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Moving charges
create magnetic
fields. What do
moving magnets
do?
Put a force on
charges by
creating electric
fields.
Electromagnetic induction
In this section, we will discuss the induction of voltage by changing the magnetic
field in loops of wire. We have seen that moving a magnet through a coil of wire
induces a current to flow in the wire. (demo?)
http://phet.colorado.edu/sims/faradays-law/faradays-law_en.html
PhET simulations
http://phet.colorado.edu/sims/faradays-law/faradays-law_en.html
http://phet.colorado.edu/en/simulation/faraday
Magnetic Induction related to Lorentz force
Wire moving in a magnetic field gets an electric field induced in it –
electromotive force or emf. More generally, a changing magnetic field
induces an electric field whose strength is related to the rate of change
of the magnetic field. This can be thought of either as a Lorentz force
on the charges in the wire moving through a magnetic field, or the emf
generated by the changing flux through the loop.
Lenz’s law
The direction of any magnetic induction effect must oppose the cause of the effect.
The change in magnetic flux in through a loop induces an emf in the
loop. The emf in the loop creates a current, which creates a magnetic
field, which opposes the original change in magnetic field.
A changing magnetic flux induces a current in a metal ring. In order to conserve energy,
the induced magnetic field is in the opposite direction from the original field.
This is the mechanism involved with the Ring Flinger demo. The opposing magnetic fields
repel each other, flinging the ring into the air. If the ring is not complete, a current cannot
flow and no opposing magnetic field is created.
Magnetic Induction
Generator – demo with hand cranks. Animation below
http://new.wvic.com/index.php?option=com_content&task=view&id=9&Itemid=46
http://phet.colorado.edu/en/simulation/faraday
Magnetic Induction and displacement current
Displacement current - If a current in a
wire is carrying charges from one
metal plate to another (charging a
capacitor), the magnetic field around
the capacitor portion of the circuit is
the same as the magnetic field around
the current carrying portion of the
circuit. Stated in another way, a
changing electric field creates a
magnetic field.
Magnetic induction – we have learned
that a changing magnetic field creates
an electric field.
An accelerated charge creates a magnetic field that is changing, so that creates an
electric field that is changing, so that creates a magnetic field that is changing, so that
creates an electric field that is changing, so … Let there be light (or at least
electromagnetic radiation). http://www.walter-fendt.de/ph14e/emwave.htm
Transformers
When two coils are placed near each other, alternating current in one (the primary,
which is the one connected to the power source) induces a changing magnetic field.
The second coil is within that field, so it has a current induced in it. If the first coil has
fewer loops than the second, the second will experience a higher induced voltage
than the first. This is called a step-up transformer. If the first coil has more loops than
the second, the second will experience a lower induced voltage than the first and it is
called a step-down transformer. Placing an iron core in the coils will intensify the
magnetic field in each.
Heat and energy loss in wires is much higher when the current is high. For power
transmitted through wire, using P = IV, we can see that higher voltage results in lower
current for the same power. Thus, we step up the voltage for tranmission and then step
it down again where we want to use the electricity. Since alternating current is more
easily stepped up and stepped down than is direct current, electricity is transported
over long distances via AC rather than DC.
The relationship between the voltages of the
primary and secondary coils is given by:
The transformer transfers energy from one coil to the other. The rate at
which the energy is transferred is the power. Since energy is conserved,
the power of the first coil equals the power of the second coil.
Rewriting this in terms of voltage and current yields:
Online induction demos from Eugenia Etkina
http://paer.rutgers.edu/pt3/experiment.php?topicid=10&exptid=91
http://paer.rutgers.edu/pt3/experiment.php?topicid=10&exptid=123
http://paer.rutgers.edu/pt3/experiment.php?topicid=10&exptid=122
http://paer.rutgers.edu/pt3/experiment.php?topicid=10&exptid=92
http://paer.rutgers.edu/pt3/experiment.php?topicid=10&exptid=93