Mutual induction is a phenomenon in physics where a changing magnetic field in one coil of wire induces a voltage across another nearby coil. When the magnetic field passing through one coil changes, it induces a voltage in the other coil, according to Faraday’s law of electromagnetic induction.
This principle is fundamental to the operation of transformers, where two or more coils of wire are used to transfer electrical energy from one circuit to another by means of a changing magnetic field.
Mutual induction is a key concept in the functioning of many electrical devices and is utilized in various applications, including power transmission, signal coupling, and wireless power transfer.
Demonstrating mutual Induction
- To investigate mutual induction, set two coils close to each other as shown
Using the variable resistor, put the current to minimum and observe the behavior on the galvanometer from the following actions:
- closing the switch K
- opening the switch K
- Increasing the current using variable resistor when the switch is closed
- decrease the current using variable resistor when the switch is closed
- replace the D.C power supply with A.C power supply.
Observations
when the switch K is closed, the pointer deflects in one direction and then comes back to zero. When K is opened, the pointer deflects to the opposite direction and then falls back to zero.
Increasing current in the primary coil causes deflection while decreasing it causes deflection to the opposite direction.
If direct current (d.c) is replaced with an alternating current source, the pointer of the galvanometer vibrates continously about the zero point.
Explanations
When the switch is closed, the current in the primary coil increases from zero to maximum value within a very short time and so the magnetic flux in the primary coil linking with the secondary coil increases from zero to maximum at the same interval of time inducing an e.m.f in the secondary coil.
The induced emf in the secondary coil causes flow of current hence deflection on the galvanometer.
The induced e.m.f lasts only for a period where current is transiting from zero to maximum and so the pointer on the galvanometer returns to zero after a very short time. This is because after the current in primary coil builds up to it’s maximum value, there is no further change in magnetic flux in the primary coil and so induced e.m.f in the secondary coil stops.
When the switch is opened, the current in the primary coil takes a very short time to fall from maximum to zero hence the magnetic flux in the primary coil linking with the secondary coil also falls from maximum value to zero inducing an e.m.f in the secondary.
The current in the circuit takes much shorter time to fall off from maximum to zero than it takes to build up from zero to maximum and therefore induced e.m.f is much higher when current is being switched off than when it is switched on.
When the current is increased continously, the magnetic flux in the primary coil which links with the secondary coil also increases at the same rate causing an e.m.f to be induced in the secondary.
When current in the primary is decreased continously, an e.m.f is induced in the secondary due to the decreasing magnetic flux of the primary coil linking with the secondary coil
The direction of current in the secondary coil is to the opposite direction to that of primary coil so that the polarities in the secondary and primary coil are such that they oppose each other. The direction of current on each coil can be determined by Lenz’s law.
When the switch is closed and current is building up, the direction of current in primary and secondary coil is as illustrated below:
when the current is decaying after switch is opened, the direction of the induced e.m.f in the secondary coil is as shown.
most often the e.m.f induced in the secondary coil is less than what was produced in the primary coil. This is mostly because of what is called flux leakage where all the magnetic flux from primary coil does not link with the secondary coil.
The induced e.m.f in the secondary coil can be increased by ensuring more flux from primary coil are linking with the secondary coil.Some of the techniques used includes:
i. Winding primary and secondary coil on a soft magnetic flux
In this method, the primary and secondary coil are linked together on one soft iron which helps to concentrate magnetic flux in both coils. Typical arrangement is as shown.
In the above arrangement, both primary and secondary coil are wound on the same iron core.
ii. Both secondary and primary coil on same soft iron ring
Magnetic flux tends to form circular loop and so a circular ring makes them work better . The arrangements is as illustrated.
The ring enables all the magnetic flux of the primary to form concentric loops within it thus reaching the secondary coil more efficiently.
iii. Having more turns in the secondary coil
we saw earlier that the total induced e.m.f in the coil is the summation of all the e.m.fs induced by individual turns in the coil. When there are more turns in a coil, it then means there will be more e.m.f in that coil. Consider the setup illustrated below.
The e.m.f is induced in each turn of the secondary coil since the magnetic flux of the primary coil links with each .
Related Topics
- Mutual Induction
- The Lenz’s law
- Fleming’s Right-hand rule
- Factors affecting magnitude of the induced e.m.f
- Hans Oersted
- Calculus Integration
- Induced Electromotive force
- Magnetic effect to an electric current
References
- Secondary Physics Student’s Book Four. 3rd ed., Kenya Literature Bureau, 2012.
- Tom D., and Heather K. Cambridge IGCSE Physics. 3rd ed., Hodder Education, 2018.
- Abbot A. F. (1980), Ordinary Level Physics, 3rd Edition, Heinemann Books International,
London. - Nelkon M. and Parker P., (1987), Advanced Level Physics, Heinemann Educational
Publishers, London.
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