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An electromagnet is an object that acts
like a magnet, but its magnetic force is created and
controlled by electricity--thus the name electromagnet.
By wrapping insulated wire around a piece of iron and
then running electrical current through the wire, the
iron becomes magnetized. This happens because a magnetic
field is created around a wire when it has electrical
current running through it. Creating a coil of wire
concentrates the field. Wrapping the wire around an iron
core greatly increases the strength of the magnetic
field.
Questions you may have include:
-
How can you make an electromagnet?
-
What factors are involved in
electromagnetism?
-
How does an iron core affect the
strength?
Making an electromagnet
If you wrap a wire around an iron core,
such as a nail, and you send electrical current through
the wire, the nail will become highly magnetized. You
can verify that by picking up small objects or by
showing its effect on a compass. This is called an
electromagnet.
Creating a simple electromagnet using a
nail
Note that the wire must be an insulated
wire. A bare wire would cause an electrical short and
the current would then run through the nail or metal
core. In some electromagnets, like in an electric motor,
the wire will look like bare copper, but it is insulated
with a thin coating of a clear material.
Also, if the wire is thin, it may get
warm from the resistance to the electricity passing
through it.
The most interesting feature of the
electromagnet is that when the electrical current is
turned off, the magnetism is also turned off. This is
especially true if the core is made of soft iron, which
quickly loses its magnetism. Hardened steel may retain
its magnetism, so you can't use the most valuable
feature of an electromagnet.
Being able to turn the magnetism on and
off has lead to many amazing inventions and
applications.
How electromagnetism works
When electricity passed through a wire,
a magnetic field is created around the wire. Looping the
wire increases the magnetic field. Adding an iron core
greatly increases the effect and creates an
electromagnet. You can create an electromagnet without
the iron core. That is usually called a solenoid.
When DC electricity is passed through a
wire, a magnetic field rotates around the wire in a
specific direction.
Magnetic field rotating around wire
Connecting a wire to a battery and
placing a compass near the wire can demonstrate a
magnetic field. When the current is turned on, the
compass-needle will move. If you reverse the direction
of the current, the needle will move in the opposite
direction.
To find the direction the magnetic field
is going, you can use the "right-hand rule" to determine
it. If you take your right hand and wrap it around the
wire, with your thumb pointing in the direction of the
electrical current (positive to negative), then your
fingers are pointing in the direction of the magnetic
field around the wire. Try it with the picture above.
Wrapping the wire in a coil concentrates
and increases the magnetic field, because the additive
effect of each turn of the wire.
Coiled wire increases magnetic field
A coil of wire used to create a magnetic
field is called a solenoid.
Wrapping the wire around an iron core
greatly increases the magnetic field. If you put a nail
in the coil in the drawing above, it would result in an
electromagnet with the a north seeking pole on the "N"
side.
If AC electricity is used, the
electromagnet has the same properties of a magnet,
except that the polarity reverses with the AC cycle.
Note that it is not a good idea to try
to make an AC electromagnet. This is because of the high
voltage in house current. Using a wire around a nail
would result in a blown fuse in the AC circuit box.
There is also the potential of an electric shock.
Strength of electromagnetic field
The strength of the electromagnetic
field is determined by the amount of current, number of
coils of wire, and the distance from the wire.
The unit of magnetic force is called the
tesla (T). Near a strong magnet the force is 1-T.
Another unit used is the gauss, where 104 gauss (10,000)
equals 1 tesla.
The strength of the magnetic field is
proportional to the current in the wire. If you double
the current, the magnetic force is doubled.
Since Voltage = Current x Resistance (V
= I*R), you can double the current in a wire by doubling
the voltage of the source of electricity.
If you wrap the wire into a coil, you
increase the magnetic force inside the coil,
proportional to the number of turns. In other words, a
coil consisting of 10 loops has 10 times the magnetic
force as a single wire with the same current flowing
through it. Likewise, a coil of 20 loops has 2 times the
magnetic force than one with 10 loops.
The magnetic force decreases with
distance. It varies inversely proportional to the square
of the distance. For example the force at 2 cm. from a
wire is 1/4 that of at 1 cm., and the force at 3 cm. is
1/9 the force at 1 cm.
Effect of iron core
When the coil is wrapped around an iron
core, the strength of the electromagnetic field is much
greater than the same coil without the iron core. This
is because the atoms in the iron line up to amplify the
magnetic effect. The orientation of the atoms in the
iron is called its domain.
When you increase the current, the
magnetic strength increases, but it is not exactly
linear as it is with the coil by itself. The
characteristics of the core cause the curve of magnetic
strength versus current to be an s-shaped hysteresis
curve.
The shape of this curve depends on how
well the material in the core becomes magnetized and how
long it remains magnetized. Soft iron loses its
magnetism readily, while hard steel tends to retain its
magnetism.
By wrapping a wire around an iron core
and applying an electric current through the wire, you
create an electromagnet. This device is magnetic only
when the current is flowing. The iron core greatly
increases the magnetic strength. |
