Sample Final Exam (Ch. 27 34)

NAME______PHYSICS 38 Sample Final Exam (Ch. 27 – 34) NOTE: THESE ARE NOT THE ONLY KINDS OF QUESTIONS THAT MAY APPEAR ON THE ACTUAL FINAL EXAM All work, including formulas, must be shown for complete credit. Round off answers to three significant digits. All questions are worth ten points unless otherwise indicated.

1. Two wires are connected in parallel with a battery, as shown below. The wires have the same dimensions: length = 3.00 meters, diam = 0.500 mm. One wire is made of copper, the other wire has one half of its length made of aluminum and the other half is iron. Find the voltage drop across the iron segment. -8 -8 -8 Resistivities are: Cu = 1.7 X 10 -m, Al = 2.44 X 10 -m, Fe = 5.6 X 10 -m

Ans: 8.36 volts 2. In the circuit shown below, the switch S has been open for a long time. It is then suddenly

closed. Take  = 10.0 volts, R1 = 50.0 k, R2 = 100 k, and C = 10.0 F. Determine the time constant (a) before the switch is closed and (b) after the switch is closed. (c) Let the switch be

closed at t = 0. Determine the current in R2 as a function of time.

(a) 1.50 sec (b) 1.00 sec (c) I = (1 X 10-4)e-t 3. A thin copper rod 1.00 meter long has a mass of 50.0 grams. What is the minimum current in the rod that would allow it to levitate above the ground in a magnetic field of magnitude 0.100 Tesla?

4.90 Amps

4. (3 pts) A copper wire in the middle of the southbound fast lane of the 405 Freeway carries a current of 20 amps. A copper wire in the middle of the northbound fast land of the same freeway, carries a current of 10 amps. Both currents are towards the south. What is the direction of the resulting net magnetic field at the median of the freeway? (north, south, east, west, up or down?) Explain. up 5. The magnetic moment of a certain coil is given by  = 5i + 2j +3k Ampere-meters2. The coil sits in a magnetic field given by B = 6i +4j  2k Teslas. Find the magnitude of the torque on the coil.

43.3 N-m

6. A rectangular loop of width a and length b is located near a long wire carrying a current I. The distance between the wire and the closest side of the loop is c. The wire is parallel to the long side of the loop. Find an expresion for the total magnetic flux through the loop due to the current in the wire. (Note: B is not constant within the loop.)  = (oIb/2)ln(1 + a/c)

7. A solenoid of length 6.00 cm produces a magnetic field of magnitude 2.00 milliTeslas at its center. If the current in the wires is 3.18 Amps, find the number of turns in the solenoid. Ans: 30 turns

8. (4 points) A loop of wire is moving near a long, straight wire, carriying a constant current I, as shown below. Determine the direction of the induced current in the loop as it moves away from the wire.

Ans: ccw 9. An LC circuit consists of a 35.0-mH inductor and a 0.750-F capacitor. When the circuit is carrying an oscillating current, the total energy in the circuit at any given time is 4.60 X 10-4 Joules. Find (a) the maximum instantaneous current in the inductor, (b) the greatest potential difference across the capacitor, and (c) Find the current in the inductor when the potential difference across the capacitor is 2.00 volts. Ans: (a) 0.162 amp (b) 35.0 volts (c) 0.162 amp

10. (3 pts) A hollow copper tube carries a current along its length. (a) Is B equal to zero inside the tube, outside the tube, both, or neither? Explain.

11. An unknown capacitor, a 0.90-Henry inductor, and a 100- resistor are all connected in series with a 120-volt (rms), 60-Hz power supply. If the rms voltage across the resistor is

Vrms = 35.0 volts, find the value of the capacitance.

Ans 236 F 12. The intensity of sunlight at the Earth’s distance from the Sun is 1370 W/m2. Assume the Earth absorbs 78% of the sunlight incident upon it. Find the total force the Sun exerts on the Earth due to radiation pressure. Radius of Earth R = 6.37 x 106 meters. Take the area of pressure to be the cross-sectional area of the Earth.

Ans: 4.54 x 108 N