Chapter 25

What important discovery did physicists Michael Faraday and Joseph Henry make?

Electromagnetic induction

What exactly does a step-down transformer step down?

Voltage

The primary of a transformer connected to 180 V has 100 turns. The secondary has 10 turns. This is a step-down transformer. Select the correct equations that show that the output voltage is 18 V

Vout = Vin ⋅ Nsecondary/Nprimary = (180V) ⋅ 10turns/100turns = 18V (the answer with Nsecondary / Nprimary & 10 / 100 turns)

A device that transforms mechanical energy into electrical energy is a

generator

What are the basic differences and similarities between a generator and an electric motor?

In both, moving electrons experience a force. The motor transforms *electrical* energy to *mechanical* energy. The generator transforms mechanical energy to electrical energy

What is the principal advantage of ac over dc?

It is easy to step up and step down voltages using ac

Who extended Faraday's law to changing electric fields?

Maxwell

What happens when a wire is held between the poles of a magnet?

Nothing happens

What name is given to the rate at which energy is transferred?

Power

Faraday's law of induction deals with how a magnetic flux induces an emf in a circuit. Recall that magnetic flux depends on magnetic field strength and the effective area the field is passing through. We'll start our investigation by looking at the field strength around a bar magnet. Position the magnet around the coil so that the region labeled A in the figure below is inside the coil. Move the magnet slowly back and forth and observe the effect on the brightness of the bulb and the needle of the voltmeter. Repeat the same process for the other two regions. For which of the regions shown in the figure is the observed effect the strongest?

Region C

What commonly supplies the energy input to a turbine?

Steam or falling water

A certain transformer doubles input voltage. If the primary coil draws 10 A of current, then the current in the secondary coil is

5 A.

What is induced by the rapid alternation of an *electric* field?

A rapidly alternating magnetic field

What happens when a wire is moved between the poles of a magnet?

A voltage is generated in the wire

Why does a transformer require ac?

A voltage is induced only when the magnetic field is changing

How do you induce a voltage in a conducting loop of wire?

Change the magnetic field in the loop

You can switch the polarity of the bar magnet by clicking on the button with the bar magnet and two semi-circular arrows on the bottom right. Note how the positions of the north and south poles are switched every time you click on this button. Drag the bar magnet from left to right through the top coil, then repeat the same process after switching the polarity of the magnet. What effect, if any, does switching the polarity have on the brightness of the bulb and the needle of the voltmeter?

Switching the polarity of the magnet has no effect on the bulb brightness, but inverts the order of the needle deflection in the voltmeter

How does the brightness of the lightbulb powered by the generator change when the speed of the generator increases?

The brightness *increases*

Now let's repeat the previous experiment with the four loop coil. Starting from the far left of the screen, move the magnet to the right so it goes through the middle of the four loops coil at a constant speed and out to the right of the coil. How does the average brightness of the light bulb depend on the number of loops of the pickup coil?

The brightness *increases* when the number of loops increases

How does the brightness of the light bulb change if the magnet is moved through the coil more quickly?

The bulb shines *more* brightly

How does the frequency of induced voltage relate to how frequently one pole of a magnet is plunged in and out of a coil of wire?

The frequencies are the same

State Faraday's law.

The induced voltage in a coil is proportional to the number of loops, to the area of the loops, and to the rate of change of the magnetic field through the loops

What happens to the induced voltage if you increase the number of loops in the wire?

The induced voltage increases

Now, let's look at a situation with changing flux. Starting from the far left of the screen, move the magnet to the right so it goes through the middle of the two loops coil at a constant speed and out to the right of the coil. Roughly where is the magnet when the light bulb is the brightest? (The brightness of the light bulb correlates with how much the needle of the voltmeter gets deflected away from the middle .)

The light bulb is brightest when *either* end of the magnet is in the middle of the coil

What must change in order for electromagnetic induction to occur in a wire coil?

The magnetic field through the coil must change

Does the magnetic force on the longer sides of the generator loop help or hinder the rotation of the generator loop?

The magnetic forces on the loop hinder the rotation of the generator loop

What are the principal differences between an MHD generator and a conventional generator?

The only moving part in an MHD generator is the plasma

In a step-down transformer, how does the input current compare with the output current?

The output current is greater than the input current

Click on the button with two coils in it in the lower part of the window. The circuit should now have two sets of coils. Place the bar magnet inside the coil containing two loops. Try to find a location where the stationary magnet induces a current in the coil and causes the light bulb to shine. Which of the following is correct?

There is no induced current in the coil, so the light bulb does not shine, if the magnet is stationary (for any location of the magnet).

Faraday's law states that induced voltage in a loop of wire depends upon the rate at which

a magnetic field in the loop changes

The current produced by a common generator is

ac.

The high voltage that is important in a power line is high voltage

between the parallel wires of the line

When voltage is stepped up in a transformer, current is stepped down. But Ohm's law tells us that greater voltage means greater current, not less current. How then can current be lower when voltage is increased?

both of these (Ohm's law applies to the circuit connected to the secondary, not to the transformer, Current in the secondary is lower when compared to the greater current drawn by the primary)

The amount of current produced by electromagnetic induction depends on the

both of these (resistance of the circuit it is connected to, amount of induced voltage)

A principle difference between an MHD generator and a conventional generator is that the MHD generator

both of these (has no moving parts, operates more efficiently at high temperatures)

In order that electromagnetic induction occurs in a circuit there must be a

change in magnetic field intensity

What ultimately transfers from one location to another in electromagnetic induction is

energy

The output power of an ideal transformer is

equal to the input power

Maxwell's counterpart to Faraday's law emphasizes electric and magnetic

fields

If you drop a bar magnet in a vertical copper pipe it will fall slowly because

it induces a magnetic field in the pipe that resists motion of the magnet

Electromagnetic induction occurs in a coil when there is a change in

magnetic field intensity in the coil

A transformer produces

none of the above (none of these: power, energy, neither but transforms mechanical energy to electric energy)

A generator normally produces ac because the magnetic field experienced by interior coils

periodically alternates

The rate at which energy is transferred is

power

What transforms from one coil to another in a transformer is

power

A common feature common of all generators is that they

require an energy source

Magnetic field strength inside a current-carrying coil will be greater if the coil encloses a

rod of iron

Fred swings a sheet of copper in pendulum fashion between the poles of a strong magnet. The sheet slows in entering the magnetic field because

the magnetic field induces swirling currents in the copper and accompanying magnetic fields that resist motion

A primary difference between a motor and a generator is

the reversed roles of input and output

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

twice as great

When a magnet is moved into a wire coil, voltage is induced. If the coil had twice as many loops, the voltage induced would be

twice the original voltage

The same magnet is plunged into a coil that has twice the number of turns as before. The magent is shown before it enters the coil in the figure. (Figure 2) If the speed of the magnet is again v, the induced current in the coil is

unchanged

At what point in time is the current produced by the generator at its maximum?

when the plane of the loop is vertical, in the same plane as the two magnetic poles

Electromagnetic induction occurs in a loop of wire

when there is relative motion of the loop and wire or when current changes in the loop when near a magnet

When a magnet is quickly pushed into a coil of wire connected to a circuit

work is needed to overcome a resistance to the push