3. (15 points) For each of the changes described, decide whether the magnitude o

Question

3. (15 points) For each of the changes described, decide whether the magnitude of the net force on charge A will increase, decrease, remain the same, or cannot be determined. Consider each change to be the only change from the situation in the diagram Change Increase Decrease Same Can't be determined Change the sign of charge C from -q to q Change sign of charge A from Q to -Q Increase the angle 0 by 50 2 Long-form Question To receive full credit for the following question, you must show your work and your final answer must include units. 4. (30 points) In the Van de Graaff generator shown in the figure below, a rubber belt carries electrons up through a small hole in a large hollow spherical metal shell. The electrons come off the upper part of the belt and drift througha wire to the outer surface of the metal shell, so that the metal shell acquires a sizable negative charge, approximately uniformly distributed over the sphere. At a time when the sphere has acquired a sizable charge Q, approximately how muclh work must be done by the motor to move one more electron from the base (a Page 3

Explanation / Answer

3. for the givne situation

net force atcharge A

kQ([q - qcos(theta)]i - [qsin(theta)]j)/r^2 = F

|F| = kQq*sqrt((1 - cos(theta))^2 + sin^2(theta)) /r^2 = kQq*sqrt(2 - 2cos(theta))/r^2 = 2kQqcos(theta/2)/r^2

hence

change of sign of C form -q to q, F will increase

change of Afrom Q to -Q, force remains the same

increase theta, Force decreases

4. given

charge on large sphere = Q

distance form base = d

hecne work done on electron = kqQ/d

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