12.4 SolenoidsBy 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 ExperimentAndre-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 ConductorsInstead 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 SolenoidsWinding 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 SolenoidsSo 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 SolenoidRight 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 SolenoidsSolenoids in SubwoofersA 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 SolenoidsSolenoids in Electric BellsThe 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