Deck 11: Magnetism and Electrodynamics

Question 1

A typical bar magnet has an "N" stamped on its north pole end and an "S"
Stamped on its south pole end. With the right tools, you might be able to change this bar magnet in one or more of the following ways: (1) remove its magnetic poles altogether, (2) reverse its magnetic poles so that it has a north pole at the end stamped "S"
And a south pole at the end stamped "N", (3) convert its north pole into a south pole so that it has south poles at both ends and no north poles. Which of those three options is physically possible?

A) Only (1) and (2)
B) None of them is possible.
C) Only (2) and (3)
D) All three are possible.

Answer 1

Only (1) and (2)

Question 2

Why can't you float one permanent magnet directly above another permanent magnet indefinitely by turning their north poles toward one another?

A) Two north poles attract one another and the two magnets will pull together until they touch.
B) That arrangement is unstable-the upper magnet will fall to the side or flip over.
C) The repulsive forces between magnets cannot overcome the gravitational forces between them, so the upper magnet cannot float.
D) Because they are stationary, the two magnets exert no forces on one another and the top magnet falls.

Answer 2

That arrangement is unstable-the upper magnet will fall to the side or flip over.

Question 3

You are working with a magnet and it breaks in half. You then end up with

A) One north and one south pole
B) Two south poles only
C) Two north poles only
D) Two magnets, each with a south and a north pole.

Answer 3

Two magnets, each with a south and a north pole.

Question 4

Which of the following is not ferromagnetic?

A) Iron
B) Nickel
C) Aluminum
D) Chromium dioxide

Question 5

Which is not a characteristic of hard magnetic materials?

A) They are hard to magnetize.
B) They easily lose their magnetization.
C) They are used in magnetic tapes.
D) They have microscopic structures and defects that impede the resizing of domains.

Question 6

A magnet factory is making bar magnets, each about the size and shape of an ordinary ruler. After forming each bar of metal, that bar must be magnetized. The bar is placed in a coil of wire and a huge pulse of current is sent through the coil. During the pulse, current is only sent in one direction through the coil: a pulse of direct current or DC. If, instead, the current reversed directions rapidly during the pulse-a pulse of alternating current or AC-then

A) the bar magnet would end up with two south poles and no north poles.
B) the poles of the bar magnet would also reverse rapidly and would end up with a magnetization that depends upon what the AC current was at the precise moment it was shut off.
C) the bar would become overmagnetized because AC is much more magnetic than DC.
D) the bar magnet would end up with two north poles and no south poles.

Question 7

The north pole of a permanent magnet is clinging to the front surface of your steel refrigerator, so the refrigerator clearly has a south pole at its surface. If you flip the permanent magnet over, so that its south pole faces the refrigerator, the refrigerator will

A) keep a south pole at its surface and attract the permanent magnet.
B) place a north pole at its surface and attract the permanent magnet.
C) place a north pole at its surface and repel the permanent magnet.
D) keep a south pole at its surface and repel the permanent magnet.

Question 8

Suppose you have a machine which has a very strong magnet inside. You have some computer disks in the room and, although you think they are far away enough from the magnet to be safe, you don't want to take any risks. What material would be appropriate for building a magnetic shielding enclosure?

A) aluminum.
B) plastic.
C) iron.
D) copper.

Question 9

You are working on an experiment involving a very strong permanent magnet, and your data suggests that your magnet's field suddenly decreased during some interval in time. Such a decrease could have been caused by the magnet

A) having overheated substantially.
B) being hit hard.
C) both (A) and (B).
D) being grounded out.

Question 10

Two permanent magnets

A) always attract.
B) can attract or repel.
C) always repel.
D) never have stable force interactions.

Question 11

A steel refrigerator door is not permanently magnetic because

A) All the magnetic domains are oriented randomly
B) There are no magnetic domains
C) The electrons are bound and can't move around much
D) The door is usually ground out and loses all its magnetism.

Question 12

Electricity and magnetism are different because

A) There are no electric monopoles.
B) There are no magnetic monopoles.
C) Electrical forces decrease with increasing separation.
D) Only magnetism has to do with electrons.

Question 13

A permanent magnet and a magnetizable material like steel Two permanent magnets

A) always attract.
B) can attract or repel.
C) always repel.
D) never have stable force interactions.

Question 14

You are working in a laboratory over the summer and you measure the magnetic field in a coil to be 3T. You insert a steel rod which has a pole strength 500 A-m. The amount of force on the rod is

A) 0
B) 3 N
C) 1500 N
D) 1500 N-m

Question 15

You are working in a laboratory over the summer and you measure the magnetic field in a coil to be 2T. You insert a steel rod which has a pole strength 5 A-m outside the magnet. The amount of force on the rod

A) is 0
B) is 2 N
C) is 1000 N
D) cannot be determined because the pole strength of the steel will change inside the magnet.

Question 16

Although you would like for your date to have a magnetic personality, the field of magnetic dating is about something else altogether. The idea is that magnetic rocks get very hot in or near a fire and then when they cool down they take on the magnetic field of the earth at whatever time they cooled. The reason heat is needed is because

A) There needs to be enough thermal fluctuations to disorient the magnetic domains and cause the rock to be non - magnetic.
B) There needs to be enough thermal fluctuations to disorient the magnetic domains and cause the rock to be magnetic.
C) The fire causes a chemical reaction on the surface of the rock that preserves magnetic field direction.
D) Fire is just dramatic.

Question 17

When you drop a strong magnet through the center of a copper pipe, the magnet

A) descends slowly because it is attracted to the magnetic copper metal.
B) descends rapidly because its motion causes currents to flow in the pipe and those currents attract the magnet.
C) falls at the usual rate because copper metal is nonmagnetic.
D) descends slowly because its motion causes currents to flow in the pipe and those currents repel the magnet.

Question 18

If you float an aluminum pie plate on the surface of a pond and move the north pole of a strong magnet in a clockwise circle just above that plate, the plate will

A) begin turning counter-clockwise, as though it were being twisted away from the magnet.
B) begin turning clockwise, as though it were being dragged along with the magnet.
C) remain stationary.
D) be lifted out of the water and will stick to the strong north pole above it.

Question 19

When you drop the north pole of a permanent magnet onto an aluminum sheet, it falls slightly slower than normal. The magnet is being repelled by the aluminum because

A) aluminum is positively charged and repels approaching magnetic poles.
B) aluminum has a net north magnetic pole that repels any approaching north poles.
C) aluminum has a net south magnetic pole that repels any approaching north poles.
D) current induced in the aluminum by the approaching north pole produces a repelling north pole on the aluminum's surface.

Question 20

If you move the north pole of a permanent magnet toward the surface of an aluminum pot, a current will flow through that pot and the pot will become magnetic-repelling your permanent magnet. If you stop the permanent magnet just before it touches the pot and then hold the permanent magnet stationary, the repulsive force between the pot and the permanent magnet will gradually disappear. The repulsive force disappears because the electric current in the pot

A) becomes non-magnetic once the permanent magnet stops moving.
B) stops increasing and becomes steady once the permanent magnet stops moving.
C) gradually slows to a stop once the permanent magnet stops moving.
D) becomes an alternating current once the permanent magnet stops moving and the pot's magnetic poles then flip back and forth rapidly.

Question 21

A step-up transformer has 20 primary turns and 400 secondary turns. If the primary voltage is 120 V, what is the secondary voltage?

A) 48000 V
B) 2400 V
C) 120 V
D) 6.0 V

Question 22

A step-up transformer has 20 primary turns and 400 secondary turns. If the primary current is 30 A, what is the secondary current?

A) 12000 A
B) 600 A
C) 30 A
D) 1.5 A

Question 23

The power supply for your answering machine is a small black cube that plugs directly into an electric outlet. The main component in this supply is a transformer and moderate current from the 120 volt power line flows through the primary coil of this transformer. If the transformer's primary coil has 20 times as many turns of wire in it as the secondary coil has, then the secondary coil provides

A) a larger voltage and a larger current than the primary.
B) a smaller voltage and a larger current than the primary.
C) a smaller voltage and a smaller current than the primary.
D) a larger voltage and a smaller current than the primary.

Question 24

The principal advantage of sending electric power across country on very high voltage transmission lines is that

A) they carry less energy per charge than low voltage transmission lines.
B) electric power lost in the wires is greatly reduced.
C) these transmission lines are less likely to get in the way than low voltage transmission lines-which are much closer to the ground.
D) they carry much more current than low voltage transmission lines.

Question 25

A transformer provides the 12,000 volts needed to operate the neon sign in a local convenience store. When AC current flows through the transformer's primary coil and experiences a voltage drop of 120 volts, current flows through the transformer's secondary coil and experiences a voltage rise of 12,000 volts. Based on this observation, it is likely that

A) the currents in the two coils are about equal.
B) the secondary coil has about 100 times as many turns in it as the primary coil.
C) the current in the secondary coil is about 100 times as large as the current in the primary coil.
D) the primary coil has about 100 times as many turns in it as the secondary coil.

Question 26

Electric power passes through a nearby power transformer on its way to your home. The current that enters your home actually receives its power as it passes through the secondary coil of that transformer. Doubling the number of turns in the secondary coil would

A) halve the voltage available at your home.
B) double the frequency (cycles-per-second) of the alternating current entering your home.
C) double the voltage available at your home.
D) halve the frequency (cycles-per-second) of the alternating current entering your home.

Question 27

Commercial electric power is sent across country using high voltage transmission lines. If low voltage transmission lines were used instead, those low voltage lines would

A) have to use direct current, which would not operate most equipment properly.
B) not be magnetic enough to transform electricity efficiently into energy.
C) have to placed closer to the ground, where they would be more hazardous.
D) have to be made of thicker metal, which would be expensive.

Question 28

Electricity produced in a generating plant passes through a large step-up transformer. This step-up transformer produces the high voltages needed to send electric power long distances across the countryside. Which of the following is transferred from the transformer's primary coil to its secondary coil while the transformer is operating?

A) Negative electric charges and power.
B) Positive electric charges, negative electric charges, and power.
C) Power alone.
D) Positive electric charges and power.

Question 29

A step-down transformer transfers power from the 120 Volt alternating current passing through its primary coil to the 12 volt alternating current passing through its secondary coil. If you interchange the primary and secondary coil, and send the 120 Volt alternating current through the new primary coil,

A) a 12 Volt alternating current will pass through its new secondary coil.
B) a 12 Volt direct current will pass through its new secondary coil.
C) no current will pass through its new secondary coil.
D) a 1200 Volt alternating current will pass through its new secondary coil.

Question 30

Around high tension power lines there are

A) electric, but no magnetic fields.
B) neither electric nor magnetic fields.
C) both electric and magnetic fields.
D) magnetic, but no electric fields.

Question 31

A television contains a small step-up transformer that produces the very high voltages needed to accelerate electrons toward the front of the picture tube. When low voltage alternating current passes through the primary coil of this transformer, power is extracted from that current and passed to an alternating current flowing through the secondary coil of the transformer. The electric current in the secondary coil

A) is greater than that in the primary coil.
B) is less than that in the primary coil.
C) is equal to that in the primary coil..
D) is direct current (DC).

Question 32

Thomas Edison's dominance in the electric power industry was struck a serious blow when his competitors, Nikola Tesla and George Westinghouse, began to use transformers with their alternating current (AC) electric power systems. Edison was unable to use transformers in his direct current (DC) electric power systems because

A) Tesla and Westinghouse held the U.S. patent rights to transformers and prevented Edison from using them.
B) direct current electric power must flow directly from the generator to a home, without passing through any intermediate devices.
C) a transformer requires changing currents in its coils in order to transfer power between those currents.
D) DC electric power must flow through thicker wires than can be used efficiently in transformers.

Question 33

Suppose you are building a metal detector that uses an iron magnet to induce electric currents in conducting objects underground whose signals are then detected.
-Explain why the metal detector would always have to be moving for it to work.

Question 34

Suppose you are building a metal detector that uses an iron magnet to induce electric currents in conducting objects underground whose signals are then detected.
-What modifications would you make to the magnet so the detector could remain stationary and still work?

Question 35

Often when patients go in the hospital to get an MRI scan, they are asked to remove all metal objects such as rings, earrings, transformers, etc. In an MRI, a very strong magnetic field pulses on and off. Why is this warning in place?

Question 36

Explain why a CD placed in a running microwave oven sparks.

Question 37

Your friend calls you up and says that he has invented a new type of transformer. He says that it is able to convert 12V AC to 120V DC while keeping current steady on both the primary (input) and secondary (output). Also, he tells you that the transformer is the next greatest thing to baked bread because it doesn't even get warm when it is in operation. State three scientific reasons why you do or do not think the transformer will work.

Question 38

An ideal step-up transformer has 10 primary turns and 400 secondary turns. If the power into the primary is 150W, the power output is

A) 400 W
B) 1500 W
C) 150 W
D) 150 J

Question 39

Copper is a metal that can behave like a magnet because

A) It has many magnetic domains oriented randomly
B) It has a very large magnetic domain pointing in a given direction
C) It is a conductor and can support electric current.
D) It has two large magnetic domains that always cancel each other out.

Question 40

The principal disadvantage of sending electric power across country on very low voltage transmission lines would be

A) the wires need to be too thick and heavy.
B) high current would cause significant power loss.
C) the low voltage can leak out, leaving none at the end.
D) they would interfere with each other too much

Question 41

When you drop a strong magnet through the center of a copper pipe with a vertical slit cut along its length, the magnet

A) descends slowly because it is attracted to the magnetic copper metal.
B) descends rapidly because its motion causes currents to flow in the pipe and those currents attract the magnet.
C) falls at the usual rate because there are no currents in the copper to cause it to act like a magnet. .
D) descends slowly because its motion causes currents to flow in the pipe and those currents repel the magnet.

Question 42

It would be possible to make a compass needle out of aluminum if

A) It were suspended and not pivoted on bearings.
B) The earth's magnetic field were rapidly changing with time
C) The earth's magnetic field were stronger
D) the needle were made to be very large.

Question 43

Around transmission lines there are generally electric and magnetic fields. If there ever were to be a situation where there is just a magnetic field near a tension line, there would need to be

A) Current without charge.
B) Charge without current
C) Charge and current
D) Nether charge nor current.