AP Physics



AP Physics B

Unit 4 Practice Problems

Multiple Choice: Choose the best answer to each question below.

For questions 1 & 2: refer to the following diagram that shows part of a closed electrical circuit.

1. The electrical resistance of the part of the circuit shown between point X and point Y is

a) 1[pic]Ω b) 2 Ω c) 2[pic]Ω d) 4 Ω e) 6Ω

2. When there is a steady current in the circuit, the amount of charge passing a point per unit time is a) the same everywhere in the circuit b) greater at point X than at point Y c) greater in the 1Ω resistor than in the 2Ω resistor d) greater in the 1Ω resistor than in the 3Ω resistor e) greater in the 2Ω resistor than in the 3Ω resistor.

3. A metal spring has its ends attached so that it forms a circle. It is placed in a uniform magnetic field, as shown in the diagram to the right. Which of the following will NOT cause a current to be induced in the spring?

a) Changing the magnitude of the magnetic field. b) Increasing the diameter of the circle by stretching the spring. c) Rotating a spring about a diameter.

d) Moving the spring parallel to the magnetic field. e) Moving the spring in and out of the magnetic field.

For questions 4 & 5: A magnetic field of 0.1 T forces a proton beam of 1.5 mA to move in a circle of radius 0.1 m. The plane of the circle is perpendicular to the magnetic field.

4. Of the following, which is the best estimate of the work done by the magnetic field on the protons during one complete orbit of the circle? a) 0 J b) 10 -22 J c) 10 -5 J d) 102 J e) 1020 J

5. Of the following, which is the best estimate of the speed of a proton in the beam as it moves in the circle? a) 10 -2 m/s b) 103 m/s c) 106 m/s d) 108 m/s e) 1015 m/s

6. A single circular loop of wire in the plane of the page is perpendicular to a uniform magnetic field B directed out of the page, as shown in the diagram. If the magnitude of the magnetic field is decreasing, then the induced current in the wire is

a) directed upward out of the paper b) directed downward into the paper

c) clockwise around the loop d) counterclockwise around the loop

e) zero (no current is induced)

Free Response: Solve each problem below. Explain or show any work necessary.

1. Lightbulbs of fixed resistance 3.0 Ω and 6.0 Ω, a 9.0 V battery, and a switch S are connected as shown in the schematic diagram to the right. The switch S is closed.

a) Calculate the current in bulb A.

b) Which lightbulb is brightest? Justify your answer.

c) Switch S is then opened. By checking the appropriate spaces below, indicate whether the brightness of each lightbulb increases, decreases or remains the same. Explain your reasoning for each lightbulb.

i. Bulb A: The brightness _____ increases _____ decreases _____ remains the same

Explanation:

ii. Bulb B: The brightness _____ increases _____ decreases _____ remains the same

Explanation:

iii. Bulb C: The brightness _____ increases _____ decreases _____ remains the same

Explanation:

2. A mass spectrometer, constructed as shown in the diagram to the right, is to be used for determining the mass of singly ionized positively charged ions. There is a uniform magnetic field B = 0.20 T perpendicular to the page everywhere inside the large box. A potential difference V = 1,500 V is applied across the parallel plates L and K, which are separated by a distance d = 0.012 m.

a) In what direction, relative to the coordinate system shown in the diagram, should the magnetic field point in order for positive ions to move along the path shown by the dashed line in the diagram?

b) Should plate K have a positive or negative voltage polarity with respect to plate L?

c) Calculate the magnitude of the electric field between the plates.

d) Calculate the speed of a particle that can pass between the parallel plates without being deflected.

e) Calculate the mass of a hypothetical singly charged ion that travels in a semicircle of radius R = 0.50 m.

f) A doubly ionized ion of the same mass and velocity as the singly charged ion enters the mass spectrometer. What is the radius of its path?

3. A student is asked to design a circuit to supply an electric motor with 1.0 mA of current at 3.0 V potential difference.

a) Determine the power to be supplied to the motor.

b) Determine the electrical energy to be supplied to the motor in 60 s.

c) Operating as designed above, the motor can lift a 0.012 kg mass a distance of 1.0 m in 60 s at constant velocity. Determine the efficiency of the motor.

To operate the motor, the student has available only a 9.0 V battery to use as the power source and the following five resistors.

d) In the space below, complete a schematic diagram of a circuit that shows how one of more of these resistors can be connected to the battery and motor so that 1.0 mA of current and 3.0 V of potential difference are supplied to the motor. Be sure to label each resistor in the circuit with the correct value of its resistance.

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