ELECTROMAGNETIC INDUCTION

Date: 2015-10-07; view: 649.

Electromagnetic induction is the production of voltage across a conductor moving through a magnetic field. It underlies the operation of generators, all electric motors, transformers, induction motors, synchronous motors, solenoids, and most other electrical machines.

Michael Faraday British Physicist and Chemist (1791–1867) Faraday is often regarded as the greatest experimental scientist of the 1800s. His many contributions to the study of electricity include the invention of the electric motor, electric generator, and transformer, as well as the discovery of electromagnetic induction and the laws of electrolysis. Greatly influenced by religion, he refused to work on the development of poison gas for the British military.

An easy way to create a magnetic field of changing intensity is to move a permanent magnet next to a wire or coil of wire. The magnetic field must increase or decrease in intensity perpendicular to the wire, or else no voltage will be induced.

Michael Faraday stated that electromotive force (EMF) produced around a closed path is proportional to the rate of change of the magnetic flux through any surface bounded by that path. In practice, this means that an electric current will be induced in any closed circuit when the magnetic flux through a surface bounded by the conductor changes. This applies whether the field itself changes in strength or the conductor is moved through it.

In mathematical form, Faraday's law states that . For the special case of a coil of wire, composed of N loops with the same area, the equation becomes , where ɛ is the electromotive force, is the magnetic flux, "d" representing rate of change of flux over time, N stands for the number of turns, or wraps, in the wire coil (assuming that the wire is formed in the shape of a coil for maximum electromagnetic efficiency).

A sequence of Faraday's Law, together with Ampère's law and Ohm's law is Lenz's law. So, there are three laws of electromagnetic induction:

Faraday's first law states that the voltage appears between the ends of a conductor when there is a change in the magnetic field around it. (In other words, any magnetic field that changes over time will produce an electric field in the space around it). It explains what electromagnetic induction is.

Faraday's second law states that the induced electromotive force in a wire loop is proportional to the rate of change of magnetic flux through the loop.

Lenz's law states that the direction of induced current is always such as to oppose the cause which produces it.

Thus, Faraday's second law gives the size of the effect and Lenz's law gives its direction. This phenomenon is put into obvious practical use in the construction of electrical generators, which use mechanical power to move a magnetic field past coils of wire to generate voltage.

If we recall that the magnetic field produced by a current-carrying wire was always perpendicular to that wire, and that the flux intensity of that magnetic field varied with the amount of current through it, we can see that a wire is capable of inducing a voltage along its own length simply due to a change in current through it. This effect is called self-induction: a changing magnetic field produced by changes in current through a wire inducing voltage along the length of that same wire. If the magnetic field flux is enhanced by bending the wire into the shape of a coil, and/or wrapping that coil around a material of high permeability, this effect of self-induced voltage will be more intense. A device constructed to take advantage of this effect is called an inductor (www.allaboutcircuits.com).

1. What is electromagnetic induction?

2. Who discovered electromagnetic induction?

3. What did he discover about the voltage?

4. What is an easy way to create a magnetic field of changing intensity?

5. What do Faraday's laws state?

6. What does Lenz's law state?

7. What is self-induction?

Exercise 91. Give the corresponding terms for the following definitions.

1. _______________________ – a magnet that retains its magnetic properties in the absence of an inducing field or current.

2. _______________________ – a component in an electric or electronic circuit that possesses inductance.

3. ________________________ – a difference in potential that tends to give rise to an electric current.

4. _______________________ – a cylindrical coil of wire acting as a magnet when carrying electric current.

5._________________________– the production of an electromotive force either by motion of a conductor through a magnetic field so as to cut across the magnetic flux or by a change in the magnetic flux that threads a conductor.

6. ________________________ – a device that converts mechanical energy to electrical energy, generally using electromagnetic induction.

7. ______________________ is a type of alternating current motor where power is supplied to the rotor by means of electromagnetic induction (also called asynchronous motor).

Exercise 92. Circle the correct answer.

1. According to Lenz's law, the resulting force produced by an induced current __________.

a) opposes the current c) contributes to the motion of the wire

b) reduces the resistance d) opposes the motion of the wire 2. __________ is the generation of current due to relative motion between a wire and a magnetic field.

a) Electromagnetic inductionc) Electrolysis

b) Magnetic flux d) Electrospectrometry

3. A wire is moving toward you through a magnetic field directed to your right. In what direction is the force?

a) downward c) toward you

b) to the right d) upward

4. A transformer is a device to increase or decrease AC _________.

a) power c) heat

b) resistance d) voltage

5. What is the unit for EMF?

a) N b) C/J c) V d) N • m