Question

Explain how an electric motor works. How does a stereo speaker use an electromagnet to produce sound? (10pts)

Answer 1

Working model of a motor is explained in detail here...

lets starts with the beginning.

What Do Electric Motors Do
Electric motors use electricity and magnets to cause rotation. Two battery leads are connected to metal pieces called brushes that allow electricity to flow into an electromagnet that is connected to the axle. As electricity is passed to the electromagnet, the field magnet attracts and repels the electromagnet which causes the axle to spin. It is this magnetic rotation that allows all mechanical devices to work.

Armature
The armature, or electromagnet, consists of an iron bar that is wrapped in coiled wire. As electricity passes through the armature, the coiled wire creates a magnetic field that interacts with the magnetic field of the field magnet. The direction that the armature spins is entirely dependent on the placement of the battery leads and the field magnet.

If electricity flows from the positive end of the battery lead to the positive end of the field magnet, the field magnet will repel the armature and spin it in one direction; if electricity flows from the positive end of the battery lead to the negative end of the field magnet, the field magnet will attract the armature and spin it in the other direction. While some mechanical devices will allow the user to automatically switch the direction of the battery leads by flipping a switch, such as a power drill, other devices are fixed in a certain direction and would require the user to physically rearrange the battery leads to change the direction of the armature.

Commutator
A commutator refers to two metal plates that are attached to the axle just above the armature that pass electricity to the electromagnet. The commutator can also be used to switch the direction of the electric field so that the electromagnet and axle spin the opposite way.

Field Magnet
The field magnet is just a permanent magnet that is placed near the armature. Depending on the flow of electricity, the field magnet will repel or attract the armature. Either way, the field magnet will cause the armature to spin. The field magnet in an electric motor can be replaced by another electromagnet but permanent magnets are easier to maintain and use less electricity.

Brushes
The brushes of an electric motor are two springy pieces of metal that make contact with the commutator. The brushes are also in direct contact with the battery leads. When electricity flows from the battery to the brushes, it is passed on to the commutator which then passes it on to the electromagnet.

DC Power
DC, or direct current, refers to a type of electricity that does not change polarity. DC power is the most common form of electricity in small mechanical devices, although it is usually derived from AC, or alternating current. Alternating current can be used to power mechanical devices but the device will need to include a rectifier that can convert the AC power into DC power. Each mechanical device will require a different amount of DC power to operate depending on the functions of the device.

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In order to translate an electrical signal into an audible sound, speakers contain an electromagnet: a metal coil which creates a magnetic field when an electric current flows through it. This coil behaves much like a normal (permanent) magnet, with one particularly handy property: reversing the direction of the current in the coil flips the poles of the magnet.

Inside a speaker, an electromagnet is placed in front of a permanent magnet. The permanent magnet is fixed firmly into position whereas the electromagnet is mobile. As pulses of electricity pass through the coil of the electromagnet, the direction of its magnetic field is rapidly changed. This means that it is in turn attracted to and repelled from the permanent magnet, vibrating back and forth.

The electromagnet is attached to a cone made of a flexible material such as paper or plastic which amplifies these vibrations, pumping sound waves into the surrounding air and towards your ears.

Inside a speaker:
1. Cone
2. Electromagnet (coil)
3. Permanent magnet

The frequency of the vibrations governs the pitch of the sound produced, and their amplitude affects the volume – turn your stereo up high enough and you might even be able to see the diaphragm covering the cone move.

To reproduce all the different frequencies of sound in a piece of music faithfully, top quality speakers typically use different sized cones dedicated to high, medium and low frequencies.

A microphone uses the same mechanism as a speaker in reverse to convert sound into an electrical signal. In fact, you can even use a pair of headphones as a microphone!