1. When a conductor is moved across a magnetic field, an electromotive force (emf) is produced in the conductor. If the conductor forms part of a closed circuit then the emf produced causes an electric current to flow round the circuit. Hence an emf (and thus current), is ‘induced’ in the conductor as a result of its movement across the magnetic field. This effect is known as ‘electromagnetic induction’.

2. Faraday’s laws of electromagnetic induction state:

3. Lenz’s law states:

4. An alternative method to Lenz’s law of determining relative directions is given by Fleming’s Right-hand rule (often called the geneRator rule) which states:

‘Let the thumb, first finger and second finger on the right hand be extended such that they are all at right angles to each other, as shown in Figure 11.1. If the first finger points in the direction of the magnetic field the thumb points in the direction of motion of the conductor relative to the magnetic field, then the second finger will point in the direction of the induced emf.’

5. In a generator, conductors forming an electric circuit are made to move through a magnetic field. By Faraday’s law an emf is induced in the conductors and thus a source of emf is created. A generator converts mechanical energy into electrical energy. (The action of a simple a.c. generator is described in para 2, page 93.)

6. The induced emf E set up between the ends of the conductor shown in Figure 11.2 is given by: E = BIv volts, where B, the flux density is measured in teslas, l, the length of conductor in the magnetic field is measured in metres, and v, the conductor velocity, is measured in metres per second. If the conductor moves at an angle θ° to the magnetic field (instead of at 90° as assumed above) then E = Blv sin θ.

7. Inductance is the name given to the property of a circuit whereby there is an emf induced into the circuit by the change of flux linkages produced by a current change.

8. The unit of inductance is the henry, H.

9. A component called an inductor is used when the property of inductance is required in a circuit. The basic form of an inductor is simply a coil of wire. Factors which affect the inductance of an inductor include:

10. Two examples of practical inductors are shown in Figure 11.3 and the standard electrical circuit diagram symbol for air-cored and iron-cored inductors are shown in Figure 11.4.

An iron-cored inductor is often called a choke since, when used in a.c. circuits it has a choking effect, limiting the current flowing through it.

11. Inductance is often undesirable in a circuit. To reduce inductance to a minimum the wire may be bent back on itself as shown in Figure 11.5 so that the magnetising effect of one conductor is neutralised by that of the adjacent conductor. The wire may be coiled around an insulator as shown without increasing the inductance. Standard resistors may be non-inductively wound in this manner.

12. An inductor possesses an ability to store energy. The energy stored, W, in the magnetic field of an inductor is given by:

$W=\frac{1}{2}L{I}^{2}joules.$

13. 

14. If a current changing from 0 to I amperes, produces a flux change from 0 to Φ webers, then ΔI = I and ΔΦ = Φ. Then, from para 13, induced emf $E=\frac{N\text{Φ}}{t}=\frac{\text{L}\text{I}}{t}$, from which inductance of coil,$\text{L}=\frac{N\text{Φ}}{\text{I}}$ henrys.

15. From para 23, page 86, reluctance $S=\frac{mmf}{f\text{L}ux}=\frac{N\text{I}}{\text{Φ}}$ from which $\text{Φ}=\frac{N\text{I}}{S}$.

16. Mutually induced emf in the second coil, $E_2=N\left(\frac{\text{Δ}{\text{I}}_1}{t}\right)$ volts, where M is the mutual inductance between two coils, in henrys, ΔI₁ is the change in current in the first coil, in amperes, and t is the time the current takes to change in the first coil, in seconds. A transformer is a device which uses the phenomenon of mutual inductance to change the value of alternating voltages. (See chapter 20, page 171.)