29.0 - Introduction
It was a chance occurrence that led the British scientist Michael Faraday (1791-
1867) to discover electromagnetic induction.
It was 1831. Scientists already knew that an electric current could be used to create
a magnetic field. Faraday and others were trying to achieve the opposite: They
wanted to use a magnetic field to create an electric current.
Faraday was conducting an experiment in which he wrapped two lengths of
insulated wire around a soft iron ring. One of the lengths was part of a circuit that
included a battery. The second was part of a different circuit containing an ammeter
that could measure any current passing through it. The wires were insulated so that
no current could flow through the iron ring between the circuits.
Faraday knew he could create a magnetic field in the iron ring by running electricity
through the first coil. His goal was to use this magnetic field to create a current in
the second coil. The illustration to the right shows a modern-day recreation of his
experiment. The upper coil is the part of the circuit that in Faraday’s experiment
contained the battery; we have replaced the battery with a slider control labeled
“Current” so that you can control the amount of current in this circuit. The coil of the second circuit is the lower one in the illustration; we have
included a light bulb to make it easier to see when a current flows in the second circuit.
Faraday had always connected the battery before he connected the ammeter, and he detected no current in the second circuit. However, on
the morning of August 29, Faraday connected the ammeter first and then connected the battery. To his delight, he detected a momentary
current in the second circuit; connecting the battery after the ammeter meant that it was measuring what happened as the current changed in
the upper circuit. Today, scientists would say he created a basic transformer, and they understand that it was the change in the magnetic field
caused by the change in current in the first circuit that caused the current in the second.
Faraday rapidly pushed his work ahead. In the next few months, he discovered that by moving a wire in a magnetic field he could also generate
a current in a circuit. Today, this principle is employed in the machinery that generates most of the electricity we use.
Faraday’s simple lab equipment yielded powerful insights that engineers continue to utilize today. Electric generators, microphones, VCRs, and
induction stoves all rely on Faraday’s discovery that a current-producing emf can be induced by changing the strength of a magnetic field, or by
moving a wire in a magnetic field.
In the simulation to the right, you can recreate Michael Faraday’s groundbreaking experiment. The simulation contains an experimental setup
similar to the one Faraday used in 1831. You use a slider to control the current in the upper circuit on the left. By setting the slider’s position,
you determine the amount and direction of the current. You will see the magnetic field lines created by the current of this circuit; the more
intense their color, the stronger the magnetic field.
When you launch the simulation, you will see that an oscilloscope, rather than Faraday’s ammeter, is attached to the bottom circuit. It displays
the potential difference across the light bulb that is part of the bottom circuit.
Experiment by changing the current in the top circuit and observing what happens in the bottom circuit. By moving the slider back and forth,
you can continuously change the current. Is there a current in the bottom circuit if the current in the top circuit is steady and unchanging? What
if the current in the top circuit is changing? Does the rate at which the current in the top circuit changes have any effect on the potential
difference you measure in the second circuit?
29.1 - Motional electromagnetic induction
Motional electromagnetic induction: Moving a
wire through a magnetic field induces an emf.
Michael Faraday demonstrated two ways to generate a current in a circuit: by changing
the strength of a magnetic field passing through a wire coil or by moving a wire through
a magnetic field. This section examines the current generated by moving a wire through
a field. The phenomenon is called motional electromagnetic induction, or just motional
induction.
We start our explanation of motional induction by making sure the diagram on the right
is clear to you. The vertical segment of conducting wire is pushed from left to right,
sliding across the horizontal wires connected to the light bulb. The wires connect to
form a complete circuit. The sliding vertical wire moves through an external magnetic
field, represented by ×’s, that points directly into the computer screen. The sliding wire
moves perpendicularly to this magnetic field.
The result is called an induced emf (Ǜ). You studied another source of emf, a battery,
earlier. The process shown here creates an emf, “induced” by sliding the vertical segment of wire through the field. Since this segment is part
Motional electromagnetic
induction
Wire moves through magnetic field
Motion induces an emf