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Home Lab Week 13 – Lenz-Faraday                                                                     1
Name________________________________________ Date_________________________

Home Lab 13
Overview
Michael Faraday was the first to appreciate the intimate connection between electricity
and magnetism. In 1831 Faraday conducted an experiment which showed that a changing
magnetic field produced an electric current. More importantly, the reverse was true: a changing
electric current produced a magnetic field. James Maxwell was able to strengthen Faraday’s
experimental ideas with a coherent theory of electromagnetism.
Faraday’s experimental setup was two coils wound around a piece of
metal. The first coil is connected to an electrical battery while the second
coil is connected to a current detector called a galvanometer. Faraday
expected from his earlier experiments with electromagnets that the battery
would produce a current in the galvanometer. As the diagram (from
http://hyperphysics.phy-astr.gsu.edu) to the left shows, the steady current
produces no induced voltage or current in the galvanometer.
Instead he found that when he connected the battery to the first coil,
there is a momentary current. When he disconnected the battery, there was
another momentary current which causes the needle to deflect in the opposite
direction. Faraday found that the faster the disconnection or connection, the
bigger the current in the galvanometer. More importantly, the direction of
the current always reversed. If connecting the battery deflected the needle to
the right, then disconnecting the battery deflected the needle to the left.
Maxwell was able to produce a consistent theory of electromagnetism
which formalized these experimental observations.
In light of this, Faraday’s experiment can be explained stepwise:
1.      As the battery connects to the first coil, an electrical current
begins to flow in the coil.
2.      This current is changing (increasing) which produces a
changing magnetic field in the coil and metal bar.
3.      The magnetic field preferentially travels through the metal bar
as opposed to the air surrounding the apparatus.
4. The changing magnetic field in the bar produces a changing electric field in the
second coil.
5. The changing electric field produces a voltage and current in the second coil.
6. The current causes the galvanometer to deflect.
When the battery is disconnected, the process is the same but with a change in direction due to
the change in the first electric field from increasing to decreasing.

University of Virginia Physics Department
Home Lab Week 13 – Lenz-Faraday                                                                       2
Name________________________________________ Date_________________________

The changing magnetic field induces a voltage in the second coil. This is summarized by
a form of the Lorentz equation for induced voltage:

Where ΔV is the induced voltage, N is the number of coils, ΔB is the change in magnetic
field, ΔA is the change in the area of the coil, θ is the angle between the coil area and the
magnetic field, and Δt is the time interval of the change. One important feature of this equation
is the minus sign on the right side. This is given its own name: Lenz’s Law.
Lenz’s Law can be stated more simply as the induced voltage will always give rise to a
magnetic field that opposes the change that produced it. The induced field will act like friction.
No matter what direction, friction always opposes the motion of an object. This phenomenon is
the basis of magnetic braking.
An excellent JAVA applet that illustrates the effect of moving a magnet near a coil

Activity 13-1: Faraday’s experiment with a simple compass
galvanometer.
Objective: Investigate the effect of changing the current in a coil on the current in a second coil

Materials:
• Iron rod 10 cm long (in the class kit)
• Magnet wire 3.0 m (26 gauge enamel-covered copper wire) (in the class kit)
• Small compass (in the class kit)
• 9-Volt battery
• Steel wool (or a blade to scrape the ends of the magnet wire)

You should take a picture of your materials and add the image to your completed activity report.

University of Virginia Physics Department
Home Lab Week 13 – Lenz-Faraday                                                                   3
Name________________________________________ Date_________________________
1. Cut an approximately 1.0 m long piece of magnet wire. Wrap one wire around one end
of the iron rod leaving approximately 10 cm of wire free at each end. Secure the coil
using transparent tape. Use steel wool or a knife blade to scrape insulation away from
both free ends of the coil.
2. Cut an approximately 2.0 m
long piece of magnet wire. Wrap
approximately 1.0 m of wire around
one end of the iron rod leaving
approximately 10 cm of wire free at
one end. Secure the coil using
transparent tape. Continue with the
other end to wrap the compass with
magnet wire so that the wire runs from
30° to 210° (just as in the experiment
11-1) Use steel wool or a knife blade
to scrape insulation away from the free
end of the coil. Twist the two ends of
this coil together to form an electrical
circuit. Secure with tape.
answer questions for each of the
following: Connect one end of the first coil to the (-) side of the 9-Volt battery. Lightly
touch remaining free end of the first coil to the (+) side of the 9-Volt battery. What
happens immediately to the compass needle when the circuit is closed? Provide a brief
explanation for the movement or lack of movement.

4. While the free end is connected to (+) side of the 9-Volt battery, continue watching the
needle. What happens to the compass needle when the circuit is closed for a while?
Provide a brief explanation for the movement or lack of movement.

5. Disconnect (+) side of the 9-Volt battery, continue watching the needle. What happens to
the compass needle when the circuit is opened? Provide a brief explanation for the
movement or lack of movement.

University of Virginia Physics Department
Home Lab Week 13 – Lenz-Faraday                                                                       4
Name________________________________________ Date_________________________

Activity 13-2: Faraday’s experiment with a transformer and LEDs.
Objective: Investigate the effect of changing the current in a coil on the current in a second coil
as demonstrated by light-emitting diodes.

Materials:
• Transformer (in the class kit)
• Volt meter (in the class kit)
• 2x red light-emitting diodes (LED) (in the class kit)
• Battery snap for 9-Volt battery (in the class kit)
• Circuit board (in the class kit)
• 9-Volt battery

You should take a picture of your materials and add the image to your completed activity report.
1. Place the transformer so that the two leads of one side
are in separate columns on one side of the divide of the
separate columns on the other side of the breadboard.
2. Place one LED so that one leg of the LED is in a tie
point of the same column as one outside leg of the
transformer. Place the other leg of the same LED so that it is
in a tie point of the same column as the other outside leg of
the transformer.
3. Place the second LED across the two tie points from
step two, but with the polarity reversed (i.e. if the long leg of
the first LED is in one column, then the short leg of the
second LED should be in the same column. Likewise if the
short leg of the first LED is in one column, then the long leg
of the second LED should be in the same column.). The
LEDs will naturally lean to the shorter side. The result should be that there is one LED
leaning to the left and the other LED is leaning to the right.
4. Connect the black lead of the battery snap
to one leg of transformer on the side away from the
LEDs.
5. Touch the red lead of the battery snap to
the second leg of the transformer on the side away
from the LEDs. What happens immediately to the
LEDs when the red lead closes the circuit?
Provide a brief explanation why one LED lights
and the other does not.

University of Virginia Physics Department
Home Lab Week 13 – Lenz-Faraday                                                                   5
Name________________________________________ Date_________________________

6. What happens to the LEDs when the circuit has been closed for a while? Provide a brief
explanation why one LED lights and the other does not.

7. Using the voltmeter, measure the voltage across the side of the transformer nearest the
battery. Measure the voltage on the side of the transformer nearest the LEDs. What is
the voltage on the battery side? What is the voltage on the LED side? Provide a brief
explanation why the voltage on the LED side is zero.

8. Disconnect the red lead of the battery snap from the second leg of the transformer on the
side away from the LEDs. What happens immediately to the LEDs when the red lead
opens the circuit? Provide a brief explanation why one LED lights and the other does
not.

9. Given that the minimum voltage to sensibly light the red LED (Mouser TLHK5100) is
approximately 1.4 V, what is the minimum voltage that the transformer is providing for
the LED?

University of Virginia Physics Department
Home Lab Week 13 – Lenz-Faraday                                                                     6
Name________________________________________ Date_________________________

10. This demonstration is clearer if the red lead is simply tapped against the transformer leg.
One LED will light as the circuit is closed and the other will light as the circuit is opened.

Activity 13-3: Magnetic braking
Objective: Investigate the effect of a non-ferrous metal in the presence of a moving magnet

Materials:
• 2x small neodymium magnets (in the class kit)
• Aluminum plate or heavy duty aluminum foil
• Scissors
• Pendulum support (such as a ring stand with cross piece)

You should take a picture of your materials and add the image to your completed activity report.
1. Tie the thread to an appropriate support so that the pendulum can swing freely. The path
of the pendulum must be free of the influence of nearby ferromagnetic materials. One
suitable procedure is to tie the thread so that the free end of thread touches the desktop or
tabletop.
2. Attach the magnets by placing one on
either side of the thread and allowing the
magnets to snap together trapping the thread
between them. Adjust the magnets so the
magnets are just above the aluminum plate
(bit do not touch the aluminum plate). Using
scissors cut the thread below the magnet so
3. Remove the aluminum plate. Pull
the magnet over a fixed distance and release
the magnet. Count the number of swings
until the magnet/pendulum bob stops. Describe the forces acting on the
magnet/pendulum bob while it is in motion. Include the direction of the forces in your
description.

University of Virginia Physics Department
Home Lab Week 13 – Lenz-Faraday                                                              7
Name________________________________________ Date_________________________
4. Replace the aluminum plate so that it is directly under the lowest point of the pendulum
swing. Pull the magnet over the same fixed distance and release the magnet. Count the
number of swings until the magnet/pendulum bob stops. Describe the forces acting on the
magnet/pendulum bob while it is in motion. Include the direction of the forces in your
description. Provide an explanation for why the magnet/pendulum bob stops sooner.

5. With the magnet/pendulum bob stopped at its lowest point, pull the aluminum plate along
the tabletop from underneath the suspended magnet/pendulum bob. Describe the motion
of the pendulum bob as the aluminum plate is pulled away. Provide an explanation for
why the magnet/pendulum bob follows the plate.

University of Virginia Physics Department

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