5tiNFFYs_ZjgjxBAWVQYaXp_2Imha4_nSyc/edit?usp=sharing Modern Physics II Final Study Guide]


== Resources ==
== Resources ==
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* A guide to representing equations in math mode [https://en.wikipedia.org/wiki/Help:Displaying_a_formula Wiki Math Mode]
* A guide to representing equations in math mode [https://en.wikipedia.org/wiki/Help:Displaying_a_formula Wiki Math Mode]
* A page to keep track of all the physics [[Constants]]
* A page to keep track of all the physics [[Constants]]
* A page for review of [[Vectors]] and vector operations
* A listing of [[Notable Scientist]] with links to their individual pages  
* A listing of [[Notable Scientist]] with links to their individual pages  




<div style="float:left; width:30%; padding:1%;">
<div style="float:left; width:30%; padding:1%;">
==Physics 1==
==Physics 1==
===Week 1===
===Week 1===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Help with VPython====
====GlowScript 101====
<div class="mw-collapsible-content">
*[[Python Syntax]]
*[[GlowScript]]
</div>
</div>
 
<div class="toccolours mw-collapsible mw-collapsed">
 
====VPython====
 
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[VPython]]
*[[VPython]]
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<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Vectors and Units====
====Vectors and Units====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
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<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Interactions====
====Interactions====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
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<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Newton's First Law of Motion]]
*[[Newton's First Law of Motion]]
*[[Mass]]
*[[Velocity]]
*[[Velocity]]
*[[Mass]]
*[[Speed]]
*[[Speed and Velocity]]
*[[Speed vs Velocity]]
*[[Relative Velocity]]
*[[Relative Velocity]]
*[[Derivation of Average Velocity]]
*[[Derivation of Average Velocity]]
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====Momentum and the Momentum Principle====
====Momentum and the Momentum Principle====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Momentum Principle]]
*[[Linear Momentum]]
*[[Newton's Second Law: the Momentum Principle]]
*[[Impulse and Momentum]]
*[[Net Force]]
*[[Inertia]]
*[[Inertia]]
*[[Net Force]]
*[[Derivation of the Momentum Principle]]
*[[Impulse Momentum]]
*[[Acceleration]]
*[[Acceleration]]
*[[Momentum with respect to external Forces]]
*[[Relativistic Momentum]]
<!-- Kinematics and Projectile Motion relocated to Week 3 per advice of Dr. Greco -->
</div>
</div>
</div>
</div>


<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Iterative Prediction with a Constant Force====
====Iterative Prediction with a Constant Force====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Newton’s Second Law of Motion]]
*[[Iterative Prediction]]
*[[Iterative Prediction]]
*[[Kinematics]]
*[[Newton’s Laws and Linear Momentum]]
*[[Projectile Motion]]
</div>
</div>
</div>
</div>
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===Week 3===
===Week 3===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Analytic Prediction with a Constant Force====
====Analytic Prediction with a Constant Force====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Analytical Prediction]]
<!-- *[[Analytical Prediction]] Deprecated -->
*[[Kinematics]]
*[[Projectile Motion]]
</div>
</div>
</div>
</div>


<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Iterative Prediction with a Varying Force====
====Iterative Prediction with a Varying Force====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Predicting Change in multiple dimensions]]
*[[Fundamentals of Iterative Prediction with Varying Force]]
*[[Spring Force]]
*[[Spring_Force]]
*[[Hooke's Law]]
*[[Simple Harmonic Motion]]
*[[Simple Harmonic Motion]]
<!--*[[Hooke's Law]] folded into simple harmonic motion-->
<!--*[[Spring Force]] folded into simple harmonic motion-->
*[[Iterative Prediction of Spring-Mass System]]
*[[Iterative Prediction of Spring-Mass System]]
*[[Terminal Speed]]
*[[Terminal Speed]]
*[[Predicting Change in multiple dimensions]]
*[[Two Dimensional Harmonic Motion]]
*[[Determinism]]
*[[Determinism]]
</div>
</div>
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<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Gravitational Force]]
*[[Gravitational Force]]
*[[Gravitational Force Near Earth]]
*[[Gravitational Force in Space and Other Applications]]
*[[3 or More Body Interactions]]
<!--[[Fluid Mechanics]]-->
*[[Electric Force]]
*[[Electric Force]]
*[[Introduction to Magnetic Force]]
*[[Strong and Weak Force]]
*[[Reciprocity]]
*[[Reciprocity]]
*[[Conservation of Momentum]]
</div>
</div>
</div>
</div>
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===Week 5===
===Week 5===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Conservation of Momentum====
<div class="mw-collapsible-content">
*[[Conservation of Momentum]]
</div>
</div>
<div class="toccolours mw-collapsible mw-collapsed">
====Properties of Matter====
====Properties of Matter====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
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*[[Boiling Point]]
*[[Boiling Point]]
*[[Melting Point]]
*[[Melting Point]]
*[[Change of State]]
</div>
</div>
</div>
</div>
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<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Free Body Diagram]]
*[[Free Body Diagram]]
*[[Inclined Plane]]
*[[Compression or Normal Force]]
*[[Compression or Normal Force]]
*[[Tension]]
*[[Tension]]
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</div>
</div>
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Curving Motion====
====Curving Motion====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
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===Week 7===
===Week 7===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Jeet Bhatkar====
====Energy Principle====
====Energy Principle====
The Energy Principle is a fundamental concept in physics that describes the relationship between different forms of energy and their conservation within a system. Understanding the Energy Principle is crucial for analyzing the motion and interactions of objects in various physical scenarios.
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Kinetic Energy]]
Kinetic energy is the energy an object possesses due to its motion.
*[[Work/Energy]]
Potential energy arises from the position of an object relative to its surroundings. Common forms of potential energy include gravitational potential energy and elastic potential energy.
*[[The Energy Principle]]
*[[The Energy Principle]]
Work and energy are closely related concepts. Work (
𝑊) done on an object is defined as the force (
𝐹) applied to the object multiplied by the displacement (
𝑑) of the object in the direction of the force:
The Energy Principle states that the total mechanical energy of a system remains constant if only conservative forces (forces that depend only on the positions of the objects) are acting on the system.
*[[Conservation of Energy]]
*[[Conservation of Energy]]
*[[Kinetic Energy]]
The principle of conservation of energy states that the total energy of an isolated system remains constant over time. In other words, energy cannot be created or destroyed, only transformed from one form to another. This principle is a fundamental concept in physics and has wide-ranging applications in mechanics, thermodynamics, and other branches of science.
*[[Work]]
*[[Power (Mechanical)]]
</div>
</div>
</div>
</div>
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*[[Center of Mass]]
*[[Center of Mass]]
*[[Multi-particle analysis of Momentum]]
*[[Multi-particle analysis of Momentum]]
*[[Momentum with respect to external Forces]]
*[[Potential Energy of a Multiparticle System]]
*[[Potential Energy of a Multiparticle System]]
*[[Work and Energy for an Extended System]]
*[[Work and Energy for an Extended System]]
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</div>
</div>
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Thermal Energy, Dissipation and Transfer of Energy====
====Thermal Energy, Dissipation, and Transfer of Energy====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Thermal Energy]]
*[[Thermal Energy]]
*[[Specific Heat]]
*[[Specific Heat]]
*[[Heat Capacity]]
*[[Calorific Value(Heat of combustion)]]
*[[Specific Heat Capacity]]
*[[First Law of Thermodynamics]]
*[[First Law of Thermodynamics]]
*[[Second Law of Thermodynamics and Entropy]]
*[[Second Law of Thermodynamics and Entropy]]
*[[Temperature]]
*[[Temperature]]
*[[Predicting Change]]
*[[Energy Transfer due to a Temperature Difference]]
*[[Transformation of Energy]]
*[[Transformation of Energy]]
*[[The Maxwell-Boltzmann Distribution]]
*[[The Maxwell-Boltzmann Distribution]]
*[[Air Resistance]]
*[[Air Resistance]]
*[[The Third Law of Thermodynamics]]
</div>
</div>
</div>
</div>
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Rotational and Vibrational Energy====
====Rotational and Vibrational Energy====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Translational, Rotational and Vibrational Energy]]
*[[Translational, Rotational and Vibrational Energy]]
*[[Rolling Motion]]
</div>
</div>
</div>
</div>
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</div>
</div>
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Models of Friction====
====Friction====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Friction]]
*[[Friction]]
*[[Static Friction]]
*[[Static Friction]]
*[[Kinetic Friction]]
</div>
</div>
</div>
</div>


===Week 12===
===Week 12===
<div class="toccolours mw-collapsible mw-collapsed">
====Conservation of Momentum====
<div class="mw-collapsible-content">
*[[Conservation of Momentum]]
</div>
</div>
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Collisions====
====Collisions====
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====Rotations====
====Rotations====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Rotation]]
*[[Rotational Kinematics]]
*[[Angular Velocity]]
*[[Eulerian Angles]]
*[[Eulerian Angles]]
</div>
</div>
</div>
</div>
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Angular Momentum====
====Angular Momentum====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
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*[[Rotational Angular Momentum]]
*[[Rotational Angular Momentum]]
*[[The Angular Momentum Principle]]
*[[The Angular Momentum Principle]]
*[[Angular Momentum Compared to Linear Momentum]]
*[[Angular Impulse]]
*[[Angular Impulse]]
*[[Predicting the Position of a Rotating System]]
*[[Predicting the Position of a Rotating System]]
*[[Angular Momentum of Multiparticle Systems]]
*[[The Moments of Inertia]]
*[[The Moments of Inertia]]
*[[Moment of Inertia for a cylinder]]
*[[Right Hand Rule]]
*[[Right Hand Rule]]
</div>
</div>
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*[[Energy graphs and the Bohr model]]
*[[Energy graphs and the Bohr model]]
*[[Quantized energy levels]]
*[[Quantized energy levels]]
*[[Electron transitions]]
*[[Entropy]]
</div>
</div>
</div>
</div>
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<div style="float:left; width:30%; padding:1%;">
<div style="float:left; width:30%; padding:1%;">
==Physics 2==
==Physics 2==
===Week 1===
===Week 1===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====3D Vectors====
====3D Vectors====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Vectors]]
*[[Vectors]]
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<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Electric field====
====Electric field====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Electric Field]]
*[[Electric Field]]
*[[Electric Field and Electric Potential]]
</div>
</div>
</div>
</div>


<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Electric force====
<div class="mw-collapsible-content">
*[[Electric Force]]
*[[Lorentz Force]]
</div>
</div>


<div class="toccolours mw-collapsible mw-collapsed">
== Electric force ==
'''Jeet Bhatkar – Fall 2025'''
 
== Big Idea ==
Electric force is the interaction between objects that have electric charge. It is:
 
* '''Long-range''': acts even when charges do not touch 
* '''Vector-valued''': has magnitude and direction 
* '''Superposable''': forces from many charges add as vectors 
 
At the intro level, the electric force between two point charges is described by Coulomb’s law, the electrostatic analog of the gravitational force between masses.
 
== Key Equations ==
 
'''Coulomb’s Law (magnitude)'''
:[math]\displaystyle{ F = k \dfrac{|q_1 q_2|}{r^2} }[/math]
 
* [math]\displaystyle{F}[/math] = magnitude of the electric force 
* [math]\displaystyle{k \approx 8.99 \times 10^9\ \text{N·m}^2/\text{C}^2}[/math] 
* [math]\displaystyle{q_1, q_2}[/math] = charges (C) 
* [math]\displaystyle{r}[/math] = separation between the charges (m)
 
'''Coulomb’s Law (vector form)'''
:[math]\displaystyle{ \vec{F}_{2 \leftarrow 1} = k \dfrac{q_1 q_2}{r^2} \,\hat{r}_{2 \leftarrow 1} }[/math]
 
* [math]\displaystyle{\vec{F}_{2 \leftarrow 1}}[/math] = force on charge 2 due to charge 1 
* [math]\displaystyle{\hat{r}_{2 \leftarrow 1}}[/math] = unit vector from 1 to 2 
 
'''Relation to Electric Field'''
:[math]\displaystyle{ \vec{F} = q \vec{E} }[/math]
 
Once you know [math]\displaystyle{\vec{E}}[/math] at a point, you can find the force on any charge [math]\displaystyle{q}[/math] placed there.
 
== Conceptual Picture ==
 
'''Sign of charges'''
* Like charges (both positive or both negative) → '''repel''' 
* Unlike charges (one positive, one negative) → '''attract'''
 
'''Distance dependence'''
* Force falls off as [math]\displaystyle{1/r^2}[/math], so doubling the distance makes the force 4 times smaller.
 
'''Superposition principle'''
If there are many charges, the net force on a given charge is the vector sum of the forces from each individual charge:
[math]\displaystyle{ \vec{F}_\text{net} = \sum_i \vec{F}_i }[/math].
 
'''Electric vs. gravitational force'''
* Both follow inverse-square laws 
* Gravity is always attractive; electric force can be attractive or repulsive 
* Electric forces are usually much stronger at the particle scale 
 
== Worked Example 1: Two Point Charges on a Line ==
 
'''Problem.''' 
Two charges are placed on the x-axis:
 
* [math]\displaystyle{q_1 = +3.0\ \mu\text{C}}[/math] at [math]\displaystyle{x = 0.00\ \text{m}}[/math] 
* [math]\displaystyle{q_2 = -2.0\ \mu\text{C}}[/math] at [math]\displaystyle{x = 0.40\ \text{m}}[/math] 
 
What is the magnitude and direction of the force on [math]\displaystyle{q_2}[/math]?
 
'''Solution (outline).'''
 
# Distance between charges: 
[math]\displaystyle{ r = 0.40\ \text{m} }[/math].
 
# Magnitude using Coulomb’s law: 
[math]\displaystyle{
F = k \dfrac{|q_1 q_2|}{r^2}
= (8.99 \times 10^9)\,\dfrac{(3.0 \times 10^{-6})(2.0 \times 10^{-6})}{(0.40)^2}
}[/math]
 
# Sign and direction: 
* [math]\displaystyle{q_1}[/math] is positive, [math]\displaystyle{q_2}[/math] is negative → force is '''attractive''' 
* On [math]\displaystyle{q_2}[/math], the force points toward [math]\displaystyle{q_1}[/math] 
* Since [math]\displaystyle{q_1}[/math] is at smaller x, the force on [math]\displaystyle{q_2}[/math] points in the '''−x''' direction 
 
You can finish by computing the numerical value and writing it as a vector, e.g. [math]\displaystyle{\vec{F}_{2 \leftarrow 1} = -F\,\hat{x}}[/math].
 
== Worked Example 2: Superposition with Three Charges ==
 
'''Problem.''' 
Three equal charges [math]\displaystyle{q}[/math] are at the corners of an equilateral triangle of side [math]\displaystyle{a}[/math]. What is the net force on one of the charges?
 
'''Idea (no full algebra).'''
 
* Each of the other two charges exerts a force of magnitude 
  [math]\displaystyle{ F = k \dfrac{q^2}{a^2} }[/math] 
* The angle between these two forces is [math]\displaystyle{60^\circ}[/math] 
* Use vector addition: 
  * Add components along the symmetry axis 
  * Perpendicular components cancel by symmetry 
 
This shows how symmetry plus superposition simplify the vector addition.
 
== Computational Model (GlowScript) ==
 
Below is a simple GlowScript (VPython) model that computes and visualizes the electric force between two point charges in 3D.
 
You can:
* Paste this into a new GlowScript Trinket (Python / VPython),
* Get the embed code from Trinket,
* Embed that code into this page so it runs directly here.
 
<syntaxhighlight lang="python">
from vpython import *
 
# constant
k = 8.99e9  # N·m^2/C^2
 
# scene setup
scene.caption = "Drag the red charge to see how the force on the blue charge changes.\n"
 
# charges (positions in meters, charges in coulombs)
q1 = 2e-6  # C (blue, fixed)
q2 = -3e-6  # C (red, movable)
 
charge1 = sphere(pos=vector(-0.5, 0, 0), radius=0.05, color=color.blue)
charge2 = sphere(pos=vector(0.5, 0, 0), radius=0.05, color=color.red, make_trail=True)
 
# arrow to show force on q2 due to q1
F_arrow = arrow(pos=charge2.pos, axis=vector(0.2, 0, 0))
 
def electric_force(q1, q2, r1, r2):
    r_vec = r2 - r1
    r = mag(r_vec)
    if r == 0:
        return vector(0, 0, 0)
    F_mag = k * q1 * q2 / r**2
    return F_mag * norm(r_vec)
 
dragging = False
 
def down():
    global dragging
    if scene.mouse.pick is charge2:
        dragging = True
 
def up():
    global dragging
    dragging = False
 
scene.bind("mousedown", lambda evt: down())
scene.bind("mouseup",  lambda evt: up())
 
while True:
    rate(60)
 
    if dragging:
        # move the red charge with the mouse in the x-y plane
        m = scene.mouse.pos
        charge2.pos = vector(m.x, m.y, 0)
 
    F = electric_force(q1, q2, charge1.pos, charge2.pos)
 
    # update arrow to show force on q2
    F_arrow.pos = charge2.pos
    # scale arrow length for visibility (purely visual)
    F_arrow.axis = F * 1e7
</syntaxhighlight>
 
You can extend this model to include more charges or show the net force on a test charge at different locations.
 
== Common Mistakes and How to Avoid Them ==
 
* '''Forgetting that force is a vector.''' 
  Always draw a diagram and keep track of directions. Use components in 2D/3D.
* '''Dropping the absolute value in the magnitude formula.''' 
  [math]\displaystyle{ F = k \dfrac{|q_1 q_2|}{r^2} }[/math] is a positive magnitude. Decide direction separately.
* '''Mixing up [math]\displaystyle{r}[/math] and [math]\displaystyle{r^2}[/math].''' 
  The force goes like [math]\displaystyle{1/r^2}[/math], not [math]\displaystyle{1/r}[/math].
* '''Using wrong units.''' 
  Convert microcoulombs to coulombs, centimeters to meters, etc. 
  [math]\displaystyle{1\ \mu\text{C} = 1 \times 10^{-6}\ \text{C}}[/math].
* '''Trying to memorize instead of understand.''' 
  Focus on inverse-square behavior, sign of charges, and superposition.
 
== Connections to Other Topics ==
 
* '''Electric Field''' – Electric force per unit charge is the electric field: 
  [math]\displaystyle{ \vec{E} = \dfrac{\vec{F}}{q} }[/math].
* '''Potential Energy and Electric Potential''' – Work done by electric forces leads to electric potential energy and voltage.
* '''Lorentz Force''' – The full force on a moving charge also includes magnetic fields: 
  [math]\displaystyle{ \vec{F} = q(\vec{E} + \vec{v} \times \vec{B}) }[/math]. 
  This page focuses on the electric part.
 
== Practice Problems ==
 
You can add your own numerical values and solve them. Consider including full solutions in a collapsible section.
 
# Two charges of [math]\displaystyle{+2.0\ \mu\text{C}}[/math] and [math]\displaystyle{+5.0\ \mu\text{C}}[/math] are 0.30 m apart. 
  * (a) Find the magnitude of the force on each charge. 
  * (b) Is the force attractive or repulsive? Explain.
 
# A charge [math]\displaystyle{q_1 = +4.0\ \mu\text{C}}[/math] is at the origin and [math]\displaystyle{q_2 = -1.0\ \mu\text{C}}[/math] is at [math]\displaystyle{x = 0.20\ \text{m}}[/math]. 
  * Find the electric force on [math]\displaystyle{q_1}[/math] (magnitude and direction). 
  * Verify that Newton’s third law holds (forces are equal and opposite).
 
# Three equal positive charges are placed at the corners of a square of side [math]\displaystyle{a}[/math]. 
  * Find the net force on one of the corner charges. 
  * Use symmetry to simplify the vector addition.
 
# A particle with charge [math]\displaystyle{q = -1.6 \times 10^{-19}\ \text{C}}[/math] experiences an electric force of [math]\displaystyle{3.2 \times 10^{-14}\ \text{N}}[/math] in the +y direction. 
  * (a) What is the electric field at that point (magnitude and direction)? 
  * (b) If the charge were positive instead, what would the force direction be?
 
== What to Review Before an Exam ==
 
* Determine force direction from a diagram, not just from algebra 
* Practice vector addition of multiple forces 
* Do quick estimates: “If I double the distance, what happens to the force?” 
* Know the difference between:
  * Force between charges (Coulomb’s law) 
  * Electric field 
  * Electric potential energy
 
====Electric field of a point particle====
====Electric field of a point particle====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
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<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Superposition====
====Superposition====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
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<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Dipoles====
====Dipoles====
<div class="mw-collapsible-content">
*[[Electric Dipole]]
*[[Magnetic Dipole]]
</div>
</div>
===Week 2===
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Electric Dipole]]
*[[Electric Dipole]]
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<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Polarization====
====Polarization====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
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===Week 3===
===Week 3===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Insulators====
====Conductors and Insulators====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Conductivity and Resistivity]]
*[[Insulators]]
*[[Insulators]]
*[[Potential Difference in an Insulator]]
*[[Potential Difference in an Insulator]]
*[[Charged Conductor and Charged Insulator]]
*[[Conductors]]
*[[Charged conductor and charged insulator]]
*[[Polarization of a conductor]]
</div>
</div>
</div>
</div>


<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Conductors====
<div class="mw-collapsible-content">
*[[Conductivity]]
*[[Charge Transfer]]
*[[Resistivity]]
*[[Polarization of a conductor]]
*[[Charged Conductor and Charged Insulator]]
*[[Charged conductor and charged insulator]]
</div>
</div>


<div class="toccolours mw-collapsible mw-collapsed">
====Charging and Discharging====
====Charging and discharging====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Charge Transfer]]
*[[Charge Transfer]]
*[[Electrostatic Discharge]]
*[[Electrostatic Discharge]]
*[[Charged Conductor and Charged Insulator]]
*[[Charged Conductor and Charged Insulator]]
*[[Charged conductor and charged insulator]]
</div>
</div>
</div>
</div>
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====Field of a charged rod====
====Field of a charged rod====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Charged Rod]]
*[[Field of a Charged Rod|Charged Rod]]
</div>
</div>
</div>
</div>
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<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Field of a charged sphere====
====Field of a charged sphere====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
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====Potential energy====
====Potential energy====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Potential Energy - Claimed by Janki Patel]]
*[[Potential Energy]]
</div>
</div>
</div>
</div>
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</div>
</div>
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
 
====Sign of a potential difference====
====Sign of Potential Difference====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Sign of Potential Difference]]
*[[Sign of a Potential Difference]]
</div>
</div>
</div>
</div>
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Potential at a single location====
====Potential at a single location====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
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</div>
</div>
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Moving charges in a magnetic field====
====Moving charges in a magnetic field====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
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</div>
</div>
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Biot-Savart Law====
====Biot-Savart Law====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
Line 559: Line 792:


====Moving charges, electron current, and conventional current====
====Moving charges, electron current, and conventional current====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Moving Point Charge]]
*[[Moving Point Charge]]
A moving point charge creates both electric and magnetic fields. As the charge accelerates or changes position, it alters the surrounding electromagnetic field, which can influence other charges nearby. This is the fundamental concept behind electromagnetic radiation and wave propagation. When many charges move collectively—such as electrons in a wire—this flow is referred to as electric current. This perfectly segues us into the next section of this page.
*[[Current]]
*[[Current]]
Current is typically measured in amperes and represents the rate at which charge flows through a surface. Although electrons carry the charge and move from negative to positive, conventional current is defined in the opposite direction: from positive to negative. This convention dates back to early scientific assumptions and remains standard in circuit diagrams and equations today.
</div>
</div>
</div>
</div>
Line 567: Line 807:
===Week 7===
===Week 7===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Magnetic field of a wire====
====Magnetic field of a wire====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Magnetic Field of a Long Straight Wire]]
*[[Magnetic Field of a Long Straight Wire]]
*[[Magnetic Field of a Curved Wire]]
</div>
</div>
</div>
</div>


<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Magnetic field of a current-carrying loop====
====Magnetic field of a current-carrying loop====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
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<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Magnetic field of a Charged Disk====
<div class="mw-collapsible-content">
*[[Magnetic Field of a Disk]]
</div>
</div>


<div class="toccolours mw-collapsible mw-collapsed">
====Magnetic dipoles====
====Magnetic dipoles====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
Line 599: Line 846:
===Week 8===
===Week 8===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Circuitry Basics====
<div class="mw-collapsible-content">
*[[Understanding Fundamentals of Current, Voltage, and Resistance]]
</div>
</div>
<div class="toccolours mw-collapsible mw-collapsed">
====Steady state current====
====Steady state current====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
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<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Node rule====
====Kirchoff's Laws====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Loop Rule]]
*[[Node Rule]]
*[[Node Rule]]
</div>
</div>
Line 614: Line 871:


<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Electric fields and energy in circuits====
====Electric fields and energy in circuits====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Series circuit]]
*[[Node Rule]]
*[[Loop Rule]]
*[[Electric Potential Difference]]
*[[Electric Potential Difference]]
</div>
</div>
Line 628: Line 883:
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Series Circuits]]
*[[Series Circuits]]
*[[Parallel CIrcuits]]
*[[Parallel Circuits]]
*[[Parallel Circuits vs. Series Circuits*]]
*[[Parallel Circuits vs. Series Circuits*]]
*[[Loop Rule]]
*[[Loop Rule]]
*[[Node Rule]]
*[[Node Rule]]
*[[Fundamentals of Resistance]]
*[[Fundamentals of Resistance]]
*[[Problem Solving]]
</div>
</div>
</div>
</div>
Line 641: Line 897:
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Charging and Discharging a Capacitor]]
*[[Charging and Discharging a Capacitor]]
*[[RC Circuit]] *CLAIMED BY MARK RUSSELL SPRING 2016
*[[RC Circuit]]  
*[[R Circuit]]
*[[R Circuit]]
*[[AC and DC]]
*[[AC and DC]]
</div>
</div>
</div>
</div>


<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Magnetic forces on charges and currents====
====Magnetic forces on charges and currents====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Magnetic Force]]
*[[Magnetic Force]]
*[[Lorentz Force]]
*[[Lorentz Force]]
*[[Motors and Generators]]
*[[Applying Magnetic Force to Currents]]
*[[Applying Magnetic Force to Currents]]
*[[Magnetic Force in a Moving Reference Frame]]
*[[Magnetic Force in a Moving Reference Frame]]
Line 662: Line 917:


<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Electric and magnetic forces====
====Electric and magnetic forces====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
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<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Velocity selector====
====Velocity selector====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
Line 680: Line 937:


===Week 10===
===Week 10===
<div class="toccolours mw-collapsible mw-collapsed">


====Student Content====
====Hall Effect====
 
<div class=“toccolours mw-collapsible mw-collapsed”>
==== Hall Effect ====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Hall Effect]]
*[[Hall Effect]]
*[[Right-Hand Rule]]
*[[Motional Emf]]
*[[Motional Emf]]
*[[Magnetic Force]]
*[[Magnetic Force]]
*[[Magnetic Torque]]
*[[Magnetic Torque]]
</div>
====Magnetic force====
<div class="mw-collapsible-content">
*[[Magnetic Force]]
*[[Lorentz Force]]
</div>
</div>


<div class="toccolours mw-collapsible mw-collapsed">
====Magnetic torque====
<div class="mw-collapsible-content">
*[[Magnetic Torque]]
*[[Right-Hand Rule]]
</div>
</div>
===Week 12===
<div class="toccolours mw-collapsible mw-collapsed">
====Gauss's Law====
<div class="mw-collapsible-content">
*[[Gauss's Flux Theorem]]
*[[Gauss's Law]]
*[[Magnetic Flux]]
</div>
</div>
<div class="toccolours mw-collapsible mw-collapsed">
====Ampere's Law====
<div class="mw-collapsible-content">
*[[Ampere's Law]]
*[[Ampere-Maxwell Law]]
*[[Magnetic Field of Coaxial Cable Using Ampere's Law]]
*[[Magnetic Field of a Long Thick Wire Using Ampere's Law]]
*[[Magnetic Field of a Toroid Using Ampere's Law]]
*[[Magnetic Field of a Solenoid Using Ampere's Law]]
*[[The Differential Form of Ampere's Law]]
</div>
</div>
</div>
</div>


===Week 13===
===Week 13===
<div class=“toccolours mw-collapsible mw-collapsed”>
<div class="toccolours mw-collapsible mw-collapsed">
==== MAXWELL'S EQUATIONS ====
====Semiconductors====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Gauss's Law]]
*[[Semiconductor Devices]]
</div>
</div>


==== Changing Field Patterns ====
<div class="toccolours mw-collapsible mw-collapsed">
====Faraday's Law====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Faraday's Law]]
*[[Faraday's Law]]
*[[Motional Emf using Faraday's Law]]
*[[Lenz's Law]]
</div>
</div>


<div class="toccolours mw-collapsible mw-collapsed">
====Maxwell's equations====
<div class="mw-collapsible-content">
*[[Gauss's Law]]
*[[Magnetic Flux]]
*[[Ampere's Law]]
*[[Faraday's Law]]
*[[Maxwell's Electromagnetic Theory]]
</div>
</div>
</div>
</div>


===Week 14===
===Week 14===
<div class=“toccolours mw-collapsible mw-collapsed”>
<div class="toccolours mw-collapsible mw-collapsed">
====Circuits revisited====
<div class="mw-collapsible-content">
 
</div>
</div>
 
<div class="toccolours mw-collapsible mw-collapsed">
 
====Inductors====
<div class="mw-collapsible-content">
*[[Inductors]]
*[[Current in an LC Circuit]]
*[[Current in an RL Circuit]]
</div>
</div>
 
===Week 15===
<div class="toccolours mw-collapsible mw-collapsed">
==== Electromagnetic Radiation ====
==== Electromagnetic Radiation ====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Electromagnetic Radiation]]
*[[Electromagnetic Radiation]]
</div>
</div>


<div class="toccolours mw-collapsible mw-collapsed">
====Sparks in the air====
<div class="mw-collapsible-content">
*[[Sparks in Air]]
*[[Spark Plugs]]
</div>
</div>


<div class="toccolours mw-collapsible mw-collapsed">
====Superconductors====
<div class="mw-collapsible-content">
*[[Superconducters]]
*[[Superconductors]]
*[[Meissner effect]]
</div>
</div>
</div>
</div>
</div>
</div>
<div style="float:left; width:30%; padding:1%;">


==Physics 3==
==Physics 3==
Line 724: Line 1,069:
====Classical Physics====
====Classical Physics====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Classical Physics]]
</div>
</div>
</div>
</div>


===Week 2===
[[Category:Which Category did you place this in?]]
 
===Weeks 2 and 3===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Special Relativity====
====Special Relativity and the Lorentz Transformation====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Frame of Reference]]
*[[Frame of Reference]]
*[[Einstein's Theory of Special Relativity]]
*[[Einstein's Theory of Special Relativity]]
*[[Time Dilation]]
*[[Time Dilation]]
*[[Twin Paradox]]
*[[Lorentz Transformations]]
*[[Relativistic Doppler Effect]]
*[[Einstein's Theory of General Relativity]]
*[[Einstein's Theory of General Relativity]]
*[[Albert A. Micheleson & Edward W. Morley]]
*[[Albert A. Micheleson & Edward W. Morley]]
Line 740: Line 1,092:
</div>
</div>


===Week 3===
===Week 4===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Photons====
====Photons and the Photoelectric Effect====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Spontaneous Photon Emission]]
*[[Spontaneous Photon Emission]]
*[[Light Scattering: Why is the Sky Blue]]
*[[Light Scattering]]
*[[Lasers]]
*[[Lasers]]
*[[Electronic Energy Levels and Photons]]
*[[Electronic Energy Levels and Photons]]
*[[Quantum Properties of Light]]
*[[Quantum Properties of Light]]
*[[The Photoelectric Effect]]
</div>
</div>
</div>
</div>


===Week 4===
===Weeks 5 and 6===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Matter Waves====
====Matter Waves and Wave-Particle Duality====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Wave-Particle Duality]]
*[[Wave-Particle Duality]]
*[[Particle in a 1-Dimensional box]]
*[[Heisenberg Uncertainty Principle]]
</div>
</div>
</div>
</div>


===Week 5===
===Week 7===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Wave Mechanics====
====Wave Mechanics====
Line 769: Line 1,124:
*[[Mechanical Waves]]
*[[Mechanical Waves]]
*[[Transverse and Longitudinal Waves]]
*[[Transverse and Longitudinal Waves]]
*[[Fourier Series and Transform]]
</div>
</div>
===Week 8===
<div class="toccolours mw-collapsible mw-collapsed">
====Schrödinger Equation====
<div class="mw-collapsible-content">
*[[The Born Rule]]
*[[Solution for a Single Free Particle]]
*[[Solution for a Single Particle in an Infinite Quantum Well - Darin]]
*[[Solution for a Single Particle in a Semi-Infinite Quantum Well]]
*[[Quantum Harmonic Oscillator]]
*[[Solution for Simple Harmonic Oscillator]]
</div>
</div>
===Week 9===
<div class="toccolours mw-collapsible mw-collapsed">
====Quantum Mechanics====
<div class="mw-collapsible-content">
*[[Quantum Tunneling through Potential Barriers]]
</div>
</div>
</div>
</div>


===Week 6===
<div class="toccolours mw-collapsible mw-collapsed">
====The Hydrogen Atom====
<div class="mw-collapsible-content">
*[[Quantum Theory]]
*[[Atomic Theory]]
</div>
</div>
 
===Week 10===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Rutherford-Bohr Model====
====Rutherford-Bohr Model====
Line 783: Line 1,168:
</div>
</div>


===Week 7===
===Week 11===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====The Hydrogen Atom====
====Many-Electron Atoms====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Quantum Theory]]
*[[Quantum Theory]]
*[[Atomic Theory]]
*[[Atomic Theory]]
*[[Pauli exclusion principle]]
</div>
</div>
</div>
</div>


===Week 8===
===Week 12===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Many-Electron Atoms====
====The Nucleus====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Quantum Theory]]
*[[Nucleus]]
*[[Atomic Theory]]
*[[Pauli exclusion principle]]
</div>
</div>
</div>
</div>


===Week 9===
===Week 13===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Molecules====
====Molecules====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Molecules]]
*[[Covalent Bonds]]
</div>
</div>
</div>
</div>


===Week 10===
===Week 14===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Statistical Physics====
====Statistical Physics====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Application of Statistics in Physics]]
</div>
</div>
</div>
</div>


===Week 11===
===Week 15===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Condensed Matter Physics====
====Statistical Physics====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Temperature & Entropy]]
</div>
</div>
</div>
</div>


===Week 12===
===Additional Topics===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====The Nucleus====
====Thermodynamics====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Maxwell Relations]]
*[[Brownian Motion]]
</div>
</div>
</div>
</div>
<div class="toccolours mw-collapsible mw-collapsed">


===Week 13===
<div class="toccolours mw-collapsible mw-collapsed">
====Nuclear Physics====
====Nuclear Physics====
<div class="mw-collapsible-content">
<div class="mw-collapsible-content">
*[[Nuclear Fission]]
*[[Nuclear Fission]]
*[[Nuclear Energy from Fission and Fusion]]
*[[Nuclear Energy from Fission and Fusion]]
*[[Radioactive Decay Processes]]
</div>
</div>
</div>
</div>
===Week 14===
<div class="toccolours mw-collapsible mw-collapsed">
<div class="toccolours mw-collapsible mw-collapsed">
====Particle Physics====
====Particle Physics====
Line 847: Line 1,235:
</div>
</div>
</div>
</div>
</div
<div class="toccolours mw-collapsible mw-collapsed">
====Solid-State/Condensed Matter Physics====
<div class="mw-collapsible-content">
*[[What is Condensed Matter]]
*[[Crystalline Structures]]
*[[Electric-Band Structure]]
</div>
</div>

Latest revision as of 23:21, 30 November 2025


Georgia Tech Student Wiki for Introductory Physics.

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  • A wiki book on modern physics Modern Physics Wiki
  • A collection of 26 volumes of lecture notes by Prof. Wheeler of Reed College [1]
  • The MIT open courseware for intro physics MITOCW Wiki
  • An online concept map of intro physics HyperPhysics
  • Interactive physics simulations PhET
  • OpenStax intro physics textbooks: Vol1, Vol2, Vol3
  • The Open Source Physics project is a collection of online physics resources OSP
  • A resource guide compiled by the AAPT for educators ComPADRE
  • The Feynman lectures on physics are free to read Feynman
  • Final Study Guide for Modern Physics II created by a lab TA Modern Physics II Final Study Guide

Resources


Physics 1

Week 1

GlowScript 101

Vectors and Units

Week 2

Iterative Prediction with a Constant Force

Week 3

Analytic Prediction with a Constant Force

Week 4

Week 5

Week 6

Week 7

Jeet Bhatkar

Energy Principle

The Energy Principle is a fundamental concept in physics that describes the relationship between different forms of energy and their conservation within a system. Understanding the Energy Principle is crucial for analyzing the motion and interactions of objects in various physical scenarios.

Week 8

Work by Non-Constant Forces

Week 9

Week 10

Choice of System

Rotational and Vibrational Energy

Week 11

Different Models of a System

Week 12

Conservation of Momentum

Week 13

Week 14

Week 15

Physics 2

Week 1

Electric force

Jeet Bhatkar – Fall 2025

Big Idea

Electric force is the interaction between objects that have electric charge. It is:

  • Long-range: acts even when charges do not touch
  • Vector-valued: has magnitude and direction
  • Superposable: forces from many charges add as vectors

At the intro level, the electric force between two point charges is described by Coulomb’s law, the electrostatic analog of the gravitational force between masses.

Key Equations

Coulomb’s Law (magnitude)

[math]\displaystyle{ F = k \dfrac{|q_1 q_2|}{r^2} }[/math]
  • [math]\displaystyle{F}[/math] = magnitude of the electric force
  • [math]\displaystyle{k \approx 8.99 \times 10^9\ \text{N·m}^2/\text{C}^2}[/math]
  • [math]\displaystyle{q_1, q_2}[/math] = charges (C)
  • [math]\displaystyle{r}[/math] = separation between the charges (m)

Coulomb’s Law (vector form)

[math]\displaystyle{ \vec{F}_{2 \leftarrow 1} = k \dfrac{q_1 q_2}{r^2} \,\hat{r}_{2 \leftarrow 1} }[/math]
  • [math]\displaystyle{\vec{F}_{2 \leftarrow 1}}[/math] = force on charge 2 due to charge 1
  • [math]\displaystyle{\hat{r}_{2 \leftarrow 1}}[/math] = unit vector from 1 to 2

Relation to Electric Field

[math]\displaystyle{ \vec{F} = q \vec{E} }[/math]

Once you know [math]\displaystyle{\vec{E}}[/math] at a point, you can find the force on any charge [math]\displaystyle{q}[/math] placed there.

Conceptual Picture

Sign of charges

  • Like charges (both positive or both negative) → repel
  • Unlike charges (one positive, one negative) → attract

Distance dependence

  • Force falls off as [math]\displaystyle{1/r^2}[/math], so doubling the distance makes the force 4 times smaller.

Superposition principle If there are many charges, the net force on a given charge is the vector sum of the forces from each individual charge: [math]\displaystyle{ \vec{F}_\text{net} = \sum_i \vec{F}_i }[/math].

Electric vs. gravitational force

  • Both follow inverse-square laws
  • Gravity is always attractive; electric force can be attractive or repulsive
  • Electric forces are usually much stronger at the particle scale

Worked Example 1: Two Point Charges on a Line

Problem. Two charges are placed on the x-axis:

  • [math]\displaystyle{q_1 = +3.0\ \mu\text{C}}[/math] at [math]\displaystyle{x = 0.00\ \text{m}}[/math]
  • [math]\displaystyle{q_2 = -2.0\ \mu\text{C}}[/math] at [math]\displaystyle{x = 0.40\ \text{m}}[/math]

What is the magnitude and direction of the force on [math]\displaystyle{q_2}[/math]?

Solution (outline).

  1. Distance between charges:

[math]\displaystyle{ r = 0.40\ \text{m} }[/math].

  1. Magnitude using Coulomb’s law:

[math]\displaystyle{ F = k \dfrac{|q_1 q_2|}{r^2} = (8.99 \times 10^9)\,\dfrac{(3.0 \times 10^{-6})(2.0 \times 10^{-6})}{(0.40)^2} }[/math]

  1. Sign and direction:
  • [math]\displaystyle{q_1}[/math] is positive, [math]\displaystyle{q_2}[/math] is negative → force is attractive
  • On [math]\displaystyle{q_2}[/math], the force points toward [math]\displaystyle{q_1}[/math]
  • Since [math]\displaystyle{q_1}[/math] is at smaller x, the force on [math]\displaystyle{q_2}[/math] points in the −x direction

You can finish by computing the numerical value and writing it as a vector, e.g. [math]\displaystyle{\vec{F}_{2 \leftarrow 1} = -F\,\hat{x}}[/math].

Worked Example 2: Superposition with Three Charges

Problem. Three equal charges [math]\displaystyle{q}[/math] are at the corners of an equilateral triangle of side [math]\displaystyle{a}[/math]. What is the net force on one of the charges?

Idea (no full algebra).

  • Each of the other two charges exerts a force of magnitude
 [math]\displaystyle{ F = k \dfrac{q^2}{a^2} }[/math]  
  • The angle between these two forces is [math]\displaystyle{60^\circ}[/math]
  • Use vector addition:
 * Add components along the symmetry axis  
 * Perpendicular components cancel by symmetry  

This shows how symmetry plus superposition simplify the vector addition.

Computational Model (GlowScript)

Below is a simple GlowScript (VPython) model that computes and visualizes the electric force between two point charges in 3D.

You can:

  • Paste this into a new GlowScript Trinket (Python / VPython),
  • Get the embed code from Trinket,
  • Embed that code into this page so it runs directly here.

<syntaxhighlight lang="python"> from vpython import *

  1. constant

k = 8.99e9 # N·m^2/C^2

  1. scene setup

scene.caption = "Drag the red charge to see how the force on the blue charge changes.\n"

  1. charges (positions in meters, charges in coulombs)

q1 = 2e-6 # C (blue, fixed) q2 = -3e-6 # C (red, movable)

charge1 = sphere(pos=vector(-0.5, 0, 0), radius=0.05, color=color.blue) charge2 = sphere(pos=vector(0.5, 0, 0), radius=0.05, color=color.red, make_trail=True)

  1. arrow to show force on q2 due to q1

F_arrow = arrow(pos=charge2.pos, axis=vector(0.2, 0, 0))

def electric_force(q1, q2, r1, r2):

   r_vec = r2 - r1
   r = mag(r_vec)
   if r == 0:
       return vector(0, 0, 0)
   F_mag = k * q1 * q2 / r**2
   return F_mag * norm(r_vec)

dragging = False

def down():

   global dragging
   if scene.mouse.pick is charge2:
       dragging = True

def up():

   global dragging
   dragging = False

scene.bind("mousedown", lambda evt: down()) scene.bind("mouseup", lambda evt: up())

while True:

   rate(60)
   if dragging:
       # move the red charge with the mouse in the x-y plane
       m = scene.mouse.pos
       charge2.pos = vector(m.x, m.y, 0)
   F = electric_force(q1, q2, charge1.pos, charge2.pos)
   # update arrow to show force on q2
   F_arrow.pos = charge2.pos
   # scale arrow length for visibility (purely visual)
   F_arrow.axis = F * 1e7

</syntaxhighlight>

You can extend this model to include more charges or show the net force on a test charge at different locations.

Common Mistakes and How to Avoid Them

  • Forgetting that force is a vector.
 Always draw a diagram and keep track of directions. Use components in 2D/3D.
  • Dropping the absolute value in the magnitude formula.
 [math]\displaystyle{ F = k \dfrac{|q_1 q_2|}{r^2} }[/math] is a positive magnitude. Decide direction separately.
  • Mixing up [math]\displaystyle{r}[/math] and [math]\displaystyle{r^2}[/math].
 The force goes like [math]\displaystyle{1/r^2}[/math], not [math]\displaystyle{1/r}[/math].
  • Using wrong units.
 Convert microcoulombs to coulombs, centimeters to meters, etc.  
 [math]\displaystyle{1\ \mu\text{C} = 1 \times 10^{-6}\ \text{C}}[/math].
  • Trying to memorize instead of understand.
 Focus on inverse-square behavior, sign of charges, and superposition.

Connections to Other Topics

  • Electric Field – Electric force per unit charge is the electric field:
 [math]\displaystyle{ \vec{E} = \dfrac{\vec{F}}{q} }[/math].
  • Potential Energy and Electric Potential – Work done by electric forces leads to electric potential energy and voltage.
  • Lorentz Force – The full force on a moving charge also includes magnetic fields:
 [math]\displaystyle{ \vec{F} = q(\vec{E} + \vec{v} \times \vec{B}) }[/math].  
 This page focuses on the electric part.

Practice Problems

You can add your own numerical values and solve them. Consider including full solutions in a collapsible section.

  1. Two charges of [math]\displaystyle{+2.0\ \mu\text{C}}[/math] and [math]\displaystyle{+5.0\ \mu\text{C}}[/math] are 0.30 m apart.
  * (a) Find the magnitude of the force on each charge.  
  * (b) Is the force attractive or repulsive? Explain.
  1. A charge [math]\displaystyle{q_1 = +4.0\ \mu\text{C}}[/math] is at the origin and [math]\displaystyle{q_2 = -1.0\ \mu\text{C}}[/math] is at [math]\displaystyle{x = 0.20\ \text{m}}[/math].
  * Find the electric force on [math]\displaystyle{q_1}[/math] (magnitude and direction).  
  * Verify that Newton’s third law holds (forces are equal and opposite).
  1. Three equal positive charges are placed at the corners of a square of side [math]\displaystyle{a}[/math].
  * Find the net force on one of the corner charges.  
  * Use symmetry to simplify the vector addition.
  1. A particle with charge [math]\displaystyle{q = -1.6 \times 10^{-19}\ \text{C}}[/math] experiences an electric force of [math]\displaystyle{3.2 \times 10^{-14}\ \text{N}}[/math] in the +y direction.
  * (a) What is the electric field at that point (magnitude and direction)?  
  * (b) If the charge were positive instead, what would the force direction be?

What to Review Before an Exam

  • Determine force direction from a diagram, not just from algebra
  • Practice vector addition of multiple forces
  • Do quick estimates: “If I double the distance, what happens to the force?”
  • Know the difference between:
 * Force between charges (Coulomb’s law)  
 * Electric field  
 * Electric potential energy

Electric field of a point particle

Week 2

Interactions of charged objects

Tape experiments

Week 3

Week 4

Field of a charged rod

Field of a charged ring/disk/capacitor

Field of a charged sphere

Week 5

Potential energy

Sign of a potential difference

Potential at a single location

Week 6

Electric field and potential in an insulator

Moving charges in a magnetic field

Moving charges, electron current, and conventional current

Week 7

Magnetic field of a current-carrying loop

Magnetic field of a Charged Disk

Magnetic dipoles

Atomic structure of magnets

Week 8

Steady state current

Kirchoff's Laws

Electric fields and energy in circuits

Week 9

Electric field and potential in circuits with capacitors

Week 10

Magnetic torque

Week 12

Week 13

Semiconductors

Week 14

Circuits revisited

Week 15

Electromagnetic Radiation

Sparks in the air

Physics 3

Week 1

Classical Physics

Weeks 2 and 3

Week 4

Weeks 5 and 6

Week 7

Week 8

Week 9

The Hydrogen Atom

Week 10

Week 11

Week 12

The Nucleus

Week 13

Molecules

Week 14

Statistical Physics

Week 15

Statistical Physics

Additional Topics

Thermodynamics

Solid-State/Condensed Matter Physics