3. Imagine moving a positively charged dust grain around in a region of space filled with a varying
electric potential. The grain has an electric charge of 5.5 μC (5.5 ×10
−6 C) and a mass of 8 milligrams
(8×10
−6 kg).
a. Does the grain have more electric potential energy when it is at a high potential (1000 V)
or at a low potential (200 V)? Explain.
b. Would your answer to part (a) change if the dust grain were negatively charged instead?
Explain.
c. How much energy do I have to expend (in the form of work) to move the positively
charged dust grain from an electric potential of 200 V to an electric potential of 1000 V?
d. If I then release the grain at 1000 V, so that it travels back “down” to 200 V, how fast is it
going when it reaches 200 V?
You may skip the next problem with no penalty. However, if you have time, work this problem to
demonstrate mastery.
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