PHY 102 Exam #2 Name ______
Instructions:
- Do all of your work in this booklet.
- Show all of your steps in problems for full credit.
- Be clear and neat in your work. Any illegible work, or scribbling in the margins, will not be graded.
- Place youranswers in a box.
- If you need more space, you may use the back of the page and write On back in the problem space.
1. Multiple Choice (8 pts) Find the answer which best fits the question and write it in the space provided.
a. At a location near the equator, the earth's magnetic field is
horizontal and points north. A proton is moving vertically
upward from the ground. What is the direction of the magnetic
force that acts on the proton?
a) North. b) South. c) East. d) West. e) The force is zero.
_____
b. In a transformer the voltage in the secondary coil is less than
that in the primary coil. This is
a) a step-up transformer. b) a step-down transformer.
c) a gobot transformer. d) an efficient transformer.
e) None of these.
_____
c. Suppose you want to produce a strong magnetic field that is
constant in magnitude and direction in a region of space. Which
one of the following would you choose to produce this field?
a) The interior region of a long current-carrying solenoid.
b) The exterior region of a long current-carrying solenoid.
c) The region outside a long, straight, current-carrying wire.
d) The region surrounding a loop of a current-carrying wire.
_____
d. The impedance has units of
a) farads. b) hertz. c) ohms. d) henries. e) None of these.
_____
e. If one doubles the length of aninductor but keeps the number of
turns the same, the inductance will
a) double. b) halve. c) be the same.d) None of these.
_____
f. If a magnetic field points out of your paper and a positive charge moves from right to left in this field, then the force is directed
a) towards the bottom. b) out of the paper. c) to the left.
d) into the paper. e) None of these.
_____
g. In aninductive circuit the voltage ______the current.
a) leads b) is in phase with c) lags
_____
h. A current carrying wire is perpendicular to your paper with the
current going into the paper from above. The magnetic field
produced looks like
a) a uniform field directed from left to right. b) a uniform
field directed from right to left. c) concentric circles directed
counterclockwise in the plane of the paper. d) concentric circles
directed clockwise in the plane of the paper. e) None of these.
_____
Constants:kg kg
Problem / Points / Score1 / 8
2 / 12
3 / 15
4 / 15
Total
/ 502. Definitions/Principles (12 pts)
a. Write the exact equations for the
i) Magnetic field inside a solenoid.
ii) Cyclotron radius.
iii) Inductive reactance.
b. Three long, equally spaced, straight wires are carrying currents that have the same magnitude. Which wire experiences the largest net force?
c. You have a beam of protons at different speeds. As a proton enters the region between to plates, it encounters electric and magnetic fields. The magnetic field is shown.
i) Show the charge on each plate to make this a velocity selector.
ii) What is the velocity of any non-deflected protons?
iii) Sketch and label the paths of the protons with speeds less than ,greater than and equal to the speed in part (ii).
Bonus:What is the resonant frequency f0 of an LC circuit with mH and F?
3. Problems (15 pts)
a. A current-carrying wire is 0.50 m long and situated in a uniform magnetic field of 1.5 T. If the field is directed at 30o to the wire and the current is 3.0 A,what is the magnitude of the magnetic force exerted on the wire?
b.Suppose you wish to construct a motor that produces a maximum torque whose magnitude isN·m. The coil of the motor has an area of m2, consists of N turns, and contains a current of 2.0 A. The coil is placed in a uniform magnetic field of magnitude 0.20 T. What must N be?
c. A magnetic field perpendicular to the plane of a wire loop with and area of 0.40 m2 decreases from 0.50 T to zero in one tenth of a second. What is the magnitude of the average value of the electromotive force induced in the loop?
d.A step-down transformer has a turns ratio of 1:10, and its primary is connected across the terminals of a standard 120-V wall socket. A 470 resistor is connected across the secondary of this transformer. What power P is dissipated in the resistor?
e. A loop of wire has a radius of R0 and carries a current I0, while a concentric loop has a radius of 2R0 and carries an oppositely directed current of 4I0.If R0 = 0.20 m and I0 = 1.5 A, determine the magnitude and direction of the net magnetic field at the center of the two loops.
4. Problems (15 pts)
a. A series LRC circuit includes a resistance of 30an inductive reactance of 90 a capacitive reactance of 50and a measured voltage source of 100.0 V. Find the rms current.
b. In the last problem, determine the phase difference and state if the current leads or lags the voltage.
c. A 20.0cm long conducting rod is moving perpendicular to a 0.25 T magnetic field. How fast must the rod move in order to produce and emf of 12.0 V across the ends?
d. What voltage is needed to create a current of 5.0 mA in a circuit containing only a 2.0 Fcapacitor, when the frequency of the source is 1.50 kHz?
e. In the figures below indicate the direction of the induced current along the path between points a and b.
(as the switch is closed)
(as the switch is closed)