PhysicsLAB Amusing Problems
The Birthday Cake

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Dr. J's birthday turns out to be an electrifying experience for everyone involved. Plans are made for a big surprise party, and everyone is invited - J. Jr., Tripod, Timex, and yes, even members of SAP. J. Jr. orders a giant cake with a model, who looks like Madame Curie dressed in a radioactive bikini, ready to jump out. However, the Atomic Energy Commission and the Committee for Public Decency both decide that this involves too much exposure. Fortunately, SAP has gone to the Optimistic Bakery, which makes cakes out of electrons (pessimistic people find them repulsive). So, as Dr. J stands over his cake at a distance of 1 meter making a wish, he does not realize that he is looking at 1.5 kg of pure electrons. If he did, he surely would not open his gift of a single electron. In no time at all, the electron is 21 meters away from the cake.
How much work is done on the electron by the cake's electric field?

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