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physics
college physics a strategic approach 2nd
College Physics Essentials Electricity And Magnetism Optics Modern Physics Volume Two 8th Edition Jerry D. Wilson, Anthony J. Buffa, Bo Lou - Solutions
41. • How long does a laser beam take to travel from the Earth to a reflector on the Moon and back?Take the distance from the Earth to the Moon to be 2.4 × 105 mi. (This experiment was done when the Apollo flights of the early 1970s left laser reflectors on the lunar surface.)
40. • A meteorologist for a TV station is using radar to determine the distance to a cloud. He notes that a time of 0.24 ms elapses between the sending and the return of a radar pulse. How far away is the cloud?
39. • In a small town there are only two AM radio stations, one at 920 kHz and one at 1280 kHz. What are the wavelengths of the radio waves transmitted by each station?
38. • Find the frequencies of electromagnetic waves with wavelengths of (a) 3.0 cm, (b) 650 nm, and (c) 1.2 fm.(d) Classify the type of light in each case.
37. •• Electrical power from a generator is transmitted through a power line 175 km long with a resistance of 1.2 Ω/km. The generator’s output is 50 A at its operating voltage of 440 V. This output is increased by a single step-up for transmission at 44 kV. (a) How much power is lost as
36. •• A small plant produces electric energy and, through a transformer, sends it out over the transmission lines at 50 A and 20 kV. The line reaches a small town over 25-km-long transmission lines whose resistance is 1.2 Ω/km. (a) What is the power loss in the lines if the energy is
35. •• The electricity supplied in Exercise 34 is transmitted over a line 80.0 km long with a resistance of 0.80 Ω/km. (a) How many kilowatt-hours are saved in 5.00 h by stepping up the voltage? (b) At $0.15/kWh, how much of a savings (to the nearest $10) is this to all the consumers the line
34. •• An ac generator supplies 20 A at 440 V to a 10 000-V power line through a step-up transformer that has 150 turns in its primary coil. (a) If the transformer is 95% efficient (see Exercise 32), how many turns are in the secondary coil? (b) What is the current in the power line?
33. IE •• The specifications of a transformer used with a small appliance read as follows: input, 120 V, 6.0 W; output, 9.0 V, 300 mA. (a) Is this transformer(1) an ideal or (2) a non-ideal transformer? Why?(b) What is its efficiency? (See Exercise 32.)
32. •• The efficiency e of a transformer is defined as the ratio of the power output to the power input, or e = Ps/Pp. (a) Show that for an ideal transformer, this expression gives an efficiency of 100% (e = 1.00).(b) Suppose a step-up transformer increased the line voltage from 120 to 240 V,
31. • An ideal transformer has 840 turns in its primary coil and 120 turns in its secondary coil. If the primary coil draws 2.50 A at 110 V, what are (a) the current and(b) the output voltage of the secondary coil?
30. •• The primary coil of an ideal transformer is connected to a 120-V source and draws 1.0 A. The secondary coil has 800 turns and supplies an output current of 10 A to run an electrical device. (a) What is the voltage across the secondary coil? (b) How many turns are in the primary coil? (c)
29. •• The transformer in the power supply for a computer’s external hard drive changes a 120-V input voltage (from a regular house line) to the 5.0-V output voltage that is required to power the drive. (a) Find the ratio of the number of turns in the primary coil to the number of turns in
28. • The primary coil of an ideal transformer has 720 turns, and the secondary coil has 180 turns. If the primary coil carries 15 A at a voltage of 120 V, what are (a) the voltage and (b) the output current of the secondary coil?
27. • An ideal transformer steps 8.0 V up to 2000 V, and the 4000-turn secondary coil carries 2.0 A.(a) Find the number of turns in the primary coil.(b) Find the current in the primary coil.
26. IE • The secondary coil of an ideal transformer has 450 turns, and the primary coil has 75 turns. (a) Is this transformer a (1) step-up or (2) step-down transformer?Explain your choice. (b) What is the ratio of the current in the primary coil to the current in the secondary coil?(c) What is
25. ••• A 240-V dc motor has an armature whose resistance is 1.50 Ω. When running at its operating speed, it draws a current of 16.0 A. (a) What is the back emf of the motor when it is operating normally? (b) What is the starting current? (Assume that there is no additional resistance in the
24. ••• The starter motor in an automobile has a resistance of 0.40 Ω in its armature windings. The motor operates on 12 V and has a back emf of 10 V when running at normal operating speed. How much current does the motor draw (a) when running at its operating speed,(b) when running at half
23. IE •• A motor has a resistance of 2.50 Ω and is connected to a 110-V line. (a) Is the operating current of the motor(1) higher than 44 A, (2) 44 A, or (3) lower than 44 A?Why? (b) If the back emf of the motor at operating speed is 100 V, what is its operating current?
22. IE •• (a) To increase the output of an ac generator, a student has the choice of doubling either the generator’s magnetic field or its frequency. To maximize the increase in emf output, (1) he should double the magnetic field, (2) he should double the frequency,(3) it does not matter
12 cm. The generator has a sinusoidal voltage output with an amplitude of 24 V. (a) If the magnetic field of the generator is 250 mT, with what frequency does the armature turn? (b) If the magnetic field was doubled and the frequency cut in half, what would be the amplitude of the output?
21. •• The armature of an ac generator has 100 turns.Each turn is a rectangular loop measuring 8.0 cm by
20. •• The armature of a simple ac generator has 20 circular loops of wire, each with a radius of 10 cm.It is rotated with a frequency of 60 Hz in a uniform magnetic field of 800 mT. (a) What is the maximum emf induced in the loops? (b) How often is this value attained? (c) If the time period
19. •• An ac generator operates at a rotational frequency of 60 Hz and produces a maximum emf of 100 V.Assume that its output at t = 0 is zero. What is the instantaneous emf (a) at t = 1/240 s? (b) at t = 1/120 s?(c) Find an expression for the instantaneous emf at any time t. (d) How much time
18. •• A simple ac generator consists of a coil with 10 turns(each turn has an area of 50 cm2). The coil rotates in a uniform magnetic field of 350 mT with a frequency of 60 Hz. (a) Write an expression in the form of Equation 20.5 for the generator’s emf variation with time.(b) Compute the
17. • A student makes a simple ac generator by using a single square wire loop 10 cm on a side. The loop is then rotated at a frequency of 60 Hz in a magnetic field of 0.015 T. (a)What is the maximum emf output? (b) If she wanted to make the maximum emf output ten times larger by adding loops,
16. • A hospital emergency room ac generator operates at a frequency of 60 Hz. If the output voltage is at a maximum value (magnitude) at t = 0, when is it next(a) a maximum (magnitude), (b) zero, and (c) at its initial value (direction and magnitude)?
15. ••• A fixed coil of wire with 10 turns and an area of 0.055 m2 is placed in a perpendicular magnetic field.This field oscillates in direction and magnitude at a frequency of 10 Hz and has a maximum value of 0.12 T.(a) What is the average emf induced in the coil during the time it takes
14. ••• The flux through a loop of wire changes uniformly from +40 Wb to −20 Wb in 1.5 ms. (a) What is the significance of the negative number attached to the final flux value? (b) What is the average induced emf in the loop? (c) If you wanted to double the average induced emf by changing
13. •• Suppose that the metal rod in Figure 20.11 is 20 cm long and is moving at a speed of 10 m/s in a magnetic field of 0.30 T and that the metal frame is covered with an insulating material. Find (a) the magnitude of the induced emf across the rod and (b) the current in the rod. (c) Repeat
12. •• A metal airplane with a wingspan of 30 m flies horizontally along a north-south route in the northern hemisphere at a constant speed of 320 km/h in a region where the vertical component of the Earth’s magnetic field is 5.0×10−5 T. (a) What is the magnitude of the induced emf between
11. IE •• A boy is traveling due north at a constant speed while carrying a metal rod. The rod’s length is oriented in the east-west direction and is parallel to the ground.(a) There will be no induced emf when the rod is (1) at the equator, (2) near the Earth’s magnetic poles, (3)somewhere
10. •• In 0.20 s, a coil of wire with 50 loops experiences an average induced emf of 9.0 V due to a changing magnetic field perpendicular to the plane of the coil.The radius of the coil is 10 cm, and the initial strength of the magnetic field is 1.5 T. Assuming that the strength of the field
9. •• When the magnetic flux through a single loop of wire increases by 30 T · m2, an average current of 40 A is induced in the wire. Assuming that the wire has a resistance of 2.5 Ω, (a) over what period of time did the flux increase? (b) If the current had been only 20 A, how long would the
8. •• The magnetic flux through one loop of wire is reduced from 0.35 Wb to 0.15 Wb in 0.20 s. The average induced current in the coil is 10 A. (a) Can you determine the area of the loop from the data given?Explain. (b) Find the resistance of the wire.
7. •• (a) A square loop of wire with sides of length 40 cm is in a uniform magnetic field perpendicular to its area.If the field’s strength is initially 100 mT and it decays to zero in 0.010 s, what is the magnitude of the average emf induced in the loop? (b) What would be the average emf if
6. •• A uniform magnetic field is at right angles to the plane of a wire loop. If the field decreases by 0.20 T in 1.0×10−3 s and the magnitude of the average emf induced in the loop is 80 V, (a) what is the area of the loop? (b) What would be the value of the average induced emf if the
5. • An ideal solenoid with a current of 1.5 A has a radius of 3.0 cm and a turn density of 250 turns/m. (a) What is the magnetic flux (due to its own field) through only one of its loops at its center? (b) What current is required to double the flux in part (a)?
4. • The plane of a conductive loop with an area of 0.020 m2 is perpendicular to a uniform magnetic field of 0.30 T. If the field drops to zero in 0.0045 s, what is the magnitude of the average emf induced in the loop?
3. • A circular loop (radius of 20 cm) is in a uniform magnetic field of 0.15 T. What angle(s) between the normal to the plane of the loop and the field would result in a flux with a magnitude of 1.4×10−2 T⋅m2?
2. • A circular loop with an area of 0.015 m2 is in a uniform magnetic field of 0.30 T. What is the flux through the loop’s plane if it is (a) parallel to the field,(b) at an angle of 37° to the field, and (c) perpendicular to the field?
1. • What is the diameter of a circular wire loop if a magnetic field of 0.15 T oriented perpendicular to its area produces a magnetic flux of 1.2×10−2 T⋅m2?
19. (a) When police radar waves bounce off an oncoming car and are received by the transmitting radar “gun,”they have a different frequency than the emitted waves.Explain. (b) Is the frequency in part (a) higher or lower than the original frequency? What about wavelength?What about wave speed?
18. Electromagnetic energy can exert a force on surfaces that it strikes (this phenomenon is called radiation pressure). (a) Will this force be greater on a shiny or a dark surface? (b) For the same surface, would you expect the force on it to be greater using a bright source rather than a fainter
17. On a cloudy summer day, you work outside and feel cool, yet that evening you find that you are sunburned.Explain how this is possible.
16. (a) An antenna has been connected to a car battery. Under these conditions, will the antenna emit electromagnetic radiation? Why or why not? Explain. (b) Repeat part (a)for the time when the battery is disconnected and the current in the antenna drops to zero.
15. A metallic plate is in the plane of this paper. A uniform magnetic field is perpendicular to the plane of the paper and creates a magnetic flux in the plate.Give the direction of the eddy currents in the plate
14. For an emergency project in his automotive workshop, a mechanic needs a step-down transformer, but has quick access to only step-up transformers in the shop. Show how he might be able to use a step-up transformer as a step-down one.
13. Explain why large-scale electric energy delivery systems operate at such high voltages when such voltages can be dangerous.
12. If you wanted to make a more compact ac generator(operating at the same frequency) by reducing the area of the coils, how would you compensate by changing its other physical characteristics in order to maintain the same emf output? Explain how each characteristic would change (larger, smaller,
11. In a dc motor, if the armature is jammed or turns very slowly under a heavy load, the coils in the motor may burn out. Explain why this can happen.
10. A student has a bright idea for a generator that apparently generates electric energy without a corresponding loss of energy somewhere else. His suggested arrangement is shown in ▼ Figure 20.22.The magnet is initially pulled down and released.When the magnet is attached to a spring, the
9. (a) Explain why the maximum emf produced by an ac generator occurs when the flux through its armature coil is zero. (b) Explain why the emf produced by an ac generator is zero when the flux through its armature coil is at its maximum.
8. (a) In Figure 20.10, what would happen to the direction of the induced current if the metal rod were moving downward instead? (b) What would happen to the direction of the induced magnetic field? Is the induced magnetic field necessarily opposite to the external field direction? [Hint: Compare
7. A circular metal loop is kept entirely within a magnetic field but moved to a region of higher field strength while not rotated. What could you do to its diameter to prevent an induced current in it? Explain.
6. If a fixed-area metal loop is kept entirely within a uniform magnetic field and a current is induced in it, what must its motion be? Explain.
5. By using the appropriate SI units, show that the units on both sides of Equation 20.2 (Faraday’s law) are the same.
4. Two identical strong magnets are dropped simultaneously by two students into two vertical tubes of the same dimensions (▼ Figure 20.21). One tube is made of copper, and the other is made of plastic. From which tube will the magnet emerge first? Why?
3. In Figure 20.7a, how could you move the coil so as to prevent any current from being induced in it? Explain.
2. In Figure 20.1b, what would be the direction of the induced current in the loop if the south pole of the magnet were approaching instead of the north pole?
1. A bar magnet is dropped through a coil of wire as shown in ▶ Figure 20.20. (a) Describe what is observed on the galvanometer by sketching a graph of induced emf versus t. (b) Does the magnet fall freely? Explain.
19. If the frequency of an orange source of light was halved, what kind of light would it then put out:(a) red, (b) blue, (c) violet, (d) UV, (e) X-ray, or(f) IR?
18. Which of these electromagnetic waves travels slowest in a vacuum: (a) green light, (b) infrared light,(c) gamma rays, (d) radio waves, or (e) they all have the same speed?
17. Which of these electromagnetic waves has the lowest frequency: (a) UV, (b) IR, (c) X-ray, or(d) microwave?
16. Relative to the blue end of the visible spectrum, the yellow and green regions have light waves of(a) higher frequencies, (b) longer wavelengths,(c) shorter wavelengths, (d) both (a) and (c).
15. A transformer located just outside a power plant before the energy is delivered over the wires would have (a) more current in the primary coil than in the secondary, (b) more current in the secondary coil than in the primary, (c) the same current in the primary and secondary coils.
14. A transformer located just outside a power plant before the energy is delivered over the wires would have (a) more windings in the primary coil than in the secondary, (b) more windings in the secondary coil than in the primary, (c) the same number of windings in the primary and secondary coils.
13. The output power delivered by a realistic step-down transformer is (a) greater than the input power,(b) less than the input power, (c) the same as the input power.
12. A transformer at the local substation of the delivery system just before your house has (a) more windings in the primary coil, (b) more windings in the secondary coil, (c) the same number of windings in the primary and secondary coils.
19. IE ••• In older computer monitors, electrons were accelerated from rest through a potential difference in an “electron gun” arrangement (▼ Figure 16.29).(a) Should the left side of the gun be at (1) a higher,(2) an equal, or (3) a lower potential than the right side? Why? (b) If the
21. • In Exercise 20, if the ground is designated as zero potential, how far above the ground is the equipotential surface corresponding to 7.0 kV?
22. • Determine the potential 2.5 mm from the negative plate of a pair of parallel plates separated by 20.0 mm and connected to a 24-V battery. Assume the negative plate is designated as zero potential.
23. • Relative to the positive plate in Exercise 22, where is the point with a potential of 14 V?
24. • If the radius of the equipotential surface due to a point charge is 10.5 m and is at a potential of +2.20 kV(compared to zero at infinity), what are the magnitude and sign of the point charge?
18. ••• What is the electric potential at (a) the center of the square and (b) a point midway between q1 and q4 in Figure 16.28?
17. ••• What is the electric potential at (a) the center of the triangle and (b) a point midway between q2 and q3 in Figure 16.27?
16. •• Find the work necessary to bring together (from a large distance) the charges in ▼ Figure 16.28. -10C 0.10 m --10MC 0.10 m 94-+5.0 C 0.10 m 0.10 m +5.0
15. •• Find the work necessary to bring together (from a large distance) the charges in ▼ Figure 16.27. 20 cm +4.0C 20 cm 42 +4.0 C 20 cm %=-4.0 C
14. •• An electron is moved from A to B and then to C along two legs of an equilateral triangle with sides of 0.25 m (▼ Figure 16.26). If the horizontal electric field is 15 V/m, (a) what is the magnitude of the work required? (b) What is the potential difference between A and C? (c) Which is
13. •• A +2.0-μC charge is initially 0.20 m from a fixed −5.0-μC charge and is then moved to 0.50 m from that fixed charge. (a) How much work is required to move the charge? (b) Does the work depend on the path through which the charge is moved?
11. •• In Exercise 10, if the charges were, instead, released from rest at their initial separation, how much kinetic energy would each have when they are very distant from one another?
25. IE • (a) The equipotential surfaces in the neighborhood of a positive point charge are spheres. Which sphere is associated with the higher electric potential:(1) the smaller one, (2) the larger one, or (3) they are associated with the same potential? (b) Calculate the amount of work (in
26. •• Calculate the voltage to accelerate a beam of protons initially at rest, and calculate their speed if they have kinetic energies of (a) 3.5 eV, (b) 4.1 keV, and(c) 8.0 × 10−16 J.
38. • A capacitor has a capacitance of 50 pF, which increases to 150 pF when a dielectric material is between its plates. What is the dielectric constant of the material?
37. ••• Two parallel plates have a capacitance of 0.17 μF when they are 1.5 mm apart. They are connected permanently to a 100-V power supply. If you pull the plates to a distance of 4.5 mm, (a) what is the electric field between them? (b) By how much has the capacitor’s charge changed? (c)
36. ••• A modern large 1.50-F capacitor is connected to a 12.0-V battery for a long time, and then disconnected.The capacitor is able to briefly run a 1.00-W toy motor for 2.00 s. After this time, (a) by how much has the energy stored in the capacitor decreased? (b) What is the voltage across
35. •• Modern capacitors are now capable of storing many times the energy of previous ones. Such a capacitor, with a capacitance of 1.0 F, after being disconnected from its battery, is found to be able to light a small 0.50-W bulb at steady full power for 5.0 s before it quits. What is the
34. •• If the plate separation of the capacitor in Exercise 33 changed to 10.48 mm after the capacitor is disconnected from the battery, what is the change in your answers?
33. •• A 12.0-V battery remains connected to a parallel plate capacitor of plate area 0.224 m2 and separation 5.24 mm. (a) What is the charge on the capacitor?(b) How much energy is stored in the capacitor?(c) What is the electric field between its plates?
30. • A parallel plate capacitor has a plate area of 0.525 m2 and a plate separation of 2.15 mm. What is its capacitance?
29. • How much charge flows from a 12-V battery when a 2.0-μF capacitor is connected across its terminals?
28. ••• Two large parallel plates are separated by 3.0 cm and connected to a 12-V battery. Starting at the negative plate and moving 1.0 cm toward the positive plate at a 45° angle (▼ Figure 16.30), (a) what value of potential would be reached, assuming the negative plate were defined as
27. •• Repeat the calculation in Exercise 26 for a beam of electrons.
10. •• What is the least amount of work required to completely separate two charges (each −1.4 μC) if they were initially 8.0 mm apart?
18. Give several reasons why a conducting material is not be a good choice as a plate separator in a capacitor.
15. If the plates of an isolated parallel plate capacitor are pulled farther apart, does the energy storage of this capacitor increase, decrease, or remain the same?Explain.
14. (a) Can the electric field at a point be zero if there is a nonzero electric potential there? (b) Can the electric potential at a point be zero while there is also a nonzero electric field there? Explain. If your answer to either part is yes, give an example.
13. If a proton is accelerated from rest by a potential difference of 1 million volts, (a) how much kinetic energy does it gain? (b) How would your answer to (a) change if the particle had twice the charge of the proton (same sign) and four times the mass?
12. Near a fixed positive point charge, if you move from one equipotential surface to another with a smaller radius, (a) what happens to the value of the potential?(b) What was your general direction relative to the electric field?
11. What shape are the equipotential surfaces in the region between two concentric metallic spherical shells that have equal but opposite charge on them?
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