Physics Block 6

In shakable flash light, the light lights after shaking the body of the flashlight up and down for about 60 seconds. This happens because

There is a magnet which moves back and forth through a coil of wire.

If you hold a strong permanent magnet inside a metal ring and then quickly remove that magnet, you create an electric field in the metal ring and an electrical current flows through the metal ring. If you repeat this procedure with a glass ring, no current flows in the glass ring because

although removing the magnet creates an electric field in the glass ring, the glass ring has no mobile electric charges to carry an electrical current.

You have a long bar magnet with ends that are clearly marked "N" and "S". You accidentally drop the bar magnet and it breaks in two exactly at the center of the magnet. If you bring the two broken ends towards each other, the two broken ends will

attract

You have two bar magnets with ends that are clearly marked "N" and "S" to show which ends are North and which ends are South poles. If you bring the end marked "S" of one bar magnet close to the ends marked "N" of the other magnet, the two magnets

attract

You have a DC motor which turns when you hook it up to a battery. That same DC motor

could also act as a generator.

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

descends slowly because its motion causes currents to flow in the pipe and those currents repel the magnet.

The principal advantage of sending electric power across long distances using low current is that

electric power lost in the wires is greatly reduced

The huge steam-powered generators found in electric power plants produce electricity by

moving magnets past coils of wire.

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

place a north pole at its surface and attract the permanent magnet.

If you rub a paper clip along a strong bar magnet one can cause the paper clip to have a permanent magnetic moment so that it can pick up other paper clips. If you hit the paper clip with a hammer then

the paper clip can become unmagnetized again.

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-it's 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

the poles of the bar magnet would also reverse rapidly and the bar wouldn't acquire a strong magnetization.

A transformer provides the 12,000 volts needed to operate the neon sign in a local convenience store. The AC voltage across the transformer's primary coil is 120 volts, and across the transformer's secondary coil is 12,000 volts. Based on this observation, it is likely that

the secondary coil has about 100 times more turns than the primary coil has.

You observe that a transformer has a primary voltage of 120 V and a secondary voltage of 2400 V. The transformer has 400 secondary turns. How many primary turns to the transformer have?

20

If the primary coil of a transformer has 200 turns and is supplied with 120-V AC power, how many turns must the secondary coil have to provide 12-V AC power?

20 turns

To read the magnetic strip on an ID or credit card, you must swipe it quickly past a tiny coil of wire. Why must the card be moving for the coil system to read it?

A moving magnet will induce a current in the coil so that the magnetic strip can be read.

Which is true of a transformer?

A transformer can keep the power constant while changing the current and voltage.

Which of the following can cause a moving charged particle to accelerate?

A) and B) each will

You have a loop of copper wire connected to a light bulb. You quickly pass a permanent bar magnet through the center of the loop. What happens?

Current flows in the loop.

You use a hand generator to light a light bulb. As you crank the generator, what happened when the light bulb is removed?

It became easier to crank because no current was flowing in the circuit leaving the magnet to move with less resistance

We saw a movie of a floating frog in lecture. Why did the frog float?

It was placed in a strong magnetic field and the frog was diamagnetic

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

Power alone

A horseshoe magnet is a bar magnet which is bent in a "U" shape. Looking at the horseshoe magnet the North pole is to the left and the South pole is to the right. In between the North and South poles a compass would point in the direction of the magnetic field, or

Right

A compass needle points approximately towards the Northern rotational axis of the earth. A compass is just a magnetized needle where the pointing part is a N pole and the opposite end is the S pole of the magnet. If the rotational axis of the Earth can be viewed as just a bar magnet, what end of the bar magnet is pointing towards the North rotational axis?

South Pole

In magnetic resonance imaging (MRI), a patient is immersed in an intense magnetic field. That field is created entirely without permanent magnets or even iron. How is this possible?

The MRI uses electric current to generate the magnetic field

A television picture tube produces its images using beams of electrons that move through empty space and strike phosphors on the inside of the screen. Those phosphors glow following the impact. Aiming the electron beams is a delicate task. Why is it important to avoid placing strong magnets near a television picture tube?

The electron beams can be deflected as they pass through the stray magnetic fields.