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CH 23-1
Another way of doing the Right-hand Rule
for Magnetic Field due to a loop
Face the loop. If current flows counterclockwise, then
the magnetic field along the axis is directed out of the
loop. If current flows clockwise, then the magnetic field
along the axis flows into the the loop.
Magnetic Flux
Flux means “flow.” Magnetic flux is the “flow” of magnetic
field lines through a closed path.
6 magnetic field lines pass
through the loop.
Maximum possible magnetic flux
Magnetic Flux
Flux means “flow.” Magnetic flux is the “flow” of magnetic
field lines through a closed path.
4 magnetic field lines pass
through the loop so there’s
less flux than when the loop
was perpendicular to the
magnetic field.
The angle  is measured between the “normal” and
the magnetic field.
Magnetic Flux
Flux means “flow.” Magnetic flux is the “flow” of magnetic
field lines through a closed path.
2 magnetic field lines pass
through the loop so there’s
less flux than before.
The angle  is measured between the “normal” and
the magnetic field.
Magnetic Flux
Flux means “flow.” Magnetic flux is the “flow” of magnetic
field lines through a closed path.
0 magnetic field lines pass
through the loop so the flux is
zero.
so the magnetic flux is zero.
Another way to change magnetic flux
Keep the orientation of the loop the same and change
the magnetic field through the loop. (This is visualized
by a change in the number of magnetic field lines that
“flow” through the loop.)
2 magnetic field lines, thus
there is less magnetic flux.
Another way to change magnetic flux
Keep the orientation of the loop the same and change
the magnetic field through the loop. (This is visualized
by a change in the number of magnetic field lines that
“flow” through the loop.)
4 magnetic field lines is more
magnetic flux.
Another way to change magnetic flux
Keep the orientation of the loop the same and change
the magnetic field through the loop. (This is visualized
by a change in the number of magnetic field lines that
“flow” through the loop.)
6 magnetic field lines is more
magnetic flux.
Pickup Coil
A pickup coil refers to a coil that is in a magnetic field
and is connected a resistor (or bulb). Ammeter can be
used to measure current through the coil or a voltmeter
can be used to measure the voltage across the resistor.
Faraday’s Law
electromagnetic induction
A changing magnetic flux through a coil will create an
emf across the coil which creates a current through the
coil. The emf is proportional to the rate of change of the
magnetic flux.
There are 3 ways to change the magnetic flux through the coil:
1. Change the strength of the magnetic field at the plane of the coil.
2. Change the angle (orientation) of the coil, with respect to the
magnetic field.
3. Change the area of the coil.
Generate an emf by changing the
orientation of the coil
Besides changing the
orientation of the coil, one can
instead leave the coil at rest and
change the orientation of the
magnetic field.
B
a
b
c
A circular loop of wire is again shown in three different
positions as it rotates at constant angular speed in a uniform
magnetic field. F is the magnetic flux (pictured as the number
of magnetic field lines) crossing the loop. The rate DF/Dt at
which F is changing is greatest in magnitude when the loop
passes ____________ and least when it passes ___________ .
A. position a … position b
B. position b … position a
C. position c … position a or b
B
a
b
c
A circular loop of wire is again shown in three different
positions as it rotates at constant angular speed in a
uniform magnetic field. The instantaneous EMF induced in
the loop is greatest in magnitude when the loop is passing
____________ and least when it is passing _____________ .
A. position a … position b
B. position b … position a
C. position c … position a or b
Generate an emf by changing the
strength of the magnetic field
1. Move a magnet back and forth through the coil and
the magnetic field at the plane of the loop “alternates”
causing an alternating emf and alternating current.
2
N
0
-
.
+
S
1
N
S
Situation a: The hand is stationary in position 1.
Situation b: The hand is moving at moderate
speed from position 1 to position 2.
Situation c: The hand is moving quickly from
position 1 to position 2.
Situation d: The hand is stationary in position 2.
Which of the following correctly compares how far the galvanometer
needle deflects during each of these situations?
A. Most in d, least in a, same in-between value in b and c
B. Most in d, less in c, still less in b, least in a
C. Most in c, less in b, a and d tied for least
D. Most in c, less in b, still less in d, least in a
Generate an emf by changing the
strength of the magnetic field
2. OR use an electromagnet with
an alternating current in the
electromagnet. This creates an
alternating magnetic field in the
pickup coil which induces an
alternating emf across the pickup
coil.
By means of a sliding contact, we can change the amount
of resistance that is included in the right hand circuit.
1
+
0
-
2
Situation a: The contact is
stationary in position 1.
Situation b: The contact is
slid at moderate speed from
position 1 to position 2.
Situation c: The contact is slid quickly from
+
position 1 to position 2.
Situation d: The contact is stationary in position 2.
2. Which of the following correctly compares how far the
galvanometer needle deflects during each of these situations?
.
A. Most in d, least in a, same in-between value in b and c.
B. Most in d, less in c, still less in b, least in a.
C. Most in c, less in b, a and d tied for least.
D. Most in c, less in b, still less in d, least in a.