The drawing shows a set of equipotential surfaces seen in cross-sections. Each i
ID: 2270387 • Letter: T
Question
The drawing shows a set of equipotential surfaces seen in cross-sections. Each is labeled according to its electric potential. A 1.5 x 10-7 C point charge is placed at position A. Find the work done on the point charge by the electric force when it is moved (a) from A to B, and (b) A to C.
Explanation / Answer
A is at +350V
B is at +550V
Change in potential (V) going from A to B is: ?V= 550-350 = +200V (i,e the potential has been raised by 200V).
By definition, the change in potential (?V) is the work done per unit positive charge AGAINST the electric field.
(a) a ---> b
w = q * ?V
= +1.5 * 10^-7(Vb - Va )
= 1.5 * 10^-7 * ( 550 - 350 )
= 3.0 x 10^-5 J
(It's a bit like finding the work done by gravity when something is pushed UP a hill. Gravity does positive work when something goes downhill, but negative work when something goes uphill.)
b.
A and C have the same potential, so ?V=0 and there is no work done by the field.
a ---> c
w = q * ?V ----> ?V = 0
= 0
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