Download Can a Magnetic Field Produce a Current?

Sierzega: Electromagnetic Induction 1
Can a Magnetic Field Produce a Current?
In our study of magnetism we learned that an electric current through a wire, or moving electrically
charged objects, produces a magnetic field. Could the reverse happen? Could a magnetic field produce a
current?
1.1 Observe and find a pattern The table describes five experiments involving a
galvanometer, a bar magnet, and a coil.
https://www.youtube.com/watch?v=qyHd4tPnC6Q&feature
Observational Experiment
a.
You hold a magnet
motionless in front of a
coil.
b.
You move the magnet
toward the coil or move
the coil toward the
magnet.
c.
You move the magnet
away from the coil or
move the coil away from
the magnet.
d.
You turn the magnet 90°
so that the poles are now
perpendicular to their
previous position.
e.
You collapse the sides of
the coil together so it’s
opening becomes very
small. You pull open the
sides of the collapsed
coil so the area becomes
large again.
f.
Patterns
Analysis
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1.2 Predict and test The following experiment uses two coils. Coil 1 on the bottom is
connected to a battery and has a switch to turn the current through coil 1 on and
off. When the switch is open, there is no current in coil 1. When the switch is
closed, the current in coil 1 produces a magnetic field whose B-field lines pass
through coil 2’s area. For each of the experiments we will use our explanation to
predict whether or not there should be an induced electric current in coil 2.
https://www.youtube.com/watch?feature=youtu.be&v=iKnB7oiRMTA
Testing Experiment
Experiment 1: The switch in
the circuit for coil 1 is open.
There is no current in coil 1.
Is there any current in coil 2?
Experiment 2: You close
the switch in the circuit for
coil 1. While the switch is
being closed, the current in
coil 1 increases rapidly from
zero to a steady final value.
Is there any current in coil 2
while the switch is being
closed?
Experiment 3: You keep the
switch in the circuit for coil
1 closed. The current in coil
1 has a steady value, Is there
current in coil 2?
Experiment 4: You open the
switch again. Is there any
current in coil 2 while the
switch is being opened?
Conclusion
Will a current be induced in coil 2? Based
on our explanation: Induced current is due
to magnetic force exerted on moving
charged particles.
Outcome
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1.3 Observe and find a pattern The table that follows describes five new experiments using a
galvanometer, a bar magnet, and a coil. The outcomes of the experiments are included.
Experiment
1. Position a magnet
perpendicular to the coil
and move it slowly toward
the coil. Repeat the
experiment, moving the
magnet quickly.
2. Position a small magnet
perpendicular to the coil
and move it slowly toward
the coil. Repeat the
experiment using a bigger
magnet.
Illustration
Outcome
The quicker the magnet’s
motion, the stronger the
induced current.
The bigger magnet induces a
bigger current than the small
magnet when they move at the
same speed with respect to the
coil.
3. Move a magnet
perpendicular to the coil.
Then move it so that it
makes an angle with the
plane of the coil. Keep the
speed the same.
When the magnet moves
perpendicular to the coil, the
biggest current is induced.
4. Make a small coil and a
large coil. Move the
magnet toward each.
A stronger current is induced in
the larger coil.
5. Make two coils of the same
area, one with two turns
and one with ten turns.
Move the magnet toward
each.
A stronger current is induced in
the coil with more turns.
Devise in words a rule that
relates the magnitude of the
induced current to various
properties of the magnet, its
motion, and properties of the
coil.
Magnetic Flux (: a physical quantity for the number of B-field lines through a coil’s area.
 How does Magnetic Flux (  depend on B-field? On area?
 How do we include the dependence of the orientation of the loop relative to the B-field lines?
Sierzega: Electromagnetic Induction 1
Did you know?
Magnetic Flux (The magnetic flux through a region of area A is
  = AB cos 
Where B is the magnitude of the uniform magnetic field throughout the area and is the angle between
the direction of the B field and a normal vector perpendicular to the area. The SI unit of magnetic flux is
the unit of the magnetic field (T) times the unit of area (m2), or T*m2. This unit is also known as the
weber (Wb).
Direction of the Induced Current
1.4 Observe and find a pattern The table that follows repeats three earlier experiments that used a
galvanometer, a bar magnet, and a coil and in which a current was induced. The direction of the
induced current is shown in the illustrations.
Draw B field vectors caused by the moving magnet. Indicate
whether the field vectors through the coil are decreasing or
Experiment
increasing. Draw Bind field vectors due to the induced current.


The coil area is
collapsing.
The coil expands
(a) Use the data in the table above to devise a rule relating the direction of the induced current in the coil
and the change of external magnetic flux through it. Fill in the table on the following page.
Hint: (1) Draw the B field vectors of the moving magnet and make a note of whether the flux due to
the magnet is increasing or decreasing though the coil. (2) Then draw B ind vectors as a result of the
induced electric current. Compare the direction of B ind vectors to the B field vectors of the moving
magnet (3)
 when the flux through the coil increases and (4) when the flux decreases.



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Direction of B-field
(
or
)
Flux
(Increasing or
Decreasing)
Induced Current
(Clockwise or
counterclockwise)
Direction of Resulting Current’s B-field
(
or
)
(b) How does the direction of the induced current in a coil relate to the change of external magnetic flux
through it?
Did you know?
Lenz’s law The direction of the induced current in a coil is such that its B-field opposes the change in
the magnetic flux through the coil’s area produced by other objects. If the magnetic flux through the coil
is increasing, the direction of the induced current’s B-field leads to a decrease in the flux. If the magnetic
flux through the coil is decreasing, the direction of the induced current’s B-field leads to an increase in
the flux.
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1.5 Reason For each situation shown in the table that follows, use the rules devised and tested in the
previous handout to predict if a current is induced through the resistor attached to the loop. If a
current is induced, indicate the direction of that induced current.
Experiment
(a) The loop is perpendicular to the page.
(b) The loop is perpendicular to page and
the magnet turns 90o.
(c) The loop and magnet are in the plane of
the page.
(d)The loop, perpendicular to page, is
pulled upward so that it collapses.
(e) The switch in the left circuit is closed
and the current increases abruptly.
Predict if a current
is induced; explain
your prediction.
If you predict that a
current is induced,
what is the direction
of the current?
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(f) A steady current flows in the left circuit.
(g) The circuit on left is rotated 90o about
the dotted line.
(h) The switch in the left circuit is opened
and the current decreases abruptly.