The size of the induced emf can be increased by increasing the number of turns and winding the coil on a soft iron core. Any coil wound like this and with properties of self-inductance can be referred to as an ‘inductor’.
Use of back emfs
This phenomenon of self-inductance has been put to use in a number of ways. It will constantly oppose a.c. given that current direction is constantly changing. It will have very little effect on d.c., since it is constant.
Because of this, it can be used to:
• smooth out fluctuations in current as sharp increases and decreases (spikes) are opposed by the self-inductance and are slower to take effect • build dimmer circuits.
You may have noticed some lights are built to gradually turn off or gradually turn on. This is due to self-inductance. For this reason, coils that are designed to use self-inductance are called ‘inductors’.
Inductors and a.c.
Inductors are coils of wire which may or may not be wound around a soft iron core. If you place an inductor into a circuit, its resistance affects both a.c. and d.c. However, in the case of a.c., the inductor not only causes resistance but also offers a back emf which has the effect of ‘slowing down’ the rise and fall of the alternating current. This can be seen in the way dimmer switches operate, as light gradually increases or decreases because of inductance. It may also be used in radio tuning, along with capacitors, in order to fine-tune stations.
Transformers
A transformer is a device used to change the value of an alternating voltage (Fig. 28.10).
soft iron core
primary coil
Vi
110 volts
primary coil
Vi
220 volts
secondary coil
220 volts
secondary coil
110 volts
Vo
FIG. 28.10 A transformer changes the value of an alternating voltage
LEAVING CERTIFICATE PHYSICS 307 Vo NOTE
Inductance may only have a significant effect on a.c. but remember that ohmic resistance affects both a.c. and d.c.
