Experiment 6: Magnetism and Coils
(Note: The "write up" for this week's lab has been integrated into the answer sheet. You don't need
to attach a separate one.)
Part I: Solenoid.
Connect the coil and a DC ammeter to the
DC terminals of your power supply. Set
the power supply so that the current is
around 4 A. Turn it off as soon as you have
it set; if you leave that much current in it
for more than 15 or 20 seconds, the coil
might start to get hot.
A. To demonstrate the principle of a
solenoid, put one end of the iron bar into the hole in the coil, as shown above. If necessary, tilt the
coil a little so that the bar is level. What happens when you turn the coil on? Again, don't leave it
on more than 15 or 20 seconds. (The word "solenoid" usually refers to a coil which is longer,
compared to its diameter, than this one is; but this shows you the basic idea of how turning on a coil
can be used to throw a switch or valve.)
B. Without changing the knob on the power supply, move the wires from the DC terminals to the
AC terminals. Leave out the meter. Does it work on AC too?
Optional: If you hold a permanent magnet nearby, you can feel the AC magnetic field push and pull
the magnet at 60 cycles per second. This is something like how a loudspeaker works.
Part II: Motor
You are given a little machine, consisting of a coil between the
poles of a horseshoe magnet, to use as a motor. Connect the
circuit shown to the DC terminals of the gray power supply.
Use meters with a needle; digital meters might not be steady
enough to read. On the ammeter, use a scale suitable for an
amp or so; on the voltmeter, a scale suitable for a few volts. Before switching on the power supply,
set its knob at the second mark on the left.
KEEP AN EYE ON THE METERS; BE SURE NEITHER GOES OFF SCALE. Turn on the
power and adjust the knob so that the motor runs at a moderate speed. You'll probably have to push
it to get it started. Record the meter readings, and answer the questions on the answer sheet.
The motor amounts to an electromagnet on a pivot. To keep it
turning in the same direction, its magnetic poles have to be
reversed every half turn, as shown. Describe the switching
mechanism used in the motor to do this. Don’t be afraid to include
a sketch; it doesn’t have to be all in words.
Part III: Induction
A. Coil and Magnet.
1. Sign out a neodymium magnet. Caution: Keep the magnet wrapped in tape and
cardboard, as you found it. Without this padding, they have pinched people's fingers, and
also gotten chipped.
2. Connect a galvanometer (a sensitive ammeter) to a coil of wire. With the magnet moving
along the axis of the coil, what difference do you observe (if any) between moving the
magnet toward the coil and moving it away, keeping all else the same? What is the
difference between approaching the coil with opposite poles of the magnet? What happens
when you approach the coil with the same pole at different speeds? Answer the questions
about this last result.
B. Transformer.
1. DC: Instead of using a magnet, use
another coil as an electromagnet to
produce the magnetic field. Include an
ammeter in the circuit with the
electromagnet and keep the current in it
around one amp (Don't go much higher).
The two coils should be placed against each other with a piece of iron through their centers.
Investigate these questions: Do transformers work on steady DC? Do transformers work on
DC if you chop it (turn it off and on)?
2. AC: Replace the DC meters with an
AC ammeter and voltmeter as shown.
Switch the primary coil from the DC to
the AC terminals of your power supply.
Do transformers work on AC?
C. Generator.
Connect the device from part two (a coil between the poles of a
horseshoe magnet) to the galvanometer, then give the motor a good
spin with your finger. Try not to send the meter off scale. Unlike
parts A and B, the strength of the field at the coil's position is not
changing. In view of that, how do you explain what you saw?
PHY 122
Report on Experiment 6: Magnetism and Coils
Name _____________________________________
Objective: To perform various demonstrations involving magnetism and induction.
Procedure & Results:
Part 1: A coil is connected to a DC power supply. The end of an iron bar is placed in the coil.
When a large enough current is turned on in the coil, (Record your observations.)
When this is repeated using AC,
Part 2: A simple DC motor is connected to a power supply and some meters. While running,
V = ________________ and I = ________________.
The power taken in by the motor is
The energy it uses in one minute is
To keep the motor turning, the spinning coil’s magnetic poles must be reversed every half turn.
This is done by (Continue on back if necessary.)
Part 3: A) A coil is connected to a galvanometer. The effect of moving a magnet toward the coil
vs. away is
The effect of approaching the coil with opposite poles is
The effect of moving the magnet quickly vs. very slowly vs. at rest at the same distance is
(In each blank put "FLUX", "FIELD STRENGTH", or "RATE THE FLUX CHANGES":) This
last result shows that the induced emf depends on the
___________________________, not ___________________________or
___________________________ because ___________________________ and
___________________________depend only on how far away the magnet is, but
___________________________depends on how fast it's moving.
B) A transformer made from two identical coils linked by an iron bar is connected to a DC power
supply. A galvanometer is connected to the secondary coil to detect any induced current. With
steady DC in the primary, the galvanometer shows
The power supply is turned off and on, simulating how current from a car battery is turned off and
on in the ignition coil's circuit. I observe
The transformer is now connected to an AC power supply and AC meters. This time, I observe
C) The device used as a motor in part two is now connected to a galvanometer instead of a power
supply. When the coil is spun, (in a constant magnetic field), the meter __________ (does/ does
not) show an induced current.
Faraday's law says there has to be a changing flux for induction to happen. But, the coil stays the
same distance from a permanent magnet, so B does not change. The explanation for what I saw is
that