AP Physics 2 Magnetism HW-2 (“Magnetic Force on a Current”): due______

Read Sections 24.3, 24.4, 24.6, & 24.7

Conceptual Questions

1. A current-carrying wire is placed in a region with a uniform magnetic field. The wire experiences zero magnetic force. Explain.

2. Consider the three wires shown in Figure 1, each carrying a current of the same magnitude. The currents in wires 1 and 3 are out of the page; the current in wire 2 is into the page. What is the direction of the magnetic force experienced by wire 3?

3. Consider the four wires shown in Figure 2, each carrying a current of the same magnitude. The currents in wires 1, 2, and 3 are out of the page; the current in wire 4 is into the page. What is the direction of the magnetic force experienced by wire 2?

Figure 1 Figure 2

4. Consider the three current-carrying wires described in Figure 1. Each of these wires is long and straight, and carries the current I either into or out of the page, as shown. What is the direction of the net magnetic field produced by these three wires at the center of the triangle? Explain.

5. Consider the four current-carrying wires described in Figure 2. Each of these wires is long and straight, and carries the current I either into or out of the page, as shown. What is the direction of the net magnetic field produced by these four wires at the center of the square? Explain.

6. For each of the current loops shown in Figure 3, indicate whether there will be a tendency for the loop to rotate clockwise, counterclockwise, or not at all. Explain. (Imagine viewing the loops from above, along the indicated axis.)

Figure 3 Figure 4

7. A loop of wire is connected to the terminals of a battery, as indicated in Figure 4. If the loop is to attract the bar magnet, which of the terminals, A or B, should be the positive terminal of the battery? Explain.

8. The number of turns in a solenoid is doubled, and at the same time its length is doubled. Does the magnetic field within the solenoid increase, decrease, or stay the same? Explain.

Problems

The Magnetic Force Exerted on a Current-Carrying Wire

1. What is the magnetic force exerted on a 2.55-m length of wire carrying a current of 0.675 A perpendicular to a magnetic field of 0.720 T?

2. The magnetic force exerted on a 1.2-m segment of straight wire is 1.6 N. The wire carries a current of 3.0 A in a region with a constant magnetic field of 0.50 T. What is the angle between the wire and the magnetic field?

3. Consider a current loop immersed in a magnetic field, as in Figure 3 (a) (Conceptual Question 6). It is given that and In addition, the loop is a square 0.46 m on a side. Find the magnitude of the magnetic force exerted on each side of the loop.

4. A 0.45-m copper rod with a mass of 0.17 kg carries a current of 11 A in the positive x direction. What are the magnitude and direction of the minimum magnetic field needed to levitate the rod?

Loops of Current and Magnetic Torque

5. Consider a current loop in a region of uniform magnetic field, as shown in Figure 3 (a) (Conceptual Question 6). Find the magnitude of the torque exerted on the loop about the vertical axis of rotation, using the data given in Problem 3.

Electric Currents, Magnetic Fields, and Ampère’s Law

6. A long, straight wire carries a current of 7.2 A. How far from this wire is the magnetic field it produces equal to the Earth’s magnetic field, which is approximately

7. Two power lines, each 250 m in length, run parallel to each other with a separation of 35 cm. If the lines carry parallel currents of 120 A in the same direction, what is the magnitude and direction of the magnetic force each exerts on the other?

8. Consider the long, straight, current-carrying wires shown in Figure 5. One wire carries a current of 6.2 A in the positive y direction; the other wire carries a current of 4.5 A in the positive x direction. (a) At which of the two points, A or B, do you expect the magnitude of the net magnetic field to be greater? Explain. (b) Determine the magnitude and direction of the net magnetic field at points A and B. (Hint: use vector superposition.)

Figure 5 Figure 6

9. Two long, straight wires are oriented perpendicular to the page, as shown in Figure 6. The current in one wire is pointing into the page, and the current in the other wire is pointing out of the page. Find the magnitude and direction of the net magnetic field at point P. (Hint: use vector superposition.)

Answers

1. F = 1.24 N

2. q = 63°

3. Ftop = Fbottom = 0 Fleft = Fright = 1.5 N

4. If up is in the positive y-direction, B = 0.34 T in the negative z-direction

5. t = 0.68 N×m

6. r = 2.9 cm

7. F = 2.1 N toward the other wire

8. (a) The magnitude of the net magnetic field is greatest at A…explain.

(b) BA = 1.3 x 10-5 T out of the page BB = 2.1 x 10-6 T into the page

9. BP = 8.9 x 10-6 T @ 297°