Magnetic Field of a Toroid Using Ampere's Law: Difference between revisions

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Claimed by Kevin McGorrey
This page explains how to use Ampere's Law to solve for the magnetic field of a toroid.
This page explains how to use Ampere's Law to solve for the magnetic field of a toroid.


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Using Ampere's Law simplifies finding the magnetic field of a toroid since using the Biot-Savart law would be extremely difficult due to having to integrate over all the current elements in the toroid.
Using Ampere's Law simplifies finding the magnetic field of a toroid since using the Biot-Savart law would be extremely difficult due to having to integrate over all the current elements in the toroid.
===Geometry of a Toroid===
===Geometry of a Toroid===
A toroid is essentially a solenoid whose ends meet. It is shaped like a doughnut (the base shape does not need to be a circle; it can be a triangle, square, quadrilateral, etc.), is symmetrical around an axis of rotation, and contains ''N'' loops around a closed, circular path with a radius of ''r'' inside of its loop.
[[File:Geometry Fig.JPG |frame|right|alt=Alt text| A toroid's geometry.]]
A toroid is essentially a solenoid whose ends meet. It is shaped like a doughnut (the base shape does not need to be a circle; it can be a triangle, square, quadrilateral, etc.), is symmetrical around an axis of rotation, and contains ''N'' loops around a closed, circular path with a radius of ''r'' inside of its loop. Mathematically, it can be described as an object or surface generated by revolving a closed plane around an external axis that is parallel to it so it does not intersect. Toroids are commonly used as electronic compartments since they  generate a magnetic field or frequency depending on the material of the ring and the type of wire and the number of loops the wire is wound around the ring.


===A Mathematical Model===
===A Mathematical Model===
 
[[File:Computation Fig.JPG | left|frame|none|alt=Alt text| A toroid's view from above.]]
First we start with solving the path integral from Ampere's law:
First we start with solving the path integral from Ampere's law:
<br /> <div style="text-align: center;"><math>{\oint\,\vec{B}&bull;d\vec{l} = &mu;_{0}&sum;I_{inside&ensp;path}}</math></div>
<br /> <div style="text-align: center;"><math>{\oint\,\vec{B}&bull;d\vec{l} = &mu;_{0}&sum;I_{inside&ensp;path}}</math></div>
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<br /> <div style="text-align: center;"><math>{B = \frac{&mu;_{0}NI}{2&pi;r}}</math></div>
<br /> <div style="text-align: center;"><math>{B = \frac{&mu;_{0}NI}{2&pi;r}}</math></div>


<br />
==Examples==
==Examples==
===Simple===
[[File:Simple Example Fig.JPG | right|frame|none|alt=Alt text| Figure for the simple example.]]
A toroid frame is made out of plastic of small square cross section and tightly wrapped uniformly with 100 turns of wire, so that the magnetic field has essentially the same magnitude throughout the plastic (radius R of the curved part is much larger than cross section width w). With a current of 2 A and radius of 5 m, what is the magnetic field inside the plastic.
[[File:Simple Example Sol.JPG | center|frame|none|alt=Alt text| The solution for the simple example.]]


Be sure to show all steps in your solution and include diagrams whenever possible
===Middling - Difficult===
[[File:Mid - Diff Example Fig.JPG | right|frame|none|alt=Alt text| Figure for the middle and difficult examples.]]
The toroid shown in the diagram has an inner radius of <math>R_{i}</math> and an outer radius of <math>R_{o}</math> and is centered at the origin in the diagram. The z-axis passes through the center of the doughnut hole. This toroid is wrapped with <math>N</math> loops of current <math>I</math> flowing up the outside surface of the toroid, radially inward, down the inner surface, and then radial outward. Assume that the magnetic field produced by this toroid has the form <math>\vec{B} = B(r,z)\hat{&phi;}</math> '''at every point in space''' where <math>r</math> is the perpendicular distance from the z-axis and <math>\hat{&phi;}</math> is a unit vector which "curls" around the z-axis, i.e., it is always tangent to any circle with rotational symmetry around the z-axis.


===Simple===
<br />
Use Ampere's law to calculate the magnetic field inside a toroid with 100 turns that has an inner radius of 5 m, an outer radius of 10 m, and carries a current of 2 A?
====Middling====
(a.)  Consider a z-axis centered Amperian loop in the plane of the toroid, at <math>z = 0</math>, with a radius <math>r < R_{i}</math> and use it to find the magnitude of the magnetic field inside the inner radius of the toroid.
 
<br />
(b.) Consider a z-axis centered Amperian loop in the plane of the toroid, at <math>z = 0</math>, with a radius <math>r > R_{o}</math> and use it to find the magnitude of the magnetic field inside the inner radius of the toroid.
[[File:Mid - Diff Example Sol ab.JPG | center|frame|none|alt=Alt text| The solution for the middle example.]]
 
<br />
====Difficult====
(c.) Consider a z-axis centered Amperian loop in the plane of the toroid, at <math>z = 0</math>, with a radius <math>R_{i} < r < R_{o}</math> and use it to find the magnitude of the magnetic field inside the inner radius of the toroid.
 
<br />
(d.) Consider a z-axis centered Amperian loop far above the toroid <math>z >> R_{o}</math>, with a radius <math>R_{i} < r < R_{o}</math> and use it to find the magnitude of the magnetic field far above the toroid.
[[File:Mid - Diff Example Sol cd.JPG | center|frame|none|alt=Alt text| The solution for the difficult example.]]
<br />


===Middling===
====Another Example====
===Difficult===
(a.) Derive the formula for the magnitude of the magnetic field inside a coil that has the shape of a torus whose minor radius is much smaller than the length of the central circle. The toroidal coil has <math>N</math> turns per unit length and current <math>I</math> flows through it.
<br/>
[[File:ToroidalExample.JPG]]


==Connectedness==
Solution:
#How is this topic connected to something that you are interested in?
<br />We determine the magnitude of the magnetic B-field by using Ampere’s law
#How is it connected to your major?
<br /> <div style="text-align: center;"><math>{\oint\,\vec{B}&bull;d\vec{l} = &mu;_{0}&sum;I_{inside&ensp;path}}</math></div>
#Is there an interesting industrial application?
We chose a circle running through the center of the hole of the toroid for Ampere’s loop. The length of this circle is
<br /> <div style="text-align: center;"><math>{l ={2&pi;r}}</math></div>
The total current flowing through the circle bordered by the Ampere’s loop is
<br /> <div style="text-align: center;"><math>{I_{c} = NlI = {2&pi;r}NI}</math></div>
The magnetic field vector <math>B</math> is parallel to the vector of the length element <math>dl</math> all the way around the integration path. Thus we can express the right-hand side of Ampere’s law as
<br /> <div style="text-align: center;"><math>{\oint\,\vec{B}&bull;d\vec{l} = B{2&pi;r}}</math></div>
The left-hand and the right-hand sides of the Ampere’s law must be equal and we can express the magnitude of the <math>B</math>-field as
<br /> <div style="text-align: center;"><math>{B2&pi;r ={&mu;_{0}NI}{2&pi;r}}</math></div>
<br /> <div style="text-align: center;"><math>{B ={&mu;_{0}NI}}</math></div>
The final answer is then <math>{B ={&mu;_{0}NI}}</math>.


== See also ==
== See also ==


Are there related topics or categories in this wiki resource for the curious reader to explore?  How does this topic fit into that context?
[[Ampere's Law]]
<br />
[[Magnetic Field of a Long Thick Wire Using Ampere's Law]]
<br />
[[Magnetic Field of Coaxial Cable Using Ampere's Law]]


===Further reading===
===Further reading===
 
Chabay, Sherwood. (2015). Matter and Interactions (4th ed., Vol. 2). Raleigh, North Carolina: Wiley.
Books, Articles or other print media on this topic


===External links===
===External links===
 
*http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/toroid.html
Internet resources on this topic
*http://www.phys.uri.edu/gerhard/PHY204/tsl242.pdf
*https://clas-pages.uncc.edu/phys2102/online-lectures/chapter-7-magnetism/7-3-amperes-law/example-magnetic-field-of-a-toroid/
*https://ocw.mit.edu/courses/physics/8-02t-electricity-and-magnetism-spring-2005/lecture-notes/ch9sourc_b_field.pdf
*http://physicstasks.eu/1784/magnetic-field-inside-a-toroid


==References==
==References==
 
Chabay, Sherwood. (2015). Matter and Interactions (4th ed., Vol. 2). Raleigh, North Carolina: Wiley. Pg 888-889.
This section contains the the references you used while writing this page
<br />
 
Fall 2014 Test 4 from Phys 2212.
[[Category:Maxwell's Equations]]
[[Category:Maxwell's Equations]]

Latest revision as of 22:12, 18 April 2018

This page explains how to use Ampere's Law to solve for the magnetic field of a toroid.

Magnetic Field of a Toroid using Ampere's Law

Using Ampere's Law simplifies finding the magnetic field of a toroid since using the Biot-Savart law would be extremely difficult due to having to integrate over all the current elements in the toroid.

Geometry of a Toroid

Alt text
A toroid's geometry.

A toroid is essentially a solenoid whose ends meet. It is shaped like a doughnut (the base shape does not need to be a circle; it can be a triangle, square, quadrilateral, etc.), is symmetrical around an axis of rotation, and contains N loops around a closed, circular path with a radius of r inside of its loop. Mathematically, it can be described as an object or surface generated by revolving a closed plane around an external axis that is parallel to it so it does not intersect. Toroids are commonly used as electronic compartments since they generate a magnetic field or frequency depending on the material of the ring and the type of wire and the number of loops the wire is wound around the ring.

A Mathematical Model

Alt text
A toroid's view from above.

First we start with solving the path integral from Ampere's law:


[math]\displaystyle{ {\oint\,\vec{B}&bull;d\vec{l} = &mu;_{0}&sum;I_{inside&ensp;path}} }[/math]


The magnetic field, [math]\displaystyle{ {\vec{B}} }[/math], is, due to the symmetry of a toroid, constant in magnitude and always tangential to the circular path (i.e. parallel to [math]\displaystyle{ {d\vec{l}} }[/math]). The path of a toroid is circular, so [math]\displaystyle{ {\oint\,d\vec{l}} }[/math] is equal to [math]\displaystyle{ {2&pi;r} }[/math]. Therefore, the path integral of the magnetic field is equal to [math]\displaystyle{ {B2&pi;r} }[/math]. The amount of current piercing the soap film (i.e. [math]\displaystyle{ {&sum;I_{inside&ensp;path}} }[/math]) is [math]\displaystyle{ NI }[/math], where [math]\displaystyle{ N }[/math] is the number of piercings (i.e. turns in the coil) and [math]\displaystyle{ I }[/math] is the current. Ampere's law is now this:


[math]\displaystyle{ {B2&pi;r = &mu;_{0}NI} }[/math]


From this, we can solve for the magnetic field for a toroid:


[math]\displaystyle{ {B = \frac{&mu;_{0}NI}{2&pi;r}} }[/math]



Examples

Simple

Alt text
Figure for the simple example.

A toroid frame is made out of plastic of small square cross section and tightly wrapped uniformly with 100 turns of wire, so that the magnetic field has essentially the same magnitude throughout the plastic (radius R of the curved part is much larger than cross section width w). With a current of 2 A and radius of 5 m, what is the magnetic field inside the plastic.

Alt text
The solution for the simple example.

Middling - Difficult

Alt text
Figure for the middle and difficult examples.

The toroid shown in the diagram has an inner radius of [math]\displaystyle{ R_{i} }[/math] and an outer radius of [math]\displaystyle{ R_{o} }[/math] and is centered at the origin in the diagram. The z-axis passes through the center of the doughnut hole. This toroid is wrapped with [math]\displaystyle{ N }[/math] loops of current [math]\displaystyle{ I }[/math] flowing up the outside surface of the toroid, radially inward, down the inner surface, and then radial outward. Assume that the magnetic field produced by this toroid has the form [math]\displaystyle{ \vec{B} = B(r,z)\hat{&phi;} }[/math] at every point in space where [math]\displaystyle{ r }[/math] is the perpendicular distance from the z-axis and [math]\displaystyle{ \hat{&phi;} }[/math] is a unit vector which "curls" around the z-axis, i.e., it is always tangent to any circle with rotational symmetry around the z-axis.


Middling

(a.) Consider a z-axis centered Amperian loop in the plane of the toroid, at [math]\displaystyle{ z = 0 }[/math], with a radius [math]\displaystyle{ r \lt R_{i} }[/math] and use it to find the magnitude of the magnetic field inside the inner radius of the toroid.


(b.) Consider a z-axis centered Amperian loop in the plane of the toroid, at [math]\displaystyle{ z = 0 }[/math], with a radius [math]\displaystyle{ r \gt R_{o} }[/math] and use it to find the magnitude of the magnetic field inside the inner radius of the toroid.

Alt text
The solution for the middle example.


Difficult

(c.) Consider a z-axis centered Amperian loop in the plane of the toroid, at [math]\displaystyle{ z = 0 }[/math], with a radius [math]\displaystyle{ R_{i} \lt r \lt R_{o} }[/math] and use it to find the magnitude of the magnetic field inside the inner radius of the toroid.


(d.) Consider a z-axis centered Amperian loop far above the toroid [math]\displaystyle{ z \gt \gt R_{o} }[/math], with a radius [math]\displaystyle{ R_{i} \lt r \lt R_{o} }[/math] and use it to find the magnitude of the magnetic field far above the toroid.

Alt text
The solution for the difficult example.


Another Example

(a.) Derive the formula for the magnitude of the magnetic field inside a coil that has the shape of a torus whose minor radius is much smaller than the length of the central circle. The toroidal coil has [math]\displaystyle{ N }[/math] turns per unit length and current [math]\displaystyle{ I }[/math] flows through it.

Solution:
We determine the magnitude of the magnetic B-field by using Ampere’s law


[math]\displaystyle{ {\oint\,\vec{B}&bull;d\vec{l} = &mu;_{0}&sum;I_{inside&ensp;path}} }[/math]

We chose a circle running through the center of the hole of the toroid for Ampere’s loop. The length of this circle is


[math]\displaystyle{ {l ={2&pi;r}} }[/math]

The total current flowing through the circle bordered by the Ampere’s loop is


[math]\displaystyle{ {I_{c} = NlI = {2&pi;r}NI} }[/math]

The magnetic field vector [math]\displaystyle{ B }[/math] is parallel to the vector of the length element [math]\displaystyle{ dl }[/math] all the way around the integration path. Thus we can express the right-hand side of Ampere’s law as


[math]\displaystyle{ {\oint\,\vec{B}&bull;d\vec{l} = B{2&pi;r}} }[/math]

The left-hand and the right-hand sides of the Ampere’s law must be equal and we can express the magnitude of the [math]\displaystyle{ B }[/math]-field as


[math]\displaystyle{ {B2&pi;r ={&mu;_{0}NI}{2&pi;r}} }[/math]


[math]\displaystyle{ {B ={&mu;_{0}NI}} }[/math]

The final answer is then [math]\displaystyle{ {B ={&mu;_{0}NI}} }[/math].

See also

Ampere's Law
Magnetic Field of a Long Thick Wire Using Ampere's Law
Magnetic Field of Coaxial Cable Using Ampere's Law

Further reading

Chabay, Sherwood. (2015). Matter and Interactions (4th ed., Vol. 2). Raleigh, North Carolina: Wiley.

External links

References

Chabay, Sherwood. (2015). Matter and Interactions (4th ed., Vol. 2). Raleigh, North Carolina: Wiley. Pg 888-889.
Fall 2014 Test 4 from Phys 2212.

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