Ampere-Maxwell Law

Claimed by Maria Rivero

Ampere-Maxwell Law

James Maxwell discovered that a time-varying electric field could be accompanied by a magnetic field. He thought of this after Faraday discovered that a time-varying magnetic field was accompanied by an electric field.

The time rate that is used for this equation is given by the derivative of the electric flux with respect to time. Because The derivative of the flux gives current over epsilon nod, the derivative of the electric flux times epsilon nod will also have units of amperes.

Where B is the magnetic field dl is the change in path The sum of I is the sum of the charges inside the path.

Example

Pick a closed, rectangular path in the xz plane with a height h and a width w. Calculate the speed v of the slab.

At a time change in t, we can calculate the area as the height (h) times the speed over the change in time. ⩟ A = v (⩟ t) h Because the electric field is constant in this region we can also calculate the change in electric flux over time as Evh(⩟ t) /(⩟ t) which is the same as Evh Calculating the path integral for the magnetic field we get that ∮B . dl = Bh cos 0 = Bh An important thing to notice is that there is no current I, so, using the Ampere-Maxwell Law we can see that

Bh = μ. [I+ ε. (vEh)]

but since there is no current,

B = μ. ε. (vE)

From Faraday's law we get that the emf equals the rate of change of the magnetic flux: Eh = Bvh

Substituting E = Bv into our previous equation we get that

B = μ. ε. (v(vB))

Solving for v we get that:

From the picture we see that the speed of light relates the a time varying electric and magnetic field.

What this implies

In the example above we saw that the speed of light relates the a time varying electric and magnetic field. This translates to the fact that an electromagnetic wave propagates at the speed of light.

Conceptual Question

The plot above shows a side and a top view of a capacitor with charge Q with electric and magnetic fields E and B at time t. The charge Q is:

1) Increasing in time

2) Decreasing in time

3) Constant in time

4) I don't know

Answer:

The unit vector N points out of the plane. Given that the magnetic field curls clockwise, the electric flux would be positive and decreasing. Hence E is decreasing. Thus Q must be decreasing, since E is proportional to Q.

Consider a circular capacitor, with an Amperian circular loop (radius r) in the plane midway between the plates. When the capacitor is charging, the line integral of the magnetic field around the circle (in direction shown) is

1) Zero

2) Positive

3) Negative

4) Can’t tell

Answer:

One thing to notice is that there is no enclosed current through the disk. When integrating in the direction shown, the electric flux is positive. Because the plates are charging, the electric flux is increasing. Therefore the line integral is positive.

Connectedness

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See also

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Further reading

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External links

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References

http://bulldog2.redlands.edu/fac/eric_hill/Phys232/Lectures/Ch%2023%20lect%201.pdf http://ocw.mit.edu/high-school/physics/exam-prep/electromagnetism/maxwells-equations/ http://ocw.mit.edu/courses/physics/8-02-physics-ii-electricity-and-magnetism-spring-2007/readings/summary_w13d1.pdf http://www.schoolphysics.co.uk/age16-19/Wave%20properties/Wave%20properties/text/Electromagnetic_radiation/index.html