Physics Chapter 20 and 21 questions

A bar magnet falling inside a vertical metal tube reaches a terminal velocity even if the tube is evacuated so that there is no air resistance. Explain

Electrons in a changing magnetic field are accelerated. Loops ofcurrent called eddy currents form in the metal pipe. These eddycurrents are small electromagnets that create a magnteic field thatopposes the change in the magnetic field due the to movement of thefalling bar magnet. There is a velocity where the force applied by the eddy currents is equal to the force of gravity on the bar magnet.

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

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

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

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

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

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.

b

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 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

A proton enters a uniform magnetic field that is perpendicular 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 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

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

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

Two parallel wires are vertical. The one on the left carries a 10-A current upward. The other carries 5-A current downward. 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

Two separate but nearby 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

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

Which of the following will not increase a generator's voltage 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

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

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 10-V, 1.0-A dc current is run through a step-up transformer 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) 0V, 0A.

e

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

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

What is the difference between magnetic flux and magnetic field?

magnetic field is a region where magnetic poles and moving charges experience a force. Magnetic flux is a measurement of how much magnetic field passes through an area.