Physics 215 Test 27 points eachMay 13, 1998

1. A single wire across the top of the compass causes a deflection of 30° in needle. If it is now curved back under the compass lined up with the wire on top, the needle will now have:

a. No deflection at all.
b. Only slightly less deflection.
c. Only slightly more deflection.
d. Same deflection in the opposite direction.
e. Twice the deflection in the same direction.

2. Predict the approximate compass deflection angle in compass 2.

3. If two bulbs are hooked in series as indicated below, B1 is the only one that lites . Give a careful explanation for why this could happen.

4. Assuming all the bulbs are identical,

a. Give comparative brightness between B3 and B5.

b. Give comparative brightness between B1 and B4.

c. If I1= 8 x 10¹⁰ electrons/s,
What is I3=?
What is I5=?


5. Here is a view from above of a portion of a circuit containing three identical light bulbs (the rest of the circuit including the batteries is not shown).

(a) The compass is placed on top of the wire, and it deflects 20 degrees away from north as shown (the wire is underneath the compass). What direction are the electrons moving at location Pl ? How do you know?

b) In the steady state, 3 x 10⁸ electrons pass location Pl every second. How many electrons pass location P2 every second? Explain briefly.

(e) Describe the relative brightness of bulbs Bl, B2, and B3. Explain briefly.

6. The charge on the plates of a D Cell battery is considerably less than the charge on a U-tape.
When the current driven by 3 Volts (2 batteries) is travelling thru a wire, it will repel:

a) a positive charged tape.

b) a negatively charged tape.

c) neither tape.

d) both tapes.

 

7. If current flow is inversely proportional to length of wire, (you can think of two bulbs as one bulb with twice the length), why doesn't the current drop in half when a second identical bulb is placed in series ?

 

 

 

8. In the circuit below, all of the wire is made of the same material, but one segment has a much smaller cross sectional area.

c Show the steady-state electric field at the locations indicated, including in the thinner segment be sure to use lengths to indicate relative magnitudes.

9. On the following sequence of diagrams at various times during discharging, show the electron current at the indicated locations, also draw arrows indicate the relative magnitudes and directions of the electric field at each location:

 

10. How is the charging time for a capacitor correlated with the initial current? That is, if the initial current is bigger, is the charging time longer, shorter, or the same? Explain briefly.

 

 

 

11. How does the final (static-equilibrium) charge on the capacitor plates depend on the kind of bulb or the length of nichrome wire in the circuit during charging? Very briefly state why this is.

 

 

12. How does the final (static-equilibrium) charge on the capacitor plates depend on the size of the capacitor plates? On the spacing between the capacitor plates? On the presence of a plastic slab between the plates?

 

 

13. Here are three circuits labeled A, B, and C. All the long bulbs, capacitors, and batteries are identical, and are like the equipment you used in class. The capacitors are initially uncharged. In each circuit the batteries are connected for a short time T and then disconnected. The time T is only twenty percent of the total charging time through a single long bulb, so that the bulb brightness doesn't change much during the time T.

(a) In which circuit (A, B, or C) does the capacitor now have the most charge? Explain.

(b) In which circuit (A, B, or C) does the capacitor now have the least charge? Explain.

14. Here is a circuit consisting of two flashlight batteries, a large air-gap capacitor, a nichrome wire, and thick copper wires. The circuit is allowed to run long enough that the capacitor is fully charged with +Q and -Q on the plates.

Next you push the two plates closer together (but the plates don't touch each other). Describe what happens, and explain why in terms of the fundamental concepts of charge and field (do not use "potential" or "capacitance"). Include diagrams.

10 points Bonus

Some students intended to run a light bulb off two batteries in series in the usual way, but they accidentally hooked up one of the batteries backwards (the bulb is shown as a thin filament):

(a) Use +'s and -'s to show the approximate steady-state charge distribution along the wires and bulb.

(b) Draw vectors for the electric field at the indicated locations inside the connecting wires and bulb.

(c) Compare the brightness of the bulb in this circuit with the brightness the bulb would have had if one of the batteries hadn't been put in backwards.