test 3

What is the magnitude Vba of the potential difference between the ends of the rod?

Vba = 0.567 V (Vba = EL)

When a generator is used to produce electric current, the resulting electric energy originates from which source?

Whatever rotates the generator's axle

A nonconducting plastic hoop is held in a magnetic field that points out of the pages. As the strength of the field increases, ____________

an induced emf will be produced but no current

A square loop moves to the right from an area where B⃗ =0, completely through a region containing a uniform magnetic field directed into the figure (Figure 1), and then out to B=0 after point L. A current is induced in the loop

as it passes line J or line L

When the charges in the rod are in equilibrium, which point, a or b, has an excess of positive charge?

b

Which point, a or b, has a higher potential?

b

As a proton moves through space, it creates __________

both an electric field & magnetic field

Two loops of wire are moving in the vicinity of a very long straight wire carrying a steady current (Figure 1). Find the direction of the induced current in loop D.

current in clockwise direction

A current in a wire points into the page. In which direction is the magnetic field at point A?

downward (c)

Which of the following statements is false? The magnetic field of a current-carrying wire __________

exists only if the current in the wire is changing

A long straight wire carries a current I as shown. A small loop of wire rests in the plane of the page. Which of the following will NOT induce a current in the loop?

moving the loop in a direction parallel to the wire

Which of the following statements about the force on a charged particle due to a magnetic field are not valid?

none of the above; all of these statements are valid

For a charged particle, a constant magnetic field can be used to change __________

only the direction of the particle's velocity

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

only when the current in the first coil changes

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.

the two wires exert the same force on each other

If there is induced current, doesn't that cost energy? Where would that energy come from in case 2?

there is less kinetic energy

A wire carries a current directly away from you. Which way do the magnetic field lines produced by this wire point?

they make circles around the wire

A current in a wire points into the page. In which direction is the magnetic field at point B?

to the left (b)

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 that changes ac to dc plus a

transformer

If the speed of the magnet is doubled, the induced voltage is ________

twice as great

The same magnet is plunged into a coil that has twice the number of turns as before. The magnet is shown before it enters the coil in the figure. If the speed of the magnet is again v, the induced current in the coil is _______

unchanged

Which of the following will produce a magnetic field: a magnet the Earth an electric charge at rest a moving electric charge an electric current the voltage of a battery not connected to anything an ordinary piece of iron a piece of any metal

a magnet the Earth a moving electric charge an electric current

Which statements about the force on a charged particle placed in a magnetic field are true?

a magnetic force is exerted only if the particle is moving

The alternating electric current at a wall outlet is most commonly produced by

a rotating coil that is immersed in a magnetic field

Rank the transformers on the basis of their rms secondary voltage (greatest to least).

G (Vp = 240 V, Np = 1000 turns, Ns = 2000 turns) & (Vp = 120 V, Np = 500 turns, Ns = 2000 turns) (Vp = 480 V, Np = 2000 turns, Ns = 1000 turns) & (Vp = 480 V, Np = 4000 turns, Ns = 2000 turns) (Vp = 240 V, Np = 1000 turns, Ns = 500 turns) L

100 A of rms current is incident on the primary side of each transformer. Rank the transformers on the basis of their rms secondary current (greatest to least).

G (Vp = 240 V, Np = 1000 turns, Ns = 500 turns), (Vp = 480 V, Np = 2000 turns, Ns = 1000 turns), & (Vp = 480 V, Np = 4000 turns, Ns = 2000 turns) (Vp = 240 V, Np = 1000 turns, Ns = 2000 turns) (Vp = 120 V, Np = 500 turns, Ns = 2000 turns) L

Which of the following will not induce a current in the loop?

Moving the loop in a direction parallel to the wire.

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

Rotating the magnetic field so that it is more closely parallel to the generator's rotation axis.

When you swipe a credit card, the machine sometimes fails to read the card. What can you do differently?

Swipe the card more quickly so that the induced emf is higher

Which of the following is true about all series ac circuits?

The current at any point in the circuit is always the same as the current at any other point in the circuit.

B⃗ is kept constant but the coil is rotated so that the magnetic field, B⃗ , is now in the plane of the coil. How will the magnetic flux through the coil change as the rotation occurs?

The flux decreases because the angle between B⃗ and the coil's axis changes

If the magnitude of B⃗ increases while its direction remains unchanged, how will the magnetic flux through the coil change?

The flux increases because the magnitude of B⃗ increases

Which of the following statements about transformers is false?

Transformers work using ac current or dc current.

look over Q 1 & Q 2 of the extra practice!!

(CH 20 extra practice)

The primary coil of a transformer contains 100 turns; the secondary has 200 turns. The primary coil is connected to a size-AA battery that supplies a constant voltage of 1.5 volts. What voltage would be measured across the secondary coil?

0 V

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?

0 V, 0 A.

A transformer supplies 60 watts of power to a device that is rated at 20 volts. The primary coil is connected to a 120-volt ac source. What is the current I1 in the primary coil?

0.5 A

A coil rests in the plane of the page while a magnetic field is directed into the page. A clockwise current is induced when ____________

1) the size of the coil decreases 2) when the magnetic field is tilted so it is no longer perpendicular to the page

A loop rests in the plane of a page of textbook while a magnetic field is directed into the page. A clockwise current is induced

1) when the magnetic field is tilted so it is no longer perpendicular to the page. 2) when the size of the loop decreases

A transformer is intended to decrease the value of the alternating voltage from 500 volts to 25 volts. The primary coil contains 200 turns. Find the necessary number of turns N2 in the secondary coil.

10 turns

In a transformer, the primary coil contains 400 turns, and the secondary coil contains 80 turns. If the primary current is 2.5 amperes, what is the secondary current I2?

12.5 A

What inductance L would be needed to store energy E=3.0kWh (kilowatt-hours) in a coil carrying current I=300A?

240 H

The primary coil of a transformer has 200 turns and the secondary coil has 800 turns. The power supplied to the primary coil is 400 watts. What is the power generated in the secondary coil if it is terminated by a 20-ohm resistor?

400 W

A transformer is intended to decrease the value of the alternating current from 500 amperes to 25 amperes. The primary coil contains 200 turns. Find the necessary number of turns N2 in the secondary coil.

4000 turns

The voltage and the current in the primary coil of a nonideal transformer are 120 volts and 2.0 amperes. The voltage and the current in the secondary coil are 19.4 volts and 11.8 amperes. What is the efficiency e of the transformer? The efficiency of a transformer is defined as the ratio of the output power to the input power, expressed as a percentage: e=100Pout/Pin.

95.4%

Which of the following can a transformer accomplish?

Changing both current and voltage.

When the charges in the rod are in equilibrium, what is the magnitude E of the electric field within the rod?

E = 2.10 V/m (E = vB)

What is the magnitude E of the motional emf induced in the rod?

E = Vba = 0.567 V

What is the direction of the electric field in the rod?

from b to a

A proton enters a uniform magnetic field that is perpendicular to the proton's velocity. What happens to the kinetic energy of the proton?

it stays the same

Which of the transformers are step-up transformers? Which of the transformers are step-down transformers?

step down: (Vp = 480 V, Np = 2000 turns, Ns = 1000 turns), (Vp = 240 V, Np = 1000 turns, Ns = 500 turns), (Vp = 480 V, Np = 4000 turns, Ns = 2000 turns) (more turns in primary than secondary) step up: (Vp = 120 V, Np = 500 turns, Ns = 2000 turns), (Vp = 240 V, Np = 1000 turns, Ns = 2000 turns) (more turns in secondary than primary)

When a charged particle moves parallel to the direction of a magnetic field, the particle moves in a _________

straight line

A wire loop moves at constant velocity without rotation through a constant magnetic field. The induced current in the loop will be

zero

Two loops of wire are moving in the vicinity of a very long straight wire carrying a steady current (Figure 1). Find the direction of the induced current in loop C.

zero