Physics 208 ST Quiz 3

Lecturer: Dr. Min Xu

May 7, 2002
LAST NAME: ___________________________________________________________________________________________________ FIRST NAME: __________________________________________________________________________________________________ SS# (last 4 digits): _____________________________________________________________________________________________

General Instruction

You have 1 hour 30 minutes to answer 4 questions on this exam. Work each problem only on the sheet provided for this problem, using the back if necessary. Please write the name and recitation section on each sheet at the top of the page.

You must indicate the reasoning and show all the steps.

If you are unable to obtain the answer to part of a problem and you need the answer to solve the subsequent parts, use an appropriate symbol for the answer to the previous part and solve in terms of that 1symbol.

1. (20 points) An electrical conductor has a circular cross section of diameter 4mm and is 20m long. The resistance between its ends is 0.12_O_.


(a) (5 points) What is the resistivity of the material?
(b) (5 points) If the electric field magnitude in the conductor is 1.5V/m, what is the total current?
(c) (5 points) If the material has 8.5 × 1028 free electrons per cubic meter, find the average drift speed under the condition of part (b).
(d) (5 points) When the conductor is cooled down to 20K, it becomes a superconductor. Draw a schematic plot showing the variation of the resistivity of the conductor with absolute temperature.

2. (25 points) Consider a multiloop circuit shown in the following figure.


(a) (15 points) Find the current in each part of the circuit.
(b) (5 points) What is the energy dissipated in the 4_O_ resistor?
(c) (5 points) What is the potential V cd?
PIC
Figure 1: Figure for question 2.

3. (25 points) A square 12-turn coil with sides of length 40cm carries a current of 3A. It lies in the x-y plane in a uniform magnetic field B = (0.3i + 0.4k)T, where i and k are unit vectors along x and z axes, respectively. The current flow in the coil is counter clockwise.


(a) (10 points) Find the magnetic moment of the coil, including its direction.
(b) (10 points) Find the torque exerted on the coil.
(c) (5 points) Draw a neat diagram showing the coil, current flow, magnetic field, magnetic moment and torque.

4. (35 points) A long, straight wire carries a current of 10A. An electron is traveling in the vicinity of the wire.


(a) (10 points) What is the direction of the magnetic field produced by the current? Draw a neat diagram showing the magnetic field lines due to the current. Remember the density of the magnetic field lines should be proportional to the magnitude of the magnetic field.
(b) (15 points) At the instant when the electron is 10cm from the wire and traveling with a speed of 1×105m/s parallel to the wire in the same direction as the current, what are the direction and magnitude of the force that the magnetic field of the current exerts on the electron?
(c) (10 points) Will the electron travel in a circular path after the instant given in (b)? Explain.