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12.4 Solenoids
By demonstrating that we could use electricity to
produce magnetism, Oersted’s discovery enabled
magnetism to be controlled;
• We can turn it on and off.
• Change its strength by increasing or decreasing
the current.
• Change the direction of the magnetic field by
reversing the direction of current.
Oersted’s discovery forever changed the world,
leading to new kinds of technologies, including the
motor and the generator.
12.4 Ampere’s Experiment
Andre-Marie Ampere was fascinated by Oersted’s
discovery. While investigating other aspects of
electricity and magnetism, he took two parallel wires
and demonstrated that their respective magnetic fields
can interact to cause a force when currents were sent
through them.
If the respective currents flow in opposing directions,
the magnetic field lines would point in the same
direction; causing the wires to repel.
If the respective currents flow in the same direction,
the magnetic field lines would point in opposing
directions; causing the wires to attract.
12.4 Coiled Conductors
Instead of two parallel wires, let us examine the magnetic field around a single loop
of a current-carrying wire. Notice that in the centre of the loop, the magnetic field
points straight through.
The +ve and –ve signs show the direction of conventional current.
12.4 Solenoids
Winding the conductor into a coil containing several loops is a solenoid.
The magnetic field around a solenoid has a shape similar to that of a bar magnet.
The field is strongest at the poles, or ends of the coils, and weakest at the sides.
12.4 Solenoids
So now we have a way to make an electrically powered bar magnet.
An electromagnet is a device that produces a magnetic field as a result of an
electric current. The benefits include:
• It can be turned on and off.
The strength of an electromagnet can be increased by:
• Increasing the number of loops in the coil.
• Increasing the current in the coil.
• Inserting a core material, such as soft iron, to concentrate the magnetic field.
Soft iron can be quickly magnetized and de-magnetized.
By combining all three factors, you can produce a very powerful electromagnet.
12.2 Right Hand Rule for a Solenoid
Right Hand Rule for a Solenoid (RHR #2):
The fingers of your right hand wrap around the coil in the direction of
conventional current, while your right thumb points in the direction of the
north magnetic pole of the coil.
12.4 Applications of Solenoids
Solenoids in Subwoofers
A subwoofer is a speaker that produces deep bass sounds.
They can produce the low frequency sound effects in
surround-sound systems.
The subwoofer has a cone made from paper or plastic that
moves outward and inward to produce compressions and
rarefactions to create these sounds.
To move the cone, a permanent magnet surrounds a
solenoid voice coil. A reversing current passes through the
voice coil, by an amplifier, producing a magnetic field that
repels and attracts the voice coil and cone away from and
towards the magnet.
12.4 Applications of Solenoids
Solenoids in Electric Bells
The design of the electric bell allows it to be rung
continuously for as long as needed.
When the switch is closed, the solenoids produce a
magnetic field that is amplified by the soft-iron cores. The
armature is attracted to the core, causing the bell to ring.
As the bell is struck, the armature moves away from the
contact, breaking the circuit. The spring pushes the
armature back in place to make contact and form the
circuit once again, repeating the process as long as the
switch is closed.
12.4 Homework
Questions # 1-5 p.562