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Exam 11: Magnetism and Electrodynamics

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Question 1

Multiple Choice

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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.

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Question 2

Multiple Choice

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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.

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C

Question 3

Multiple Choice

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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

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.

Which is not a characteristic of hard magnetic materials?

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

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

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

Copper is a metal that can behave like a magnet because

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

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 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

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 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?

Two permanent magnets

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

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

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

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