ELECTROMAGNETIC INDUCTION

1. Electromagnetic induction

current is caused to flow in the circuit to which the coil is attached. If the magnet is withdrawn, the direction of the current is reversed. Such currents are called INDUCED CURRENTS. The current flows in the conductor as a result of an electromotive force that is induced - the so-called INDUCED EMF, which is often given the symbol ε.

The size of the current depends on how fast the magnet moves in or out of the coil, and the number of loops in the coil.

The phenomenon of inducing a current by changing the magnetic field in a coil of wire is known as ELECTROMAGNETIC INDUCTION.

This phenomenon underpins the design of all electric generators.

Electromagnetic induction takes place whenever the magnetic field in the vicinity of a circuit changes - this can happen when -

2. The induced emf

2.1 The magnetic flux

In order to proceed further, we need to look at a quantity called the MAGNETIC FLUX, Φ (greek letter "phi"). If a magnetic field is applied to a surface of area A, and the component of that magnetic field that is perpendicular to the surface is B, the the flux is given by the expression:

Note that both B and Φ are vector quantities (). The SI unit of magnetic flux is the WEBER, Wb.


2.2 Faraday's law of eletromagnetic induction

Faraday's law states that:

Faraday's Law of Electromagnetic Induction
"The magnitude of the induced emf in a coil is the negative of the rate of change in magnetic flux through that coil multiplied by the number of turns in that coil."

So, for a coil with N turns, with cross-sectional area A subjected to a changing magnetic flux Φ directed perpendicularly to the opening of the coil, the average induced emf will be:

where Δt is the time interval during which the change in flux ΔΦ took place. The change in the flux may be effected in various ways, such as:

The above principles underlie the functioning of devices such as electric motors and generators, discussed in Grade 12.

3. Lenz's law

Lenz's law enables us to determine the direction of the induced current:

Lenz's Law
"The direction of the induced current is such as to oppose the change causing it."

   
The above diagram shows the north pole of a bar magnet approaching a solenoid. According to Lenz's law, the current which is thereby generated in the coil must cause an effect which opposes the approaching magnetic field. This is achieved if the direction of the induced current creates a north pole at the end of the solenoid closest to the approaching magnet, as the induced north pole tends to repel the approaching north pole.     The above diagram shows the north pole of a bar magnet withdrawing from a solenoid. According to Lenz's law, the current which is thereby generated in the coil must cause an effect which opposes the departing magnetic field. This is achieved if the direction of the induced current creates a south pole at the end of the solenoid closest to the departing magnet, as the induced south pole tends to attract the departing north pole.

One may also easily determine the direction of the current using the "right-hand rule" or "dynamo rule", illustrated in the diagram on the right: if the thumb points in the direction of motion of the field relative to the conductor, and the first (index) finger points in the direction of that field, then the direction of the induced current will be indicated by the second (middle) finger.

4. The transformer

When an alternating current passes through a conductor wound around one side of an iron core shaped as in the diagram above (forming a so-called PRIMARY COIL), the alternating voltage causes an alternating magnetic field to be set up inside the iron core. In turn, this will induce an alternating voltage in a coil wrapped around the opposide side of the core (the SECONDARY COIL). The circuit of which the secondary coil is part will therefore carry an alternating current. It is important to note that transformers only work with alternating currents, about which you will learn more in Grade 12.

The size of the voltage in the secondary coils will depend on the number of turns which are wrapped around the sides of the iron core, according to the formula

If the input voltage is greater than the output voltage, the transformer is called a STEP-DOWN TRANSFORMER, while if the output voltage is greater than the input voltage, the transformer is a STEP-UP TRANSFORMER.

Transformers are very useful devices, found wherever the voltage of alternating power supplies have to be modified. The computer you are using has one, for example.

5. Additional questions


Alternating current

If the field across the conductor changes in a periodic fashion, as shown in the diagram below, then an ALTERNATING CURRENT is set up in which the electrons accelerate first in one direction then slow down, stop and then accelerate in the other direction, slow down and stop and then repeat this cycle:

Domestic electricity is supplied in South Africa as an alternating current of 50 cycles per second (50 Hz) with a supply voltage is quoted as 220 V.