Phy 214         Test 2    7  points each 2pts Absolutely free!           Spring 2007

1.   Connect each of these circuits.   Make sure you have completely discharged the capacitor before each trial.

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

b.  Explain these results.

 

2.  What happens to the capacitance of a capacitor when :

a.  The size of the plates is doubled?

b.  The distance between the plates is doubled ?

c.  A dielectric with dielectric constant 4 is placed between the plates ?

3.    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 pull the two plates further apart. Which way do electrons flow?  Eexplain why in terms of the fundamental concepts of charge and field (do not use "potential" or "capacitance"). Include diagrams.

                 

4. Two particles, with charges of 20.0 nC and -20.0 nC, are placed at the points with coordinates (0, 4.00 cm) and (0, -4.00 cm), as shown in the figure below. A particle with charge 10.0 nC is located at the origin. (a) Find the electric potential energy of the configuration of the three fixed charges. (b) A fourth particle, with a mass of

2.00 x 10^-13 kg and a charge of 40.0 nC, is released from rest at the point (3.00 cm, 0). Find its speed after it has moved freely to a very large distance away.

                  

5.   If the charge on each plate is 4 C , the distance between the plates is 6 m, and the area of each plate is 6 m²:

                     

a.  What is the capacitance of this capacitor?

b.  What is the potential difference between the plates? 

c. What will the potential difference between the plates be after a 2m thick piece of metal is centered between the two plates (represented by the dotted rectangle )?

 

d.  5 pts Bonus!  What will be the potential difference between the plates if the piece  of metal is replaced by a piece of plastic whose dielectric constant is 5?

6.  a.  What is DV_BC  = V_C-V_Bin the figure below?

                

b.  What is the work required to move a -2 C charge from B to C in this figure?

c.   The voltage V at C is actually the change in voltage DV from where to C ?

d.  What is DV_AC   ?

7.     Rank these bulbs in order of brightness from highest to least .  All bulbs are identical.

               

8.   Find the  total capacitance between a and b.

                

9.  Fnd the total resistance between a and b.

                               

10.  The resistance of a platinum wire is to be calibrated for low-temperature measurements. A platinum wire with resistance 1.00 Ω at 20.0°C is immersed in liquid nitrogen at 77 K (-196°C). If the temperature response of the platinum wire is linear, what is the expected resistance of the platinum wire at -196°C?

(a for platinum = 3.92 x 10^-3/°C)

11.  The ammeter shown in Figure below reads 2.00 A. Find the system of equations that determine I1, I2, and e.

                     12.  The quantity of charge q (in coulombs) that has passed through a surface of area 2.00 cm² varies with time according to the equation q = 4t³ + 5t + 6, where t is in seconds.  What is the instantaneous current through the surface at t = 1.00 s?

13.   Bulbs are identical in mechanical construction.   The steady state electron current with three bulbs in the circuit is 3x 10¹⁷ electrons / sec.   Next, the middle bulb, B2 is removed and replaced by a wire.   Now what is the electron current in the circuit .   Show your work.

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14.  When a round bulb and long bulb are connected in series, the long bulb lights and the round bulb does not.  Explain why.