Other team members:
Station 1: Flux change versus number of turns in coil
Two coils with different numbers of turns
Magnet on rotatable mount
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?
What is Faraday's Law?
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).
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
Two large coils
Bar of iron and bar of carbon
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
Hand cranked generator with slip rings for AC
1 F capacitor
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 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
Transformer, consisting of two coils (N = 10,000 and N = 1,000) linked by an iron yoke
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
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.