Physics 42 Exam 3 Fall 2013 PRINT Name:

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1 Physics 42 Exam 3 Fall 2013 PRINT Name: Conceptual Questions : Circle the BEST answer. (1 point each) 1. A small plastic ball has an excess negative charge on it. If a magnet is placed close to this ball, it will either attract or repel the ball, depending upon which pole is closer to the ball. A) TRUE B) FALSE 2. A long straight wire has a constant current flowing to the right. A square ring is situated above the wire, and also has a constant current flowing through it (as shown below). Which of the following statements is true? A) The net magnetic force on the ring is upward, and there is also a net torque on the ring. B) The net magnetic force on the ring is zero, and the net torque is zero. C) The net magnetic force on the ring is downward, and there is also a net torque on the ring. D) The net magnetic force on the ring is zero, but there is a net torque on the ring. E) The net magnetic force on the ring is downward, and the net torque is zero. 3. After the switch has been closed for a long time, the current in the LR circuit is zero. a. TRUE b. FALSE 4. A solenoid consists of 100 circular turns of copper wire. Parts of three turns, A, B and C, are shown below. When a current flows through the coil, a. both A and C are repelled by B. b. A is attracted to B; C is repelled by B. c. neither A nor C is attracted to or repelled by B. d. A is repelled by B; C is attracted to B. e. both A and C are attracted to B. 5. As an airplane files north from San Francisco to Seattle, it passes through the Earth s magnetic field. As a result, a motional emf is developed between the wingtips. Which wingtip is positively charged? a) the left wing b) the right wing c) chicken wing d) neither 6. When a ferromagnetic material is placed in an external magnetic field, the magnetic field strength is A) increased B) decreased C) no change. 7. When a ferromagnetic material that has been magnetized is brought to a temperature greater than the Curie temperature, what happens to its residual magnetism. a. Nothing happens to the residual magnetism. b. The residual magnetism disappears. c. The residual magnetism reaches it s highest value. d. All the magnetic domains causing magnetism become a single domain. e. The material of the magnet melts causing currents that are magnetic. 8. A current-carrying wire is pulled away from a conducting loop in the direction shown. As the wire is moving, is there a cw current around the loop, a ccw current or no current? a. There is a clockwise current around the loop. b. There is a counterclockwise current around the loop. c. There is no current around the loop. 9. If a magnetic compass is placed between the plates of a fully charged capacitor, the compass will align itself along the electric field lines. A) TRUE B) FALSE 10. The electric field strength in the figure is a. increasing b. decreasing c. not changing.

2 Short Problems 10 points each. Show work and box your answers. Circle the BEST answer. 1. If the radiant energy from the sun comes in as a plane EM wave of intensity 1340 W/m 2, calculate the peak value of E. a. 300 V/m b V/m c. 225 V/m d. 111 V/m e. 711 V/m 2. A rectangular loop (area = 0.15 m 2 ) turns in a uniform magnetic field with B = 0.20 T. At an instant when the angle between the magnetic field and the normal to the plane of the loop is ( /2) rad and increasing at the rate of 0.60 rad/s, what is the magnitude of the emf induced in the loop? a. 24 mv b. zero c. 18 mv d. 20 mv e. 6.0 mv. 3. A 2.0-m wire carries a current of 15 A directed along the positive x axis in a region where the magnetic field is uniform and given by B = (30 40 ) mt. What is the resulting magnetic force on the wire? a. (+1.2 ) N b. ( 1.2 ) N c. ( 1.5 ) N d. (+1.5 ) N e. (+0.30 ) N

3 4. A Tangent Galvanometer like we used in lab has 5.0 loops and a radius of 6.40cm.. If the wire carries a 1.2-A current, the magnitude of the magnetic field at the center is a. 79 T b. 69 T c. 59 T d. 89 T e. 9.4 T 5. A deuteron is accelerated from rest through a 10-kV potential difference and then moves perpendicularly to a uniform magnetic field with B = 1.6 T. What is the radius of the resulting circular path? (deuteron: m = kg, q = C) Sketch the situation, labeling everything. Show how the units work out. a. 19 mm b. 13 mm c. 20 mm d. 10 mm e. 9.0 mm

4 Longer Problems. DO TWO. 20 points each. You can do a third one if you have time for 10 points extra credit. You MUST label it EXTRA CREDIT. No Partial Credit on Extra Credit. Completeness and neatness count. Report to THREE Significant Figures. Box final answers. 1.A segment of wire (total length including portions of incoming and outgoing wire = 6R) is formed into the shape shown and carries a current I. Use Bio-Savart Law to find the magnetic field at the point P and express it as a vector. You need to show all the vectors on the path too. Justify your steps. Completeness and neatness counts.

5 2. A long, straight conducting wire of radius R has a non-uniform current density J=J 0 r/r where J 0 is a constant. The wire carries total current I. a. Derive an expression for J 0 in terms of I and R. b. Use Ampere s Law to derive an expression for the magnetic field strength inside the wire at radius r. You must draw everything and make it very clear. Justify steps. Completeness and neatness count.

6 3. A long straight wire is parallel to one edge and is in the plane of a single-turn rectangular loop as shown. If the loop is moving in the plane shown so that the distance x changes at a constant rate of 20 cm/s, derive an expression for the magnitude of the emf induced in the loop at the instant x = 5.0 cm? Let I = 50 A, a = 50 cm, b = 6.0 cm. What is the direction of the induced current? Completeness and neatness count.

PHYSICS 3204 PUBLIC EXAM QUESTIONS (Magnetism &Electromagnetism)

PHYSICS 3204 PUBLIC EXAM QUESTIONS (Magnetism &Electromagnetism) NAME: August 2009---------------------------------------------------------------------------------------------------------------------------------