PHY 1409 Unit 2 Misconceptual Questions
In a vacuum, what is the difference between a radio wave and an X-ray? (a) Wavelength. (b) Frequency. (c) Speed.
(a, b) All electromagnetic waves have the same velocity in a vacuum. The velocity is the product of the wavelength and frequency. Since X-rays and radio waves have different wavelengths but the same speed, they will also have different frequencies.
Which of the following travel at the same speed as light? (Choose all that apply.) (a) Radio waves. (b) Microwaves. (c) Radar. (d) Ultrasonic waves. (e) Infrared radiation. (f) Cell phone signals. (g) Gamma rays. (h) X-rays.
(a, b, c, e, f, g, h) All electromagnetic waves travel at the speed of light. The only listed wave that is not an electromagnetic wave is (d ) ultrasonic wave, which is a sound wave and would travel at the speed of sound.
In which direction (see above) is the magnetic field at point B?
(b) The right-hand rule indicates that the magnetic field is clockwise around the current, so at point B the current is to the left.
A coil rests in the plane of the page while a magnetic field is directed into the page. A clockwise current is induced (a) when the magnetic field gets stronger. (b) when the size of the coil decreases. (c) when the coil is moved sideways across the page. (d) when the magnetic field is tilted so it is no longer perpendicular to the page.
(b, d) The right-hand rule shows that a clockwise current will create a flux in the loop that points into the page. Since the initial magnetic flux is into the page, the current will be induced when the magnetic flux decreases. This happens when the size of the coil decreases or the magnetic field becomes tilted. Increasing the magnetic field, as in answer (a), will create a counterclockwise current. Moving the coil sideways, as in answer (c), does not change the flux, so no current would be induced.
If you shine a light through an optical fiber, why does it come out the end but not out the sides? (a) It does come out the sides, but this effect is not obvious because the sides are so much longer than the ends. (b) The sides are mirrored, so the light reflects. (c) Total internal reflection makes the light reflect from the sides. (d) The light flows along the length of the fiber, never touching the sides.
(c) A common misconception is that the light travels down the fiber without touching the edges. However, since the index of refraction of the fiber is greater than the index of refraction of the surrounding air, the light inside the fiber can totally internally reflect off of the sides. Since the ends are perpendicular to the sides, the angle of the light relative to the ends is less than the critical angle and the light exits the ends.
Which of the following will not increase a generator's volt- age output? (a) Rotating the generator faster. (b) Increasing the area of the coil. (c) Rotating the magnetic field so that it is more closely parallel to the generator's rotation axis. (d) Increasing the magnetic field through the coil. (e) Increasing the number of turns in the coil.
(c) Increasing the rotation frequency increases the rate at which the flux changes; therefore, answer (a) will increase the output voltage. Answers (b), (d), and (e) all increase the maximum flux in the coil. Since the generator's voltage output is proportional to the rate of change of the flux through the coil, increasing the maximum flux results in a greater voltage output. When the magnetic field is parallel to the generator's axis, little to no flux passes through the coil. With less flux passing through the coil, there will be a lower output voltage.
A current in a wire points into the page as shown at the right. In which direction is the magnetic field at point A (choose below)? (a) →term-31 (b) ← (c) ↓ (d) ↑ (e) None
(c) It is common to confuse the directions of the magnetic fields with electric fields and thus indicate that the magnetic field points toward or away from the current. Since the current is flowing into the page, the right-hand rule indicates that the magnetic field is in the clockwise direction around the current. At point A the field points downward.
Which of the following statements is false? The magnetic field of a current-carrying wire (a) is directed circularly around the wire. (b) decreases inversely with the distance from the wire. (c) exists only if the current in the wire is changing. (d) depends on the magnitude of the current.
(c) Section 20-5 shows that the magnetic field from a current is directed circularly around the wire, is proportional to the current flowing in the wire, and is inversely proportional to the distance from the wire. A constant current produces a magnetic field, so the current does not need to be changing.
A nonconducting plastic hoop is held in a magnetic field that points out of the page (Fig. 21-54). As the strength of the field increases, (a) an induced emf will be produced that causes a clockwise current. (b) an induced emf will be produced that causes a counterclockwise current. (c) an induced emf will be produced but no current. (d) no induced emf will be produced.
(c) Since the flux through the loop is increasing, an emf will be produced. However, since plastic is not a conductor, no current will be induced. The emf is produced regardless of whether there is a conducting path for current or not.
When the reflection of an object is seen in a flat mirror, the image is (a) real and upright. (b) real and inverted. (c) virtual and upright. (d) virtual and inverted.
(c) Since the image from a flat mirror is behind the mirror, the image is virtual. Experience shows that when looking through a flat mirror, the image is upright.
The radius of an atom is on the order of 10-10 m. In com- parison, the wavelength of visible light is (a) much smaller. (b) about the same size. (c) much larger.
(c) Visible light has a wavelength on the order of 10−7 to 10−8 m. This is over a thousand times larger than the size of an atom.
Which of the following can a transformer accomplish? (a) Changing voltage but not current. (b) Changing current but not voltage. (c) Changing power. (d) Changing both current and voltage.
(d) A common misconception is that a transformer only changes voltage. However, power is conserved across a transformer, where power is the product of the voltage and current. When a transformer increases the voltage, it must also proportionately decrease the current.
A lens can be characterized by its power, which (a) is the same as the magnification. (b) tells how much light the lens can focus. (c) depends on where the object is located. (d) is the reciprocal of the focal length.
(d) A common misconception is to equate power with magnification. However, power is the term used for the reciprocal of the focal length.
Which of the following statements about the force on a charged particle due to a magnetic field are not valid? (a) It depends on the particle's charge. (b) It depends on the particle's velocity. (c) It depends on the strength of the external magnetic field. (d) It acts at right angles to the direction of the particle's motion. (e) None of the above; all of these statements are valid.
(e) Equation 20-3 shows that the magnetic force depends upon the particle's charge, its velocity, and the strength of the external magnetic field. The direction of the force is always perpendicular to the magnetic field and the velocity of the particle. Therefore, all four statements are accurate.
A converging lens, such as a typical magnifying glass, (a) always produces a magnified image (taller than object). (b) always produces an image smaller than the object. (c) always produces an upright image. (d) always produces an inverted image (upside down). (e) None of these statements are true.
(e) If the object distance is greater than twice the focal length, then the image will be reduced, so (a) is incorrect. If the object distance is between the focal length and twice the focal length, then the image will be magnified, so (b) is incorrect. If the object is placed between the lens and the focal length, then the virtual image will be upright, so (c) is incorrect. If the object is placed at a distance greater than the focal length, then the image will be inverted, so (d) is incorrect. Therefore, none of the given statements are true.
For a charged particle, a constant magnetic field can be used to change (a) only the direction of the particle's velocity. (b) only the magnitude of the particle's velocity. (c) both the magnitude and direction of the particle's velocity. (d) None of the above.
(a) A common misconception is that a constant magnetic field can change the magnitude of the particle's velocity. However, the magnetic force is always perpendicular to the velocity, so it can do no work on the particle. The magnetic force only serves as a centripetal force to change the particle's direction.
Which statements about the force on a charged particle placed in a magnetic field are true? (a) A magnetic force is exerted only if the particle is moving. (b) The force is a maximum if the particle is moving in the direction of the field. (c) The force causes the particle to gain kinetic energy. (d) The direction of the force is along the magnetic field. (e) A magnetic field always exerts a force on a charged particle.
(a) A stationary charged particle does not experience a force in a magnetic field. Therefore, the particle must be moving to experience a force. The force is a maximum when the particle is moving perpendicular to the field, not parallel to the field. Since the force is perpendicular to the motion of the particle, it acts as a centripetal force, changing the particle's direction but not its kinetic energy. That is, since the force is perpendicular to the motion, it does no work on the particle. The direction of the force is always perpendicular to the direction of motion and also perpendicular to the magnetic field.
Which of the following statements about transformers is false? (a) Transformers work using ac current or dc current. (b) If the current in the secondary is higher, the voltage is lower. (c) If the voltage in the secondary is higher, the current is lower. (d) If no flux is lost, the product of the voltage and the current is the same in the primary and secondary coils.
(a) In a transformer with no lost flux, the power across the transformer (product of current and voltage) is constant across the transformer. Therefore, if the voltage increases, then the current must decrease across the transformer. If the current increases, then the voltage must decrease across the transformer. A transformer works due to the induced voltage created by the changing flux. A dc circuit does not have a changing flux, so a transformer does not work with dc current.
Suppose you are standing about 3 m in front of a mirror. You can see yourself just from the top of your head to your waist, where the bottom of the mirror cuts off the rest of your image. If you walk one step closer to the mirror (a) you will not be able to see any more of your image. (b) you will be able to see more of your image, below your waist. (c) you will see less of your image, with the cutoff rising to be above your waist.
(a) It is easy to think that as someone approaches the mirror, they will see less of themselves since their image would be larger or they will see more of themselves as the angle of reflection becomes larger. Actually, they will see the exact same amount, as the two properties (larger image and greater angle) exactly cancel each other. A diagram is included with the solution to Problem 1 showing that a ray entering the eye that has reflected from the bottom of the mirror originates at the waist, regardless of how far the person is from the mirror.
You want to create a spotlight that will shine a bright beam of light with all of the light rays parallel to each other. You have a large concave spherical mirror and a small light- bulb. Where should you place the lightbulb? (a) At the focal point of the mirror. (b) At the radius of curvature of the mirror. (c) At any point, because all rays bouncing off the mirror will be parallel. (d) None of the above; you can't make parallel rays with a concave mirror.
(a) Parallel rays that reflect off of a mirror will reflect out through the focal point. Therefore, if the source is placed at the focal point, then all of the reflected rays will exit the mirror parallel to the principal axis of the mirror.
To shoot a swimming fish with an intense light beam from a laser gun, you should aim (a) directly at the image. (b) slightly above the image. (c) slightly below the image.
(a) Students may note that due to refraction the fish is actually lower in the water than it appears and may answer that you should shoot below the image. However, the laser is a light beam that will also refract at the surface, so to hit the fish you would need to aim directly at the fish.
When a charged particle moves parallel to the direction of a magnetic field, the particle travels in a (a) straight line. (b) circular path. (c) helical path. (d) hysteresis loop.
(a) The charged particle only experiences a force when it has a component of velocity perpendicular to the magnetic field. When it moves parallel to the field, it follows a straight line at constant speed.
An electromagnetic wave is traveling straight down toward the center of the Earth. At a certain moment in time the electric field points west. In which direction does the mag- netic field point at this moment? (a) North. (b) South. (c) East. (d) West. (e) Up. (f) Down. (g) Either (a) or (b). (h) Either (c) or (d). (i) Either (e) or (f).
(a) The direction of the magnetic field is perpendicular to the direction the wave is traveling and perpendicular to the electric field, so only north and south are possible answers. The right-hand rule can be used to determine which direction is correct by pointing fingers in the direction of the electric field (west) and bending them in the direction of the magnetic field (north), resulting in the thumb pointing in the direction of the wave (down).
As an object moves from just outside the focal point of a converging lens to just inside it, the image goes from _____ and _____ to _____ and _____. (a) large; inverted; large; upright. (b) large; upright; large; inverted. (c) small; inverted; small; upright. (d) small; upright; small; inverted.
(a) When the object is outside the focal length, the image is real and inverted. When the object is inside the focal length, the image is virtual and upright. And the closer the object is to the focal length, the greater the magnification, so the image is large in both cases.
Indicate which of the following will produce a magnetic field: (a) A magnet. (b) The Earth. (c) An electric charge at rest. (d) A moving electric charge. (e) An electric current. (f ) The voltage of a battery not connected to anything. (g) An ordinary piece of iron. (h) A piece of any metal.
(a, b, d, e) A common misconception is that only permanent magnets (such as a magnet and the Earth) create magnetic fields. However, moving charges and electric currents also produce magnetic fields. Stationary charges, ordinary pieces of iron, and other pieces of metal do not create magnetic fields.
When you swipe a credit card, the machine sometimes fails to read the card. What can you do differently? (a) Swipe the card more slowly so that the reader has more time to read the magnetic stripe. (b) Swipe the card more quickly so that the induced emf is higher. (c) Swipe the card more quickly so that the induced currents are reduced. (d) Swipe the card more slowly so that the magnetic fields don't change so fast.
(b) A common misconception by users of credit card readers is that they are swiping their cards too quickly, so they slow down the swipe. The credit card has a magnetic stripe with information encoded in the magnetic field. Swiping the card more rapidly increases the induced emf, due to the greater rate of change of magnetic flux.
A long straight wire carries a current I as shown in Fig. 21-55. A small loop of wire rests in the plane of the page. Which of the following will not induce a current in the loop? (a) Increasing the current in the straight wire. (b) Moving the loop in a direction parallel to the wire. (c) Rotating the loop so that it becomes perpendicular to the plane of the page. (d) Moving the loop farther from the wire without rotating it. (e) Moving the loop farther from the wire while rotating it.
(b) A current will be induced in the loop whenever the magnetic flux through the loop is changed. Increasing the current in the wire will increase the magnetic field produced by the wire and therefore the magnetic flux in the loop. Rotating the loop changes the angle between the loop and wire, which will change the flux. Since the magnetic field strength decreases with distance from the wire, moving the loop away from the wire, either with or without rotation, will decrease the flux in the wire. If the loop is moved parallel to the wire, then the flux through the loop will not change, so no current will be created in the loop.
When a generator is used to produce electric current, the resulting electric energy originates from which source? (a) The generator's magnetic field. (b) Whatever rotates the generator's axle. (c) The resistance of the generator's coil. (d) Back emf. (e) Empty space.
(b) A generator converts mechanical energy into electric energy. The generator's magnetic field remains unchanged as the generator operates, so energy is not being pulled from the magnetic field. Resistance in the coils removes electric energy from the system in the form of heat—it does not provide the energy. In order for the generator to work, an external force is necessary to rotate the generator's axle. This external force does work, which is converted to electric energy.
If electrons in a wire vibrate up and down 1000 times per second, they will create an electromagnetic wave having (a) a wavelength of 1000 m. (b) a frequency of 1000 Hz. (c) a speed of 1000 ms. (d) an amplitude of 1000 m.
(b) Frequency is the measure of the number of oscillations made per second. The rate at which the electrons oscillate will be equal to the oscillation frequency of the radiation emitted.
When moonlight strikes the surface of a calm lake, what happens to this light? (a) All of it reflects from the water surface back to the air. (b) Some of it reflects back to the air; some enters the water. (c) All of it enters the water. (d) All of it disappears via absorption by water molecules.
(b) If all of the light reflected off of the surface, then the water under the lake would be completely dark. However, when you swim underwater on a moonlit night, it is possible to see the Moon. If all of the moonlight entered the water, then you could not see the Moon reflecting off the surface. Therefore, some of the light must enter the water and some must reflect off of the surface.
Which of the following is true about all series ac circuits? (a) The voltage across any circuit element is a maximum when the current is a maximum in that circuit element. (b) The current at any point in the circuit is always the same as the current at any other point in the circuit. (c) The current in the circuit is a maximum when the source ac voltage is a maximum. (d) Resistors, capacitors, and inductors can all change the phase of the current.
(b) In a series circuit (whether ac or dc), the current is the same at every point in the circuit. In an ac circuit, the current is out of phase with the voltage across an inductor and the voltage across a capacitor, so (a) is incorrect. In nonresonant ac circuits, there is a phase difference between the current and the voltage source, so (c) is not correct. The current and voltage across a resistor are always in phase, so the resistor does not change the phase in a circuit, so (d) is incorrect.
The alternating electric current at a wall outlet is most commonly produced by (a) a connection to rechargeable batteries. (b) a rotating coil that is immersed in a magnetic field. (c) accelerating electrons between oppositely charged capacitor plates. (d) using an electric motor. (e) alternately heating and cooling a wire.
(b) Many people may not realize that generators (rotating coils in a magnetic field) are the heart of most electric power plants that produce the alternating current in wall outlets.
A proton enters a uniform magnetic field that is perpen- dicular to the proton's velocity (Fig. 20-51). What happens to the kinetic energy of the proton? (a) It increases. (b) It decreases. (c) It stays the same. (d) It depends on the velocity direction. (e) It depends on the B field direction.
(c) A common misconception is that a force always does work on the object. Since the magnetic force is perpendicular to the velocity of the proton, the force acts as a centripetal force, changing the proton's direction, but not doing any work and thus not changing its kinetic energy.
A wire loop moves at constant velocity without rotation through a constant magnetic field. The induced current in the loop will be (a) clockwise. (b) counterclockwise. (c) zero. (d) We need to know the orientation of the loop relative to the magnetic field.
(c) A common misconception is that a moving loop would experience a change in flux. However, if the loop is moving through a constant field without rotation, then the flux through the loop will remain constant and no current will be induced.
You cover half of a lens that is forming an image on a screen. Compare what happens when you cover the top half of the lens versus the bottom half. (a) When you cover the top half of the lens, the top half of the image disappears; when you cover the bottom half of the lens, the bottom half of the image disappears. (b) When you cover the top half of the lens, the bottom half of the image disappears; when you cover the bottom half of the lens, the top half of the image disappears. (c) The image becomes half as bright in both cases. (d) Nothing happens in either case. (e) The image disappears in both cases.
(c) A common misconception is that each half of the lens produces half of the image. Actually, light from each part of the image passes through each part of the lens. When half of the lens is covered, the image is less intense, but the full image is still present on the screen.
Starting in 2009, TV stations in the U.S. switched to digital signals. [See Sections 22-7, 17-10, and 17-11.] To watch today's digital broadcast TV, could you use a pre-2009 TV antenna meant for analog? Explain. (a) No; analog antennas do not receive digital signals. (b) No; digital signals are broadcast at different frequencies, so you need a different antenna. (c) Yes; digital signals are broadcast with the same carrier frequencies, so your old antenna will be fine. (d) No; you cannot receive digital signals through an antenna and need to switch to cable or satellite.
(c) A common misconception is that the digital broadcast uses a different frequency than the analog broadcast. This is incorrect—the carrier wave frequency is the same. Since the antenna is designed for operation at the broadcast frequency, the antenna will work just as well with a digital signal.
When you look at a fish in a still stream from the bank, the fish appears shallower than it really is due to refraction. From directly above, it appears (a) deeper than it really is. (b) at its actual depth. (c) shallower than its real depth. (d) It depends on your height above the water.
(c) A common misconception is that the fish will appear to be located at its actual depth. However, due to refraction, light from the fish bends away from the normal as it exits the surface. This refraction makes the fish appear closer to the surface than it actually is. See Fig. 23-25 in the text.
If the intensity of an electromagnetic wave doubles, (a) the electric field must also double. (b) the magnetic field must also double. (c) both the magnetic field and the electric field must increase by a factor of 12 . (d) Any of the above.
(c) A misconception that can arise is thinking that the wave intensity is proportional to just one of the field amplitudes. However, the intensity is proportional to the product of the amplitudes, with the field amplitudes proportional to each other. If the intensity doubles, then each field (electric and magnetic) must increase by the same factor, √2.
As a proton moves through space, it creates (a) an electric field only. (b) a magnetic field only. (c) both an electric field and magnetic field. (d) nothing; the electric field and magnetic fields cancel each other out.
(c) Electric fields are created by charged objects whether the charges are moving or not. Magnetic fields are created by moving charged objects. Since the proton is charged and moving, it creates both an electric field and a magnetic field.
In empty space, which quantity is always larger for X-ray radiation than for a radio wave? (a) Amplitude. (c) Frequency. (b) Wavelength. (d) Speed.
(c) In empty space, X-ray radiation and a radio wave both travel at the speed of light. X-ray radiation has a much smaller wavelength than a radio wave. Since wave speed is equal to the product of the wavelength and the frequency, and both waves travel at the same speed, the X-ray radiation has a higher frequency than a radio wave. The amplitude depends upon the intensity of the wave, so either the X-ray radiation or the radio wave could have the greater amplitude.
If there is induced current in Question 18 (see Fig. 21-51), wouldn't that cost energy? Where would that energy come from in case (a)?(a) Induced current doesn't need energy. (b) Energy conservation is violated. (c) There is less kinetic energy. (d) There is more gravitational potential energy.
(c) The induced current creates a magnetic field that opposes the motion of the magnet. The result is that the magnet has less kinetic energy than it would have had if the loop were not present. The change in gravitational potential energy provides the energy for the current.
Two parallel wires are vertical. The one on the left carries a 10-A current upward. The other carries 5-A current down- ward. Compare the magnitude of the force that each wire exerts on the other. (a) The wire on the left carries twice as much current, so it exerts twice the force on the right wire as the right one exerts on the left one. (b) The wire on the left exerts a smaller force. It creates a magnetic field twice that due to the wire on the right; and therefore has less energy to cause a force on the wire on the right. (c) The two wires exert the same force on each other. (d) Not enough information; we need the length of the wire.
(c) This question requires a consideration of Newton's third law. The force that one wire exerts on a second must be equal in magnitude, but opposite in direction, to the force that the second exerts on the first.
Two separate but near by coils are mounted along the same axis. A power supply controls the flow of current in the first coil, and thus the magnetic field it produces. The second coil is connected only to an ammeter. The ammeter will indicate that a current is flowing in the second coil (a) whenever a current flows in the first coil. (b) only when a steady current flows in the first coil. (c) only when the current in the first coil changes. (d) only if the second coil is connected to the power supply by rewiring it to be in series with the first coil.
(c) When a steady current flows in the first coil, it creates a constant magnetic field and therefore a constant magnetic flux through the second coil. Since the flux is not changing, a current will not be induced in the second coil. If the current in the first coil changes, then the flux through the second will also change, inducing a current in the second coil.
Two loops of wire are moving in the vicinity of a very long straight wire carrying a steady current (Fig. 21-53). Find the direction of the induced current in each loop. For C: (a) clockwise. (b) counterclockwise. (c) zero. (d) alternating (ac). For D: (a) clockwise. (b) counterclockwise. (c) zero. (d) alternating (ac).
(c, a) The magnetic field near a long straight wire is inversely proportional to the distance fromthe wire. For C, the loop remains at the same distance from the wire, so the magnetic flux through the wire remains constant and no current is induced in the loop. For D, the magnetic field from the long wire points into the page in the region of the loop. As the loop moves away from the wire, the magnetic flux decreases, so a clockwise current is induced in the loop to oppose the change in flux.
If all else is the same, for which surface would the radiation pressure from light be the greatest? (a) A black surface. (b) A gray surface. (c) A yellow surface. (d) A white surface. (e) All experience the same radiation pressure, because they are exposed to the same light.
(d) A common misconception is that every color surface will experience the same radiation pressure. However, the pressure is related to the change in momentum of the light. With a black object, all of the light is absorbed, so the radiation pressure is proportional to the incident momentum. With a colored object, some of the light is reflected, thus increasing the change in momentum of the light and therefore increasing the radiation pressure as compared to the black object. For a white object, the maximum amount of light is reflected off the surface, creating the greatest change in momentum of the light and the greatest radiation pressure.
If the Earth-Sun distance were doubled, the intensity of radiation from the Sun that reaches the Earth's surface would (a) quadruple. (b) double. (c) drop to 1/2 . (d) drop to 1/4 .
(d) A common misconception is to think that the radiation decreases linearly with distance, such that the intensity would decrease by a factor of two. However, the radiation intensity decreases as the square of the distance. Doubling the distance will decrease the radiation intensity to one-fourth the initial intensity.
A square loop moves to the right from an area where B = 0, completely through a region containing a uniform magnetic field directed into the page (Fig. 21-52), and then out to B = 0 after point L. A current is induced in the loop MisConceptual Question 4. (a) only as it passes line J. (b) only as it passes line K. (c) only as it passes line L. (d) as it passes line J or line L. (e) as it passes all three lines.
(d) A current is induced in the loop when the flux through the loop is changing. As the loop passes through line J it enters a region with a magnetic field, so the flux through the loop increases and a current will be induced. When the loop passes line K, the flux remains constant, as there is no change in field, so no current is induced. As the loop passes line L, the magnetic field flux through the loop decreases and a current is again induced.
A laptop computer's charger unit converts 120 V from a wall power outlet to the lower voltage required by the laptop. Inside the charger's plastic case is a diode or rectifier (discussed in Chapter 29) that changes ac to dc plus a (a) battery. (b) motor. (c) generator. (d) transformer. (e) transmission line.
(d) If the charger unit had a battery, then it could run the laptop without being plugged in. A motor converts electrical energy into mechanical energy, but the laptop charger output is electrical energy, not mechanical energy. A generator converts mechanical energy into electric energy, but the input to the laptop is electrical, not mechanical. The cables to and from the charger unit can be considered transmission lines, but they are not the important component inside the charger unit. A transformer can convert a high-voltage input into a low-voltage output without power loss. This is a significant function of the charger unit.
Which of the following types of electromagnetic radiation travels the fastest? (a) Radio waves. (b) Visible light waves. (c) X-rays. (d) Gamma rays. (e) All the above travel at the same speed.
(e) A common misconception is that electromagnetic waves travel at different speeds. They do not, as they all travel at the speed of light.
A wire carries a current directly away from you. Which way do the magnetic field lines produced by this wire point? (a) They point parallel to the wire in the direction of the current. (b) They point parallel to the wire opposite the direction of the current. (c) They point toward the wire. (d) They point away from the wire. (e) They make circles around the wire.
(e) It is common to confuse the direction of electric fields (which point toward or away from the charges) with magnetic fields, which always make circles around the current.
A 10-V, 1.0-A dc current is run through a step-up trans- former that has 10 turns on the input side and 20 turns on the output side. What is the output? (a) 10 V, 0.5 A. (b) 20 V, 0.5 A. (c) 20 V, 1 A. (d) 10 V, 1 A. (e) 0 V, 0 A.
(e) It may appear that (b) is the correct answer if the problem is interpreted as an ac step-up transformer with twice as many loops in the secondary coil as in the primary. It is true that an ac current would double the voltage and cut the current in half; however, this is a dc current. A dc current does not produce a changing flux, so no current or voltage will be induced in the secondary coil.
Which of the following can form an image? (a) A plane mirror. (b) A curved mirror. (c) A lens curved on both sides. (d) A lens curved on only one side. (e) All of the above.
(e) It might seem reasonable that a lens must be curved on both sides to produce an image. However, images can be produced by plane mirrors, curved mirrors, and lenses curved on one or both sides.
Virtual images can be formed by (a) only mirrors. (b) only lenses. (c) only plane mirrors. (d) only curved mirrors or lenses. (e) plane and curved mirrors, and lenses.
(e) Virtual images are formed by plane mirrors, convex mirrors, concave mirrors (when the object is within the focal distance), diverging lenses, and converging lenses (when the object is within the focal distance). Therefore, virtual images can be formed with plane and curved mirrors and lenses.
Parallel light rays cross interfaces from medium 1 into medium 2 and then into medium 3 as shown in Fig. 23-51. What can we say about the relative sizes of the indices of refraction of these media? (a) n₁ > n₂ > n₃ (b) n₃ > n₂ > n₁ (c) n₂ > n₃ > n₁ (d) n₁ > n₃ > n₂ (e) n₂ > n₁ > n₃ (f) None of the above.
(e) When light travels from a low index to a higher index, it bends toward the normal. When it travels from a high index to a low index, it bends away from the normal. As it passes from region 1 to region 2, it bends toward the normal, so n₂ > n₁ . As it passes from region 2 to region 3, it bends away from the normal, so n₂ > n₃. Since the surfaces are parallel and the angle in region 3 is greater than the initial angle in region 1, n₁ > n₃. Combining these three relations gives n₂ > n₁ > n₃.
