Ch 17 Problem Set 31. A toaster is rated at 600 W when connected to a 120-V source. What current does the toaster carry, and what is its resistance? 33. How many 100-W lightbulbs can you use in a 120-V circuit without tripping a 15-A circuit breaker? (The bulbs are connected in parallel, which means that the potential difference across each lightbulb is 120 V.) 13. Find the current in the 12-Ω resistor in Figure P18.13. 40. A small motor draws a current of 1.75 A from a 120-V line. The output power of the motor is 0.20 hp. (a) At a rate of $0.060/kWh, what is the cost of operating the motor for 4.0 h? (b) What is the efficiency of the motor? 52. Birds resting on high-voltage power lines are a common sight. The copper wire on which a bird stands is 2.2 cm in diameter and carries a current of 50 A. If the bird s feet are 4.0 cm apart, calculate the potential difference across its body. Ch 18 Problem Set 14. Calculate the power delivered to each resistor in the circuit shown in Figure P18.14. 15. (a) You need a 45-Ω resistor, but the stockroom has only 20-Ω and 50-Ω resistors. How can the desired resistance be achieved under these circumstances? (b) What can you do if you need a 35-Ω resistor? 9. Consider the circuit shown in Figure P18.9. Find (a) the current in the 20.0-Ω resistor and (b) the potential difference between points a and b. 16. The ammeter shown in Figure P18.16 reads 2.00 A. Find I 1, I 2, and ε. 11. The resistance between terminals a and b in Figure P18.11 is 75 Ω. If the resistors labeled R have the same value, determine R. 17. Determine the current in each branch of the circuit shown in Figure P18.17.
27. Find the current in each resistor in Figure P18.27. Chapter 19 19. Figure P18.19 shows a circuit diagram. Determine (a) the current, (b) the potential of wire A relative to ground, and (c) the voltage drop across the 1 500-Ω resistor. 1. An electron gun fires electrons into a magnetic field that is directed straight downward. Find the direction of the force exerted by the field on an electron for each of the following directions of the electron s velocity: (a) horizontal and due north; (b) horizontal and 30 west of north; (c) due north, but at 30 below the horizontal; (d) straight upward. (Remember that an electron has a negative charge.) 2. (a) Find the direction of the force on a proton (a positively charged particle) moving through the magnetic fields in Figure P19.2, as shown below. (b) Repeat part (a), assuming the moving particle is an electron. 20. In the circuit of Figure P18.20, the current I 1 is 3.0 A and the values of ε and R are unknown. What are the currents I 2 and I 3? 23. (a) Find the current in each resistor shown in Figure P18.23 and (b) find the potential difference between points c and f. 3. Find the direction of the magnetic field acting on the positively charged particle moving in the various situations shown in Figure P19.3 below, if the direction of the magnetic force acting on it is as indicated.
4. Determine the initial direction of the deflection of charged particles as they enter the magnetic fields shown in Figure P19.4 below. 15. A wire carries a current of 10.0 A in a direction that makes an angle of 30.0 with the direction of a magnetic field of strength 0.300 T. Find the magnetic force on a 5.00-m length of the wire. 5. At the Equator near Earth s surface, the magnetic field is approximately 50.0 μt northward and the electric field is about 100 N/C downward in fair weather. Find the gravitational, electric, and magnetic forces on an electron with an instantaneous velocity of 6.00 10 6 m/s directed to the east in this environment. 6. A proton travels with a speed of 3.0 10 6 m/s at an angle of 37 with the direction of a magnetic field of 0.30 T in the +y direction. What are (a) the magnitude of the magnetic force on the proton and (b) the proton s acceleration? 9. A proton moves perpendicularly to a uniform magnetic field B at 1.0 10 7 m/s and experiences an acceleration of 2.0 10 13 m/s 2 in the +x direction when its velocity is in the +z direction. Determine the magnitude and direction of the field. 11. A current I = 15 A is directed along the positive x axis and perpendicularly to a magnetic field. The conductor experiences a magnetic force per unit length of 0.12 N/m in the negative y direction. Calculate the magnitude and direction of the magnetic field in the region through which the current passes. 14. A wire carries a steady current of 2.40 A. A straight section of the wire is 0.750 m long and lies along the x axis within a uniform magnetic field of magnitude 1.60 T in the positive z direction. If the current is in the + x direction, what is the magnetic force on the section of wire? 20. A thin, horizontal copper rod is 1.00 m long and has a mass of 50.0 g as shown in the figure above. What is the minimum current in the rod that can cause it to float in a horizontal magnetic field of 2.00 T? 27. A particle with a +2.0 μc charge and a kinetic energy of 0.090 J is fired into a uniform magnetic field of magnitude 0.10 T. If the particle moves in a circular path of radius 3.0 m, determine it s mass. 29. Figure P19.29a above is a diagram of a device called a velocity selector, in which particles of a specific velocity pass through undeflected but those with greater or lesser velocities are deflected either upward or downward. An electric field is directed perpendicularly to a magnetic field. This produces on the charged particle an electric force and a magnetic force that can be equal in magnitude and opposite in direction (Fig. P19.29b above), and hence cancel. Show that particles with a speed of v = E/B will pass through undeflected.
37. At what distance from a long, straight wire carrying a current of 5.0 A is the magnetic field due to the wire equal to the strength of Earth s field, approximately 5.0 10 5 T? 30. Consider the mass spectrometer shown schematically in Figure P19.30 above. The electric field between the plates of the velocity selector is 950 V/m, and the magnetic fields in both the velocity selector and the deflection chamber have magnitudes of 0.930 T. Calculate the radius of the path in the system for a singly charged ion with mass m = 2.18 10 26 kg. (Hint: See Problem 29.) 31. A singly charged positive ion has a mass of 2.50 10 26 kg. After being accelerated through a potential difference of 250 V, the ion enters a magnetic field of 0.500 T, in a direction perpendicular to the field. Calculate the radius of the ion s path in the field. 34. Find the direction of the current in the wire in Figure P19.34 that would produce a magnetic field directed as shown, in each case. 44. Two parallel wires are 10.0 cm apart, and each carries a current of 10.0 A. (a) If the currents are in the same direction, find the force per unit length exerted by one of the wires on the other. Are the wires attracted or repelled? (b) Repeat the problem with the currents in opposite directions. 47. What current is required in the windings of a long solenoid that has 1 000 turns uniformly distributed over a length of 0.400 m in order to produce a magnetic field of magnitude 1.00 10 4 T at the center of the solenoid? Chapter 20 1. A magnetic field of strength 0.30 T is directed perpendicular to a plane circular loop of wire of radius 25 cm. Find the magnetic flux through the area enclosed by this loop. 2. A circular loop with a radius of 0.200 m is placed in a uniform magnetic field of magnitude 0.850 T. The normal to the loop makes an angle of 30.0 with respect to the direction of B. If the field increases to 0.950 T, what is the increase in magnetic flux through the loop? 4. A long, straight wire carrying a current of 2.00 A is placed along the axis of a cylinder of radius 0.500 m and a length of 3.00 m. Determine the total magnetic flux through the cylinder. 5. A long, straight wire lies in the plane of a circular coil with a radius of 0.010 m. The wire carries a current of 2.0 A and is placed along a diameter of the coil. (a) What is the net flux through the coil? (b) If the wire passes through the center of the coil and is perpendicular to the plane of the coil, find the net flux through the coil. 6. A solenoid 4.00 cm in diameter and 20.0 cm long has 250 turns and carries a current of 15.0 A. Calculate the magnetic flux through the circular cross-sectional area of the solenoid. 35. A lightning bolt may carry a current of 1.00 10 4 A for a short period of time. What is the resulting magnetic field 100 m from the bolt? Suppose that the bolt extends far above and below the point of observation. 8. A circular coil of radius 20 cm is placed in an external magnetic field of strength 0.20 T so that the plane of the coil is perpendicular to the field. The coil is pulled out of the field in 0.30 s. Find the average induced emf during this interval.
9. A 25-turn circular coil of wire has a diameter of 1.00 m. It is placed with its axis along the direction of Earth s magnetic field of 50.0 μt, and then in 0.200 s it is flipped 180. An average emf of what magnitude is generated in the coil? Figure P20.23 23. A bar magnet is positioned near a coil of wire as shown in Figure P20.23. What is the direction of the current through the resistor when the magnet is moved (a) to the left? (b) to the right? 10. The flexible loop in Figure P20.10 has a radius of 12 cm and is in a magnetic field of strength 0.15 T. The loop is grasped at points A and B and stretched until its area is nearly zero. If it takes 0.20 s to close the loop, find the magnitude of the average induced emf in it during this time. 11. A strong electromagnet produces a uniform field of 1.60 T over a cross-sectional area of 0.200 m 2. We place a coil having 200 turns and a total resistance of 20.0 Ω around the electromagnet. We then smoothly decrease the current in the electromagnet until it reaches zero in 20.0 ms. What is the current induced in the coil? Figure P20.24 24. A bar magnet is held above the center of a wire loop in a horizontal plane, as shown in Figure P20.24. The south end of the magnet is toward the loop. The magnet is dropped. Find the direction of the current through the resistor (a) while the magnet is falling toward the loop and (b) after the magnet has passed through the loop and moves away from it. 18. Consider the arrangement shown in Figure P20.18 above. Assume that R = 6.00 Ω and = 1.20 m, and that a uniform 2.50-T magnetic field is directed into the page. At what speed should the bar be moved to produce a current of 0.500 A in the resistor? Figure P20.25 25. What is the direction of the current induced in the resistor when the current in the long, straight wire in Figure P20.25 decreases rapidly to zero?
Figure P20.26 26. In Figure P20.26, what is the direction of the current induced in the resistor at the instant the switch is closed? Figure P20.28 28. Find the direction of the current through the resistor in Figure P20.28, (a) at the instant the switch is closed, (b) after the switch has been closed for several minutes, and (c) at the instant the switch is opened.