Physics 114, Summer 2008

Participation Points #3

To be turned in at the end each discussion session, stapled on top of #2 & #1

-TA (HW grading, tutoring, solution manual,…) information: Naresh Jallipalli (Naresh.jallipalli@umb.edu), S-3-013

-Tutoring schedule (in room S-3-013): 10:30 -5:30 (Mondays to Thursdays)

Student’s Name:____________________ Team #________

1.- (a) The electric potential is constant throughout an entire volume, what must be true of the electric field in that region? What happens if instead the potential increases linearly with distance?:

(b) Is the potential at the center of a hollow, uniformly charged spherical shell higher, lower, or the same as that at the surface?. Explain:

2.- Electric potential. Find the on-axis potential for a charged ring (Q) and calculate the electric field from the potential. Compare with the field obtained using vector superposition. (Hint: use scalar superposition for potential from infinitesimal elements of charge)

3.- (a) A solid conducting slab with thickness a little bit smaller than the separation between the two plates of a capacitor is introduced between them. Does the capacitance increase, decrease, or stay the same? Explain.

(b) A capacitor is said to carry a charge Q, what is the net charge on the entire capacitor?

4.- Capacitors. A solid conducting slab is inserted between the plates of a charged capacitor, separated at a distance d. The slab thickness is 50% of the plates spacing, with area that of the plates. (a) what happens to the capacitance (Hint: the thickness of the field region is reduced with the introduced slab)

(b) What happens to the stored energy is the capacitor is isolated?

5.- (a) A 50W and a 100W light bulb are both designed to operate at 120V. Which has a lower resistance?

(b) How would you connect a pair of equal resistors across an ideal battery in order to get the most power dissipation in the resistors?

6.- Electric circuits. A battery’s voltage is measured with a voltmeter whose resistance is 1000 Ohm, the result is 4.5V. When the measurement is repeated with a 1500 Ohm meter, the result is 4.2V. (a) What is the battery voltage? (Hint: the internal resistance of the battery and the resistance of the voltmeter are in series with the battery’s emf)

(b) What is the battery’s internal resistance?

7.- An electron moves through a region in a straight line at constant speed. Can you conclude that there is no magnetic field in the region? Yes No. What if there were no electric or gravitational forces on the electron? Explain.

8.- The magnetic field. In 2 μs, an electron moves 15 cm in the direction of a 0.1 T magnetic field. If the electron’s velocity components perpendicular and parallel to the field are equal (a) what is the length of its actual spiral trajectory?

(b) How many orbits about the field direction does it complete? (Hint: the cyclotron period is T=2πm/(eB)).

9.- Sources of Magnetic Field. A conducting slab extends infinitely in the x and y directions and has thickness h in the z direction. It carries a uniform current density J= J i. Find the magnetic field strength (a) inside the slab (Hint: use Ampere’s Law)

(b) outside the slab as a function of the distance z from the center plane of the slab.