Download Electromagnetic Induction In earlier work, we learned that an electric

Electromagnetic Induction
In earlier work, we learned that an electric current produces a magnetic field, and that a
magnetic field exerts a force on an electric current or a moving electric charge. These
discoveries were made in the 1820's. Scientists began to wonder that if electric currents
produce a magnetic field, then would it be possible for a magnetic field to produce an electric
current. Ten years later, the American Joseph Henry (1797-1878) and the Englishman Michael
Faraday (1791-1867) independently found that this was possible. It was actually Henry who
made the discovery first, but Faraday published his results earlier and investigated the subject in
more detail.
In attempting to produce an electric current from a magnetic field, Faraday used an apparatus
similar to the one shown below.
A coil of wire at X was connected to a battery. The current that flowed through X produced a
magnetic field inside the coil. The iron core intensifies this magnetic field. Faraday hoped that by
using a strong enough battery, the steady current in X would produce a great enough magnetic
field to produce a current in the second coil at Y. The second circuit, Y, contained a
galvanometer to detect any current but it did not contain a battery. When Faraday used steady
currents from the battery, the galvanometer would not register any current.
Faraday did finally see a current in the galvanometer in circuit Y at the moment he closed the
switch in circuit X. Furthermore, the galvanometer deflected strongly in the opposite direction
when he opened the switch. Thus a steady current in X did not produce a current in Y. A current
in Y was produced only when the current in X was starting or stopping.
Faraday concluded that although a steady magnetic field does not produce a current, a
changing magnetic field could produce a current. The current in the coil in circuit Y is called an
induced current because it is brought about (or "induced") by a changing magnetic field.
Since a source of emf, like the battery in Faraday's experiment, is always needed to produce a
current, the coil behaves as if it were a source of emf. This emf is known as the induced emf.
We can summarize the effects above by stating that an induced emf or an induced current is
produced by a changing magnetic field.
In general, we can define electromagnetic induction as the production of an induced current
or emf in a conducting circuit brought about by a changing magnetic field.
Inducing EMF and the North Pole of a Magnet
Faraday did further experiments on electromagnetic induction. These experiments illustrated
other ways to induce emf in a circuit.
In the left diagram above, the north pole of a magnet is moving upwards towards the coil of wire
that is connected to a galvanometer. This induces a current to flow in the coil and the needle of
the galvanometer deflects to the right. This occurs because the strength of the magnetic field is
changing as the magnet approaches the coil.
In the right diagram above, the north pole of the magnet is moving downwards away from the
coil of wire. The induced current flows in the opposite direction, and the needle of the
galvanometer moves to the left.
Notice in both diagrams that the magnetic field lines of the magnet point away from the north
pole of the magnet. This magnetic field direction does not change as the magnet is moved but
the strength of the field does change.
Suppose that in the left diagram, the magnet was held steady and the coil moved downwards. In
that case, the current would move in the same direction as before and the needle would deflect
to the right. In the right diagram, a similar effect would be observed. If the magnet was held
stationary and the coil was moved away from it, then the current in the coil would be the same
as it was before and the needle would deflect to the left. It does not matter whether the coil or
the magnet moves to produce an induced current or emf in the coil.
Inducing EMF and the South Pole of a Magnet
Similar effects can be observed if the bar magnet is turned around and the south pole is
used.
Notice that the direction of the magnetic field lines is towards the south pole of the magnet. This
is opposite to the direction for the north pole of the magnet. So when the south pole of the
magnet is moved upwards, the direction of the current is opposite to what it was when the north
pole of the magnet moved upwards.
The right diagram shows that when the south pole of the magnet moves away from the coil, the
current is once again opposite to what it was when the north pole of the magnet moved
downwards.
As was the case with the north pole of the magnet facing upwards, the same effects in current
direction can be achieved if the coil moves towards the magnet instead of the magnet moving
towards the coil. What is important is that relative motion occurs.
Induced EMF and the Area or Orientation of a Coil
Another way to induce an emf in a coil of wire is to change the area of the coil. For
example, assume that a magnetic field is directed into the paper. A coil of wire with a
galvanometer attached is initially held in this field.
The diagram to the right shows that the ends of the coil have been stretched so that the area of
the coil facing the magnetic field is less. This causes a current to flow in the coil, and this is
registered on the galvanometer. Keep in mind that as long as the area is changing, an induced
current and emf exist; they vanish when the area is no longer changing. If the distorted coil is
returned to its original circular shape, thereby increasing the area, an oppositely directed current
is generated while the area is changing and the galvanometer needle will move in the opposite
direction.
An induced emf is also generated when a coil of constant area is rotated in a constant magnetic
field and the orientation of the coil changes with respect to the field. When the rotation stops, the
emf, and hence the current, vanishes.
In the above examples, both an emf and a current are induced in the coil because the coil is part
of a complete closed circuit. If the circuit were open, as in the case of an open switch, there
would not be an induced current. However, there would still exist an induced emf whether the
current exists or not.
Thus changing the magnetic field, changing the area of the coil, and changing the orientation of
the coil are all methods that can be used to create an induced emf.
1. Descirbe three ways in which a current or emf can be induced.
2. What is electromagnetic induction?