PHY 101 Lab 7: Electric power generation, induction, transformers

Other team members:

#### Goals:

Electric power generation is one of the most important forms of energy transformation in modern life. In this lab you will study various aspects of the physics behind this transformation, as well as some of the key technological devices involved in power generation and distribution such as generators and transformers.
For this Lab, you'll move from station to station after you complete the oscilloscope tutorial and use a variety of different sets of apparatus. Again, you will need to complete the oscilloscope tutorial (please see a separate sheet with a picture of an oscilloscope on it) BEFORE going to stations 1 thru 4.

Station 1: Flux change versus number of turns in coil
Materials:
Two coils with different numbers of turns
Magnet on rotatable mount
Oscilloscope

Changing the magnetic flux through a coil causes a voltage to be induced in it, by Faraday's Law. Since flux depends on the number of turns in the coil, so does the induced voltage. You'll see this graphically on an oscilloscope.
What is the definition of flux?

First, inspect the two coils, and find the numbers of turns that are marked on them. Write the numbers of turns here, and also give the ratio between the larger and smaller number of turns.

Hook up the coil with 23,000 turns to the oscilloscope, by plugging the two banana cables into the terminals of the coil. Don't use the connection labelled "M" (for middle of the coil), use the connections labelled "A" and "E". Hook up the coaxial end of the cable to the "Channel 1" input of the oscilloscope (indicated as 9 on the figure of o-scope tutorial). Ask your TA for help on this, if it sounds confusing.
Place the magnet on its rotatable mount just adjacent to one end of the coil, so it is as close as you can get it and still spin the magnet on its mount. Now spin the magnet. Look at the trace on the oscilloscope. What is the largest voltage that you see as the magnet spins? What is the smallest value?

Now, connect the second coil (with a smaller number of turns) using the second "double banana to coaxial" cable, hooking up the coaxial end to Channel 2 on the scope (indicated by 10 on the figure). Leave the first coil hooked up and in place. Put the second coil as close to the magnet as the first coil, but on the other side of the magnet. Now, spin the magnet on its mount again. Using the oscilloscope (set to display Channel 2 only (knob 21)), measure the largest and smallest voltages that you see as the magnet spins. (Again, ask your TA to show you where the appropriate switches are on the scope, if you can't find them.) What are the largest and smallest voltages during a spin?

Compare the voltages that you see in the two coils. Set the scope to display Ch 1 and Ch 2 at the same time, using the "Chop" setting on the Display knob 21.
What did you expect the voltage ratio to be? How close are your measurements to your expectation?

Station 2: Flux change due to permanent magnet and due to electromagnet
Materials:
Two large coils
Permanent magnet
Bar of iron and bar of carbon
Signal generator
Oscilloscope

One coil's terminals should be hooked up to the oscilloscope. Let's call it the secondary coil. Move the permanent magnet near the secondary, and look at the voltage induced across the coil by studying the trace on the oscilloscope. How large is the voltage you can make?

The other coil should be hooked up the the signal generator. Let's call this one the primary coil. Remove the magnet from the vicinity of the secondary, and move the primary up next to the secondary coil, so that they are placed end to end.
The voltage applied by the signal generator to the primary should be attached to Channel 2 of the oscilloscope. What is the amplitude of the voltage?

Now look at the amplitude of the voltage that is induced on the secondary. How big is it?

What did you expect?

How can you change the voltage induced on the secondary by the primary. Try moving the coils apart, changing their orientation, and inserting the iron and carbon rods. Describe what you find.

Can you make sense of what you see?

Station 3: AC generator
Materials:
Hand cranked generator with slip rings for AC
1 F capacitor
Power supply
Oscilloscope

Inspect the generator. Can you find the key parts that make it a generator (mechanical power input, magnet, rotatable coil, connections between coil and outside world, electrical power output)? Make as accurate a sketch as you can, labeling these key parts.

Are the brushes hooked up to the slip rings that make this an AC generator? If not, move them to the proper locations.

The power supply should be connected to the coils that produce the magnetic field of the generator's stationary magnet. Turn on the power supply if it is not turned on. Set the current to be about 1.0 A. Be sure to SLOWLY increase the power output of the power supply. In other words, slowly turn the knob to the right, otherwise you will blow a fuse.
The electrical power output of the generator should be hooked up to the oscilloscope.
Turn the crank of the generator. Make a sketch, accurately labeled, of what you see on the o-scope.

Are you generating an AC voltage? What is its amplitude? What is its frequency? Does the voltage have as large an excursion in the positive direction as the negative direction, or is there some mean value?

What control do you have over the character of the voltage? What happens as you change how fast you turn the crank?

Find the split-ring commutator that lets you generate DC instead of AC. Move the brushes so that the outside world is hooked up to the coil through the commutator.
Now, turn the crank, and observe the output of the generator on the o-scope. Make a sketch, accurately labeled, of what you see.

Are you generating a DC voltage? Is it strictly constant, or does it vary with time? Does it have a mean value different from 0? What is the amplitude and the frequency of any voltage variations that you see?

You can try to make an output that looks more like a constant voltage (more like true DC) by hooking a capacitor across (in parallel with) the output terminals. Use the alligator clips to connect the 1 F capacitor's leads to the two output terminals. Be sure to hook the lead marked with the black lines to the terminal that produces the more negative voltage. (Ask the TA to check your connections.)
What do you see as the output of the generator, once the capacitor is included? Draw a sketch, with good labels.

Can you explain what you see?

Station 4: Transformer
Materials:
Transformer, consisting of two coils (N = 10,000 and N = 1,000) linked by an iron yoke
Signal generator
Oscilloscope

Hook up the coil with 10,000 turns to the signal generator. Also hook up the voltage to the oscilloscope. Make a sketch, accurately labeled, of what you see.

Set the frequency of the signal generator's output to about 500 Hz. What is the amplitude of the voltage across the coil that you are driving with the signal generator?

Now, hook up the terminals of the second coil to the oscilloscope. Make a sketch, accurately labeled, of what you see.

What is the amplitude of the voltage across the second coil? What is the frequency?

What did you expect for the amplitude of the voltage on the second coil? How does what you saw compare to what you expected?

Now, change the roles of primary and secondary coil. Drive the N = 1,000 coil with the signal generator, and observe its voltage with the oscilloscope. What do you see?

What is the voltage induced on the N = 10,000 coil? (Use the oscilloscope.) What do you see?

What did you expect to see? How does what you see compare with your expectations?

What happens if you disassemble the yoke? You can remove one arm, or even remove the coils entirely. How much of a change can you make in the voltage on the secondary coil?

Thinking back on what you have seen, answer the following:

Regarding Station 3: Explain the operation of a generator, using Faraday's Law.

Regarding Activities 1, 2, and 4: If you want a lot of voltage induced in a coil from Faraday's Law, what things can you do to ensure that? List as many as you can.