Phy 214 Test 2 12 pts each 2 points free! Spring 2004

1. Find the system of equations that will obtain current in each of the resistors in the figure below.

2. A certain capacitor made up of two circular disks of cross-sectional area A , and distance s between them is connected to a 10 V battery and receives a total charge Q on the plates. By what factor does the charge on the plates change when :

a) The cross sectional area A of each plate is doubled ?

b) The distance between the plates s is doubled?

c) A piece of plastic with dielectric constant 5 is inserted between the plates?

d. After the capacitor is fully charged and current ceases to flow, which way does the current flow when the plates of the capacitor are pulled further apart, backward or forward?

3. Two nuclei, each of mass m and charge q, are travelling straight toward each other. They each have an initial speed of v when they are far apart. As they approach they slow down and come to a momentary halt a distance d from each other, before rebounding straight away from each other. Ignoring the effects of gravity,

a. Write an energy equation that could be solved for the distance d of closest approach.

b. Solve the equation for d in terms of q, v and m.

4. If the charge on each plate is 3 C , the distance between the plates is 3 m, and the area of each plate is 4 m²:

a. What is the capacitance of this capacitor?

b. What is the potential difference DV_AB ?

5. 50 cm long, 0.25 mm diameter

15 cm long, 0.35mm diameter

Here is a circuit consisting of two flashlight batteries and two nichrome wires of different lengths and thicknesses as shown .

The energy input by each battery in moving an electron from the "+" end of the battery to the "-" end is 2.4 x 10^-19 Joules. Determine the steady - state electric field inside each nichrome wire. Remember that in steady state you must satisfy both current conservation and energy conservation. These two principles give you the two equations necessary to obtain the two unknown fields.

6. Here is a circuit composed of two batteries, two identical long bulbs, and an initially uncharged capacitor. Three compasses are placed on the desktop with the wires running on top of the compasses. All the compasses point north before the gap is closed. Then the gap is closed, and the compass on the left immediately shows about a 20 degree deflection.

a. Draw the compass needle positions on all three compasses at the instant when the leftmost compass shows a 20- degree deflection, and write the approximate angle of deflection beside the other two compasses.

b. When this experiment was performed with one of the bulbs removed from its socket, the single bulb glowed for 15 seconds. Predict how long the two bulbs would glow when both are in their sockets (starting with thecapacitor discharged).

7. Connect each of these circuits.

a. Give the time it takes for the bulb to go out for each circuit.

b. Explain these results in terms of electron flow and fringe electric field.

8. When a long bulb and short bulb are connected in series with a battery, the long bulb lights and the round one doesn’t. Explain why in terms of electron current and electric field.