General Physics (PHYC 252) Exam 4

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1 General Physics (PHYC 5) Exam 4 Multiple Choice (6 points). Circle the one best answer for each question. For Questions 1-3, consider a car battery with 1. V emf and internal resistance r of. Ω that is supplying current to a circuit connected to a resistor R = 6. Ω. The bold lines mark off the car battery in the diagram below. r 6. Ω 1. What is the current through the 6.-Ω resistance? (a) 6. A (b) 4. A (c). A (d) 1.5 A (e).67 A. While the battery is being used as described in the introduction above, the voltage across the outer terminals will be: (a) 3. V (b) 9. V (c) 1. V (d) 15. V (e) 18. V 3. While the battery is being used as described in the introduction above, what percent of the total energy is wasted as heat in the battery due to its internal resistance? (a) 5% (b) 1% (c) 5% (d) 33% (e) 67% Use the circuit indicated below for Questions 4-6. In this circuit R a = R b = R c. R a R b R c 4. The current through R b is the current through R a. (a) equal to (b) less than (c) greater than 5. The potential difference across R b is the potential difference across R a. (a) equal to (b) less than (c) greater than 6. If the resistance of R c is increased, the potential difference across R b will: (a) increase (b) decrease (c) not change 1

2 7. A resistor R 1 is connected to a battery. A second resistor R, where R > R 1, is then connected in parallel with R 1. After R is connected, the total current through R 1 and R will be it was before R was connected. (a) less than (b) greater than (c) the same as 8. A 4-W light bulb is connected to a 1-V power source with another light bulb. The second bulb could be either 4 W or 1 W. The different ways of connecting are as follows W in series with another 4-W. 4-W in parallel with another 4-W 3. 4-W in series with a 1-W 4. 4-W in parallel with a 1-W Arrange these according to how bright the first 4-W bulb shines. The order should be from brightest to dimmest. (a) 4,, 1, 3 (b) 3, 1,, 4 (c) 1 and 3 tie, and 4 tie (d) and 4 tie, 3, 1 (e) and 4 tie, 1, 3 For Questions 9-1, refer to the following diagram showing the motion of three particles in a magnetic field. Each particle has the same amount of charge, although it may be positive or negative as shown. The speeds of +q 1 and q are equal to each other, but only half of the speed of +q. +q 1 +q θ θ =45 o B-field indicated by dashed arrows -q Velocity of charged particles, at the moment they enter the magnetic field, shown by solid arrows 9. The magnitude of force on +q will be the magnitude of force on +q (a) times (b) times (c) equal to (d) times (e) times 1. What direction of force will the -q charge experience? (a) out of the paper (b) into the paper (c) toward top of the page (d) toward bottom of the page (e) It will experience zero force 11. Semi-circle in the diagram below shows the path of a moving charged particle as it enters and then exits a magnetic field that is perpendicular to the plane of the paper. What path would this same charged particle take if the strength of the magnetic field increased by four times and the particle s initial velocity doubled? (Each of the semi-circles shown have radius that is twice that of the next smaller one) (a) 1 (b) (c) 3 (d) 4

3 For Questions 1, refer to the diagram below showing a simple electrical motor. The motor has a coil of one turn of wire with current flowing in the direction indicated by the arrows. The pivot, about which the loop can rotate, is indicated by the two thin solid lines. S N 1. If there is a clockwise turning torque on the loop, the current in the loop must be in the direction indicated by the arrow on the front part of the loop. (a) solid (b) dashed 13. The diagram shows a square arrangement of four parallel wires with equal current, all going straight into the plane of the paper. What is the direction of the magnetic field at the center? (a) towards top of the page (b) straight out of the page (c) towards the lower left corner (d) straight into the page (e) field is zero; no direction 14. The diagram shows a square arrangement of four parallel wires, each carrying a current. The two currents coming out of the paper are equal to each other, and the two currents going into the page are equal to each other. However, the two currents coming out of the page are twice as much as the two going into the page. What is the direction of the magnetic field at the center? (a) left to right (b) right to left (c) towards the lower left corner (d) towards the lower right corner (e) field is zero; no direction 15. The diagram shows long, straight wire carrying current i 1 =4 A, near a rectangular loop of current i =1 A. The distance a=.1 m, and the loop dimensions are b=.3 m and c=. m. What direction is the net force on the loop? (a) out of the plane of page (b) into the plane of page (c) left to right (d) right to left (e) net force is zero; no direction i 1 a i b c 16. What is the magnitude of the net force on the loop shown in Question 15? (a) 5µ 15µ (b) (c) 5µ (d) 3µ (e) zero 3

4 17. How would the answer to Question 16 change if the values of i 1 and i were switched so that i 1 =1 A and i =4 A? The net force would. (a) increase (b) decrease (c) be unchanged 18. The diagram shows four arrangements of long, parallel wires with each currents going either into or out of the plane of the paper. Which sequence correctly ranks the net force on the central wire, from least to greatest? (a) 3, 4, 1, (b), 1, 3, 4 (c) 4,, 3, 1 (d), 4, 3, 1 (e) 4, 3, 1, The diagram shows a central wire (inner shaded circle) surrounded by a conducting cylinder (outer shaded region). The central wire carries. A of current out of the plane of the paper; the cylinder carries 1. A of current into the plane of the paper. The currents in both of the two conductors are distibuted uniformly throughout the conductor. What is the magnitude of B ρ dl ρ for the outer dashed circle? µ (a) (b) (d) µ (e) 3µ (c) µ. The smallest dashed circle in the diagram for the previous question has a radius that is half the radius of the inner wire. What list gives the magnitude of B dl for each of the three Amperian loops (dashed circles) from smallest to greatest? (a) outer, middle, inner (b) middle and inner tie, outer (c) middle, inner, outer (d) inner, outer, middle (e) inner, middle, outer Extra Credit 1. What is the direction of the net force on the segment of wire carrying a current flowing in the direction shown in the diagram if placed in a magnetic field that is directed perpendicularly out of the plane of the paper? (a) right to left (b) left to right (c) toward top of page (d) toward bottom of page (e) There is no net force I B 4

5 Problem Solving (6 points). Solve each problem in the space provided 1. ( points) The diagram shows an arrangement of eight resistors: R 1 = 4. Ω, R = 4. Ω, R 3 = 8. Ω, R 4 = 3. Ω, R 5 = 3. Ω, R 6 = 1. Ω, R 7 = 4. Ω, and R 8 = 4. Ω. Determine: (a) the equivalent resistance R eq of the eight resistors; (b) the current through R 1 ; (c) the voltage across R 3 ; and (d) the rate at which heat is dissipated in this circuit while in use. R 3 R R 4 R 6 R 7 R 8 R 1 4 V R 5 5

6 . (15 points) What is the current in each of resistors R 1, R, R 3, and R 4? State clearly the amount of current and the direction of current in each resistor. The values of resistances are R 1 =. Ω, R =1. Ω, R 3 =. Ω, and R 4 =. Ω. 1. V R R 1 5. V 5. V R 4 R 3 6

7 3. (15 points) A wire of radius r =. m has a current density that varies with r and is given by J r. Find the magnetic field B at distances (a).1 m and (b).3 m from the center of the wire. Leave your answers in terms of µ and J. 4. (1 points) An electric motor plugged into a 1-V outlet has a back emf of 8 V and a current of 1. A when running at normal speed. (a) What is the resistance of the motor coil windings, if a current of 1. A runs through the coil while it runs at normal speed? (b) How much current flows through the windings when it is initially turned on, before the motor has time to move? (c) How much more quickly is heat is generated in the coil windings when the motor is plugged in, but in a stalled position, compared to when it is plugged in and running at its regular speed? 7

PRACTICE EXAM 2 for Midterm 2

PRACTICE EXAM 2 for Midterm 2 Multiple Choice Questions 1) In the circuit shown in the figure, all the lightbulbs are identical. Which of the following is the correct ranking of the brightness of the bulbs?