MIRAMAR COLLEGE
PHYSICS 126 LAB REPORT
Name_________________________
Date __________Time___________
Partners ______________________
Electromagnetic Induction and Lenz Law
PRE-LAB QUESTIONS
1. What is a solenoid?
2. What is a galvanometer?
3. Draw the magnetic field associated with a solenoid with the current direction shown (use your text for reference, if
needed):
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Lab Objective: Investigation of electromagnetic induction using solenoids and magnets and circuits with transformers and
inductors.
Part 1: Current travel direction in the solenoid and compass needle deflection.
Equipment: solenoid, compass, power supply, alligator leads.
A solenoid is a long coil of wire made up of many turns. When a current flows through a solenoid, the
resulting magnetic field that is very similar to that of a bar magnet, with a north and a south pole. The
direction of the magnetic field at either end of the solenoid can be determined with the Right-Hand Rule
for the magnetic field. This determination can be checked using by using the north arrow of a compass
needle.
If the current is clockwise (looking into one end of the solenoid), which way should the magnetic field (i.e. compass needle)
point, into or out of the solenoid? ______________________________
If the current is counterclockwise, which way should the magnetic field (i.e. compass needle) point, into or out of the
solenoid?______________________________________
Get acquainted with your solenoid.
Note there are two terminals, one where the connected wire leaves the terminal and goes under the unit (“the bottom
terminal”) and one where the wire goes around the top (“the top terminal”). Using a compass, investigate the area around
the solenoid. Is there any magnetic field associated with the solenoid without current flowing? Why or why
not?__________________________
__________________________________________________________________________________________________
________________________________________________________________________
Set up a circuit with the power supply and the solenoid.
1. Orient the solenoid so the top terminal is closest to you, looking into one end of the solenoid.
2. Use a current setting of 1.0 amp. Use a red lead for the positive side of the power supply and black for the negative
side (the solenoid doesn’t have a positive/negative side). With the small amount of current, the solenoid will not be
dangerous to touch when connected to the power supply.
3. Connect the solenoid to the power supply in such a way that the magnetic field points out of the end of the solenoid
pointing towards you.
4. Turn on power supply and verify magnetic field direction with the compass.
5. Which solenoid terminal (top or bottom) must be connected to the positive terminal of the power supply for the compass
needle to point out of the solenoid?____________________.
6. Which way does the current flow, cw or ccw?___________________________
7. Draw a diagram of your setup below, showing top terminal, bottom terminal on solenoid, how they are connected to the
power supply, the direction of the current and the direction the magnetic field lines from the solenoid are pointing. Orient
the drawing sideways, like the one shown at the top of the page.
8. Repeat the setup so the compass needle points into the solenoid.
9. Which solenoid terminal (top or bottom) must be connected to the positive terminal of the power supply for the compass
needle to point into the solenoid?____________________.
10. Which way does the current flow?___________________________
11. Draw a diagram of your setup, showing top terminal, bottom terminal on solenoid, how they are connected to the power
supply, the direction of the current and the direction the magnetic field lines from the solenoid are pointing. Orient the
drawing sideways, as before.
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Part 2: Current travel direction in the solenoid and galvanometer deflection.
Equipment: galvanometer, solenoid, connecting wires, bar magnet
Note: For the next part of the experiment, disconnect the power supply; it will no longer be used.
A galvanometer is a device that tests for the presence of current. The deflection of the pointer also shows current direction.
If current enters at the positive (right-hand) terminal of the galvanometer, the needle will have a positive deflection (i.e to the
right). If current enters at the negative (left-hand) terminal of the galvanometer, the needle will have a negative deflection (to
the left). Connect the top solenoid terminal to the positive terminal of the galvanometer.
With your solenoid connections, a ccw current will make the galvanometer deflect pos/neg ___________
A cw current will make the galvanometer deflect pos/neg______________________.
Bring the bar magnet near the end of the solenoid with the top terminal. Fill in the following chart with your results.
LEADING
POLE
B MAGNET
POINTS
PUSH IN
PULL OUT
STATIONARY
IN/OUT OF
SOLENOID
N
N
N
S
S
S
Φ FROM
MAGNET
INCREASE
/DECREASE
THROUGH
COIL*
GALVAN.
DEFLECTION
INDUCED
CURRENT
DIRECTION
POS/NEG
B
INDUCED
BY
CURRENT
CW/CCW
IN/OUT
STATIONARY
PUSH IN
PULL OUT
STATIONARY
PUSH IN
PULL OUT
*flux from magnet may increase even though magnetic field points away from solenoid
There was no current induced when the magnet was held stationary. Why not?
If the magnetic flux through the coil’s area, caused by the bar magnet, increased, was the induced B field in the same
direction, or in the opposite direction?
If the magnetic flux through the coil’s area, caused by the bar magnet, decreased, was the induced B field in the same
direction, or in the opposite direction?
Explain how your results agree with (or do not agree with) Lenz Law:
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Part 3 – Circuits with transformers and inductors
A transformer is a circuit element that makes use of Faraday’s Law. If you wrap two wires from different circuits around
different ends of an iron bar, a changing magnetic flux from a change in current in the first circuit will induce a potential
difference and a current in the wire from the second circuit! This only occurs when the current is changing (e.g.
turning on or turning off) . Once the current stabilizes, curren t will cease in the second coil. If the second
coil has twice as many turns (more inductance) as the first coil then the second coil will have twice the voltage but half
the current as the first coil. The inductance is expressed in henrys (H, named after the Am eric an phys ic is t Joseph
Henry), or more commonly in millihenrys (mH, thousandths of a henry). A device like this is called a transformer. The
transformer used in the circuits consists of a 400 mH coil (called the primary) and a 2mH coil (called the secondary)
wrapped around an iron bar. The primary coil has a middle tap point allowing use of half the coil. The symbol for a
transformer is shown on the right:
TRANSFORMER
Symbol for TRANSFORMER
P
THE MAGNETIC BRIDGE EXPERIMENT
Parts Needed
breadboard
switch
a 9V battery
two LEDs
one transformer
short and long wires
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S
Connect the circuit. Note the orientation of the transformer; the primary side (p) has 3 pins and the secondary side (s)
has 2 pins. The negative (flat) sides of the LEDs face inward, towards each other. If the circuit is connected correctly, the
LED-left blinks when the switch is pressed and LED-right blinks when the switch is released.
Q1: Which way is the current in the battery circuit when the switch is closed, clockwise or counterclockwise (circle correct
answer)?
Q2: Which way is the current in the LED circuit when the switch is closed? How can you explain this in terms of Lenz
Law?
Q3: If you keep the switch closed, the left LED goes out. Explain why.
Q4: Is there a current in the LED circuit circuit when the switch is opened?, If so, in which direction? Explain this in terms
of Lenz Law:
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