HPP Activity 75v1
Faraday’s Law
Exploration
Obtain
50 or 100 turn wire coil
bar magnet
galvanometer
Connect the coil to the galvanometer so that a clockwise current will produce a leftward
deflection of the galvanometer needle.
You need to be aware of one characteristic of the galvanometer: it reacts slowly. The needle will
deflect away from zero when there is a current, but then it can take several seconds to return to
zero after the current has decreased to zero. Therefore, while investigating the currents produced
by the magnet, if the needle is moving toward zero, most likely the current has dropped to zero.
GE 1.
Lay the coil flat on the table. Take a bar magnet and move it over the coil.
1. What happens?
2. Is any correlation with what you have to do with the bar magnet and getting
a current reading on the galvanometer?
Activity Guide
 2010 The Humanized Physics Project
Supported in part by NSF-CCLI Program under grants DUE #00-88712 and DUE #00-88780
HPP Activity 75v1
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Now hold the bar magnet over the coil with the N pole towards the coil, as
shown below.
S
Magnet
N
Coil
3. Pull the magnet vertically upward at a constant speed. Observe the direction
of current and indicate it in the “During” figure below. Indicate direction and
strength of the magnetic field in the interior of the coil. (Double click on
figure.)
before
during
after
4. Now hold the bar magnet far from the coil with the N pole pointing towards
the coil. Bring the magnet towards the coil at a constant speed. Indicate the
direction of current of the strength of the field in the interior of the coil.
Activity Guide
 2010 The Humanized Physics Project
HPP Activity 75v1
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before
during
after
5. Obtain a 50 turn coil and a 100 turn coil with the same radius. Pull the
magnet away from each at the same speed. How does the amount of current
produced compare?
Invention
Changing the magnetic field going through a closed circuit induces a current. To state a
quantitative law about this situation requires that we define magnetic flux. Suppose a magnetic
field of strength B is oriented at an angle  measured with respect to the normal to a small
rectangle of surface area A. The magnetic flux Bgoing through the area is defined as
 B  B A  BA cos  
Faraday’s law says that changing the magnetic flux going through a closed circuit will produce
an induced emf in the circuit. The amount is given by
E=
 B
t
If the circuit has N turns instead of one the induced emf is
E=N
 B
t
The circuit with an induced emf acts as if a battery has been placed in the circuit: a current is
induced. The direction of the induced current is given by Lenz’s Law.
The direction of the induced current in a circuit that experiences a magnetically induced emf is
such as to oppose the change that is producing it.
Application
Activity Guide
 2010 The Humanized Physics Project
HPP Activity 75v1
4
Obtain
a coil
multimeter
galvanometer
bar magnet
GE 2.
1. Measure the resistance of the coil with the multimeter.
2. Lay the coil on the table and connect it to the galvanometer, as in GE 1.
Place the bar magnet directly over the coil. Pull the magnet about one meter
away while timing how long you take to move it and noting the maximum
current. Record the values here.
3. Use Faraday’s law to make a rough estimate of the field strength at the end
of the bar magnet.
GE 3.
Conside the following scenarios
1. As the resistance is increased in the left hand loop of the figure below, what
is the direction of the induced current in the right hand loop? Explain.
2. A current I in the long straight wire of the figure below decreases with time.
Activity Guide
 2010 The Humanized Physics Project
HPP Activity 75v1
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I
a. Determine the direction of the current induced in the loop next to the wire.
Explain.
b. If the loop were doubled in length, how would the induced EMF change?
Explain.
3. A copper bar slides to the right at a constant speed along copper rails in the
shape of an angle. A uniform magnetic field is directed into the page.
a. What is the direction of the induced current in the triangle made by the three
conductors? Explain.
b. Is the EMF induced in the triangle increasing, decreasing or constant?
Explain.
Activity Guide
 2010 The Humanized Physics Project