Physics 1B Week 5 Questions Fall 1998
1. An infinite metal plane has uniformly distributed positive charges on it. The charge density is 1 C/m2. (a) Find out the direction and the magnitude of the electric field on each side of the plane. (b) If we put an electron at the right side of the plane, find out the electric force experienced by the electron. (c) If another infinite metal plane with a uniform charge density -1 C/m2 is inserted between the original plane and the electron, what is the electric force experienced by the electron?
(a) and (b) (c)
2. Figure (a) shows the electric potential on a grid. Draw the 50V, 75V and 100V equipotential contours. Figure (b) shows an equipotential contour, find out where the electric field is the strongest and the weakest. If you are pushing a proton from the outmost contour to the innermost one, between the route where the field is the strongest and the route where the field is the weakest, which one would you like to take? Do you save energy by choosing one instead of the other?
(a)
(b)
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Physics 1B Week 5 Questions Fall 1998
3. In this figure, each capacitance is 2mF, calculate the equivalent capacitance of the network. If the voltage across A and B is 10V, calculate the charge on each capacitor.
4.
Will a bird get a shock sitting on one bare high voltage line?
5. Consider a simple capacitor made of a close pair of conducting plates. Suppose the plates are charged + and -, and then discharged to produce a spark. Next, the plates are charged again exactly as they previously are, only this time after being charged they are pulled further apart. If they are shorted out a second time, the spark produced will be (a) bigger than the first spark; (b) smaller than the first one; (c) the same.
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Physics 1B Week 5 Solutions Fall 1998
1.
(a) E=s/2e0=6.6 x 1010 N/C. Points to the right at right side of the plane, left at the left side. (b) q=-1.6 x 10-19C. F=Eq=1.06 x 10-8 N, points toward the plane. (c) 0.
2. (b) Route (1) has the strongest field. Route (2) has the weakest, and is the easiest. But choosing different routes doesn’t save energy.
(a) (b)
3. Ceq = C+C/3 = 2.67 mF. Q(left)=CVAB=2 x 10-5C. Q(upper right)=Q(middle right)=Q(lower right)=CVAB/3=6.7 x 10-6 C.
4. No. Although the line is high voltage w.r.t. the ground, the voltage across the bird is tiny and wouldn’t cause any harm.
5. (a) is right. The potential energy stored=Q2/2C. Q doesn’t change when the capacitor was pulled further apart, but C decreases (because Cµ 1/distance), therefore the potential energy increases.