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# Deduce Eq. 11.100 from Eq. 11.99. Here are three

ISBN: 9780321856562 45

## Solution for problem 20P Chapter 11

Introduction to Electrodynamics | 4th Edition

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Problem 20P

Deduce Eq. 11.100 from Eq. 11.99. Here are three methods:

(a) Use the Abraham-Lorentz formula to determine the radiation reaction on each end of the dumbbell; add this to the interaction term (Eq. 11.99).

(b) Method (a) has the defect that it uses the Abraham-Lorentz formula—the very thing that we were trying to derive. To avoid this, let F(q) be the total d-independent part of the self-force on a charge q. Then

where Fint is the interaction part (Eq. 11.99), and F(q/2) is the self-force on each end. Now, F(q) must be proportional to q2, since the field is proportional to q and the force is qE. So F(q/2) = (1/4)F(q). Take it from there.

(c) Smear out the charge along a strip of length L oriented perpendicular to the motion (the charge density, then, is λ = q/L); find the cumulative interaction force for all pairs of segments, using Eq. 11.99 (with the correspondence q/2 → λ dy1, at one end and q/2 → λ dy2 at the other). Make sure you don’t count the same pair twice.

Step-by-Step Solution:

a.)

Step 1 of 3</p>

We have to deduce the equation from  .

The equation for   can be deduced using the Abraham-Lorentz formula for the radiation reaction force.

The radiation reaction on each end of the dumbbell is

So,

Therefore, we have deduced the required equation .

b.)

Step 2 of 3</p>

We have to deduce the same equation from using the suggested method.

The total -independent part of the self-force on a charge  is given as

and

Thus,

Therefore, we have deduced the required equation .

c.)

Step 3 of 3

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