Download Intro to Electromagnetic Induction

Electromagnetic Induction
Chapter 20
If a current can create a magnetic field,
can a magnetic field create a current?
• YES, but not all the time.
How do we know when a current will
be induced?
• Induced current in a loop of wire occurs only
when the magnetic field through that loop
CHANGES!
• Called electromagnetic induction.
• Instead of talking about the current, we often
talk about the emf (electromotive force,
voltage)
Demonstration
• As the magnet gets closer to the loop of wire,
the magnetic field lines through the loop
increase and current is induced in the wire
The current alternates direction, so as
you move the magnet back and forth,
you produce alternating current!
What does the induced current
depend on?
• Number of turns in the loop
• Relative velocity between the magnet & loop
• Magnetic field strength
Can we solve the ENERGY CRISIS?
• How might we be able to generate electrical
power using a magnet and a loop of wire?
• Swinging loop demo
Lenz’s Law
• An induced emf in a wire loop or coil has a
direction such that the current it creates
produces its own magnetic field that opposes
the change in the magnetic flux through that
loop or coil.
An example:
Which way is current
induced in the loop?
Why can’t we solve the energy crisis
with a pendulum?
What are some different ways of
inducing current?
• Bring a magnet closer and farther from a
stationary loop of wire.
• Bring a loop of wire closer and farther from a
stationary magnet.
• Change the magnetic field strength.
• Change the shape of the loop of wire.
• Rotate the wire.
Is there a current in the loop?
X X X X X X X X X X X X X X X
X X X X X X X X X X X X X X X
X X X X X X X X X X X X X X X
X X X X X X X X X X X X X X X
X X X X X X X X X X X X X X X
X X X X X X X X X X X X X X X
X X X X X X X X X X X X X X X
X X X X X X X X X X X X X X X
X X X X X X X X X X X X X X X
X X X X X X X X X X X X X X X
X
X
X
X
X
Is there
magnetic
• • Yes,
thereaisCHANGING
a current in
the loopfield?
–
X X X X X X X X X X X X X X
SOMETIMES!
•
What is the direction of change?
X X X X X X X X X X X X X X
X •X•XBut
X
X when
X X direction
X X XisXthere
X X Xopposes
a current
and in
What
change?
X X X X X X X X X X X X X X
• what
Whatdirection?
direction is induced current?
Emf induced in a moving conductor
We can create a circuit and utilize
this motional emf.
But wait… can’t I just make a
current since once I get the bar
going, it should keep going by
Newton’s 1st Law, right?!
How can we utilize induction?
• Even though we can’t create energy (can’t get
somethin’ for nothin’), we can still use
induction to our benefit.
• We can change the
voltage using a
transformer.
N P I S VP


N S I P VS
Power lost in the transport of power
• We learned earlier that less power is lost in
transmission with a higher voltage.
• We use transformers
200,000-400,000V
to change the
voltage several
times from the
20,000V
power plant to
your home.
120V (US)
What the heck is this?