Transcript Slide 1
Let’s look in detail at each of these four ways
of using motion to produce an emf.
Method 2…
Another Kind of Generator: A Slidewire Generator
Recall that one of the ways to induce an emf is to change the
area of the loop in the magnetic field. Let’s see how this
works.
v
B
A U-shaped conductor and a
moveable conducting rod are placed
in a magnetic field, as shown.
ℓ
The rod moves to the right with a
dA
constant speed v for a time dt.
vdt
x
The area through which flux passes
increases by dA.
The loop is perpendicular to the magnetic field, so the
magnetic flux through the loop is B = B dA = BA. Calculate
the induced emf using Faraday’s law:
dB
ε = -N
dt
d BA
ε = 1
dt
ε =
B dA
dt
ε = B
dx
dt
ε = B v.
B
x
v
ℓ
dA
vdt
B and v are vector magnitudes, so
they are always +. Wire length is
always +. You use Lenz’s law to
get the direction of the current.
Direction of current?
The induced emf causes current to flow in the loop.
Magnetic flux inside the loop
increases (more area).
System “wants” to make the flux
stay the same, so the current gives
rise to a field inside the loop into
the plane of the paper (to
counteract the “extra” flux).
B
I
x
v
vdt
Clockwise current!
What would happen if the bar were moved to the left?
ℓ
dA
As the bar moves through the magnetic field, it “feels” a force
FM = I B
A constant pulling force, equal in
magnitude and opposite in direction,
must be applied to keep the bar
moving with a constant velocity.
FP = FM = I B
v
B
I
x
F
M
FP
ℓ
Power and current.
If the loop has resistance R, the current is
ε B v
I = =
.
R
R
And the power is
P = FP v =I Bv
P = I IR = I2R
(as expected).
B
I
v
x
Mechanical energy (from the pulling force) has been
converted into electrical energy, and the electrical energy is
then dissipated by the resistance of the wire.
ℓ
You might find it useful to look at Dr. Waddill’s lecture on
Faraday’s Law, from several semesters back. Click here to view
the lecture.
If the above link doesn’t work, try copying and pasting this
into your browser address bar:
http://campus.mst.edu/physics/courses/24/Handouts/Lec_18.ppt