• Is a process by which an e.m.f is induced in a coil by changing current or magnetic field in another current near it.
  • The coil connected to the source is called a primary coil and the coil where e.m.f is induced is called the secondary coil.

                                                The setup.

When the switch is on, the current in the primary coil increases from zero to maximum. The magnetic field due to this current increases from zero to maximum. This change in magnetic fields are linked with the secondary coil and hence they induce an e.m.f in the secondary coil causing a current to flow through the galvanometer which reflect. The e.m.f in the secondary coil is momentary since when the current in the primary coil reach maximum, there is no change in the magnetic flux hence the induced e.m.f falls to zero. When the switch is opened, the current in the primary takes a very short time to fall from maximum to zero. The magnetic flux also takes a very short time to fall from maximum to zero. Since there is a change of magnetic flux, an e.m.f is induced in the secondary coil hence the galvanometer deflects again. The induced e.m.f is higher when the current is switched off than when it is switched on because the current in a circuit takes a very short time to decrease to zero thanto increase to maximum. The work of the rheostat is to control the amount of current.

                                                            Self- induction.

  • Is a process whereby a changing current in a conductor induces e.m.f in the same conductor which induces a current.
  • This second current flows in opposite direction in the conductor. It is called Eddy current.
  • The induced e.m.f is called back e.m.f.
  • The Eddy current opposes the motion of the conductor in a magnetic field and also causes heating effect.

                                                            Application of mutual induction.


  • Is a static electrical device where both input and output are electrical energy.
  • It uses a.c to transfer power from one alternating current circuit to another.
  • Its operation depends on voltage induced in its winding by an alternating magnetic flux set up by the a.c from the a.c source.
  • The induced voltage can vary in magnitude but the frequency remains the same.
  • Transformers operate under th principal of mutual induction and self- induction.
  • There are two types of transformers
  1. Step up transformer.
  2. Step down transformer.

Step up transformer.

  • It raises the input voltage to a higher value.
  • It has more turns in the secondary coil than the primary coil.
  • Its turn ratio is greater than one.

Step down transformer.

  • It lowers the input voltage.
  • It has more turns in the primary coil than the secondary coil.
  • Its turn ratio is less than one.
  • Transformers are never 100% efficient. There is power lost in transformers.
  • There are four ways in which power lost in a transformer.
  1. Resistance of the coil. The coil cannot be assumed to have negligible resistance. Heat is then produced due to resistance.
  2. Loss of magnetic flux between the primary and the secondary winding.
  3. Heating due to Eddy current in the core.
  4. Hysteresis loss in the iron core which get heated by repeatedly magnetizing and demagnetizing the core. The dipoles vibrate 50 times per second.

How to minimize power loss in a transformer.

  1. For resistance, use thick copper wires in the windings since they have low resistance.
  2. Wind the secondary coil on top of the primary coil. This kills maximum flux linkage.
  3. The soft iron core should be laminated. That is, use thin sheets of insulated soft iron plates. Each sheet has a high resistance hence less Eddy current flow on it causing little intake.
  4. Use soft iron core which is easily magnetized and demagnetized.

Uses of transformers.

Stet up transformer.

  1. Used where large voltage is required at low current e.g. transmission line for distribution of electrical energy.
  2. Used in TV’s.
  3. Used in X- rays.
  4. Used in some circuits where high voltage is required.

Step down transformer.

  1. Used where large current is required at low voltage e.g. in welding.
  2. Used in electric furnaces.
  3. Used in lighting system.
  4. Used in transmission line.

Induction coil.

  • Is a device for obtaining a high e.m.f across the ends of the secondary coil by using comparatively low e.m.f in the primary.
  • It consists of;
  1. A primary coil P1 of few turns of insulated wire wound on a soft iron core in form of strips of iron.
  2. A secondary coil S1 of several thousand turns of fine insulated wires wound over the primary coil.
  3. A make and break device or contact breaker X.
  4. A condenser C which helps the action- capacitor.
  5. An accumulator of low e.m.f e.g. 6V.

How it works.

It uses mutual induction. When the circuit is closed by pressing the switch, the primary current magnetizes the soft iron wires or the core which attracts the soft iron armature. The circuit is then open as the armature moves and the magnetic field disappears. The spring pulls the armature and the contact is made again causing the current in the primary to flow magnetizing the soft iron core. The process of switching on and off the primary current is repeated again at high rate. This results in rapid changing of magnetic flux associated with the primary coil. This causes an e.m.f to be induced in the secondary turns. The induced e.m.f is very high since the secondary turns are very many. It is much higher when the primary current is switched off because current takes a very short time to fall to zero then raise to maximum. The high e.m.f causes a spark to jump across the gap as current flows across the air in the gap. The high resistant air offers to the flow of current between two gaps causes a lot of heat to be generated and thus appears as sparks. The capacitor prevents sparks from forming between the contact breaker and discharges when the contact is complete. It allows the primary current and the magnetic flux to decay to zero.



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