# Mutual Inductance and Transformer Physics Short Questions

## Mutual Inductance

In this post, we have discussed short questions and answers of Mutual Inductance and transformer.

#### 1. A coil is wound on an iron core and looped back on itself so that the core has two sets of closely wound wires in series carrying current in the opposite direction. Why ?

A coil is wound on an iron core and looped back on itself so that the iron core has two sets of closely wound wires in series carrying current in the opposite direction because the self-inductance will be small due to cancellation of inductive effects. It is because the two sets of loops have the same current but in opposite directions and they have the self-induction in opposite direction.

#### 2. what is meant by mutual induction?

When two coils are placed close to each other and a varying current is passed through one of them, then the magnetic flux passing through the second coil will be changed due to a change in flux in the first coil which induces emf in the second coil. The phenomenon according to which an opposing emf is produced in a coil as a result of a change of current or magnetic flux linked with its neighboring coil is called Mutual Induction.

#### 3. On what factor does the mutual inductance between a pair of the coil depends?

The mutual inductance between a pair of the coil depends upon the following factors:

(a) The number of the primary coils and the number of secondary coils.

(b) The relative orientation of the coils.

(c) The permeability of the material of the core.

(d) The rate of change of current in the primary coil.

(e) The cross-sectional area of the coil.

#### 4. Define the coefficient of mutual induction for a pair of coils.

The property of two coils by virtue of which each opposes any change of current flowing in the other is called Mutual Inductance between the coils. We know ϕ   I where ϕ and I be the magnetic flux through the secondary coil and the current through the primary coil respectively.

or ϕ = MI

where M is a proportionality constant and is known as the coefficient of mutual inductance. M=ϕ/I Hence the coefficient of mutual inductance of two coils is numerically equal to the magnetic flux linked with the coil when unit current flows through the primary coil. Mutual inductance is measured in Henry in the SI system.

#### 5. How mutual induction is produced? Explain.

If a current is passed through primary coil P, a magnetic flux is set and a part of it links with secondary coil S. If the current in P is changed magnetic flux also changes, and hence an emf is produced in the secondary coil which is indicated by a galvanometer. The emf thus induced secondary coil is termed mutually induced emf and the process is called mutual induction. This induced emf also opposes the change in current in the primary coil.

#### 6. What are the advantages of a.c. over d.c.

A.c. has the following advantages over d.c. (a) A.c. can be transmitted at distant places without much loss in a.c. power but d.c. cannot be transmitted in this way. (b) Alternating voltages can be easily increased or decreased by using a transformer. (c) The generation of a.c. is economical than the generation of d.c. (d) A.c. can be easily converted into d.c. by the use of a rectifier.

#### 7. What is Fleming’s right-hand rule?

Fleming’s right-hand rule is used to determine the direction of induced emf (current) in a conductor. It states that if the thumb, the middle finger, and the forefinger of the right hand are stretched mutually perpendicular to each other in such a way that the thumb points in the direction of motion of the conductor and the fore-finger points in the direction of the magnetic field, the middle finger points in the direction of the induced emf in the conductor.

#### 8. A lamp connects with a coil of a larger number of turns and a battery in series does not light up to full brilliancy instantly on switching on the circuit. Why.

When the circuit is switched on, the current start growing through the coil at a rate (dl/dt), and a growing magnetic field is set up in it. It in turn produces an induced emf in the coil in the direction such that it opposes the growth of current in the circuit. As a result, the lamp glows less bright for an instant. When the current grows up to its maximum value, the current becomes constant and hence dl/dt becomes zero. The lamp then glows with full brilliancy as there is no induced emf in the coil.

#### 9. Two identical rings, one of the metallic and another non-metallic, are dropped from the same height. Which will reach the ground earlier?

There is a horizontal component of the earth’s magnetic field everywhere around the rings. When the metallic ring falls, it crosses the earth’s magnetic field. As a result, an induced emf is produced on it which opposes its downward motion. But, when a non-metallic ring falls, there is no induction in it, therefore falls faster than the metallic ring.

## Mutual Inductance Units

#### 10. What is the flux meter?

A flux meter is an instrument that can be used for measuring magnetic flux and hence magnetic field strength directly. It is a modified moving coil galvanometer having negligibly small mechanical damping and restoring couple but large electromagnetic damping.

#### 11. What is a transformer? On what principle does it work?

An electrical device that is used to convert low alternating voltage at high current into high alternating works g voltage at low current or vice-versa. It on the principle of mutual induction which states that if two coils are inductively coupled and when the current or magnetic flux is changed through one coil, then emf is induced in the second coil.

#### 12. Define step up and step down transformer. OR Distinguish between step-up and step-down transformer.

Step-up transformer – A transformer that covers low alternating voltage at high current into a high alternating voltage at low current. It has more number of turns in the secondary coil than in the primary turn. The primary coil is made of thick insulated copper wire whereas the secondary coil is made of then insulated copper wire. Step-up transformer is symbolized as Step down transformer – A transformer which converts in the primary coil. The high alternating voltage at low current into a low alternating voltage at high current. It has fewer turns in secondary coil than secondary coil is made of thick wire made of copper and the primary coil is made of thin copper wire.

#### 13. What is the ideal transformer?

A transformer which does not make any kind of energy loss is known as an ideal transformer. Here the input power is always equal to the output power and its efficiency is always 100%.

#### 14. We can not change the value of dc by the use of a transformer. Why?

When a d.c. the source is connected to the transformer, it provides a steady current to the primary coil and the magnetic field produced by it will also be steady. So there will be no mutual induction taking place between the primary and secondary coils which are necessary to change the value of current or voltage in output than in input. Also, the back emf will not be produced. So current will be maximum in the primary coil only which produces a large amount of heat in it. It can destroy the coil. Hence we can not change the value of dc by the use of a transformer.

#### 15. Write down the principle of a transformer.

A transformer works on the principle of mutual induction. It states that if two coils are inductively coupled and when current or magnetic flux is changed through one of the two coils, then emf is induced in the other coil.

#### 16. Why should the core of a transformer made of magnetic material of high permeability?

When the core of the transformer made of material of high permeability, the magnetic lines of force will crowd through the core, and the leakage of magnetic flux between the coils will be less which makes the magnetic flux through the secondary coil maximum and the energy loss due to the flux leakage will be minimum.

#### 17. In a step-up transformer, the primary coil is made from a thick wire. Why?

In a step-up transformer, the primary coil has less voltage than in the secondary coil and the current in the primary coil should be more than that in the secondary coil. So the resistance of the primary coil should be less than 1 and to get the resistance low, it should be thick since R A.

#### 18. Why is soft iron core whose hysteresis loop is narrow is used in making the core of a transformer?

When alternating current is passed through a transformer, the magnetization and demagnetization of the soft iron core take place. During this process, some amount of energy will be lost in a complete cycle which is equal to the area of the hysteresis loop. So if the soft iron core of a narrow hysteresis loop is used, the energy loss will be less. Hence to minimize the loss of energy, it is used in a transformer.

#### 19. What are eddy currents? Write down its uses. [HSEB 2063]

When a metal e.g. soft iron block is placed in a varying magnetic field, it induces a current in the metal whose direction is given by Lenz’s law. This induced current is called the Eddy Current or Foucault’s Current. Since this current opposes the cause which produces it, energy is lost in the form of heat and it can be reduced by laminating that metal. But it is useful for many purposes. They are used in

(a) Electromagnetic damping

(b) Induction motors

(c) Eddy current damping.

(d) Energy meters

(e) Induction heating

#### 20. Why is the core of a transformer laminated?

In electrical machines, metallic cores are used to increase magnetic flux but the changing magnetic flux produces large eddy currents and the cores are heated due to the flow of large eddy currents. The heat so produced is likely to damage the internal insulation besides causing loss of energy. To reduce the eddy currents, the cores are split into thin insulated sheets into planes parallel to the magnetic field. As the insulating layers offer very high resistances and the area of the conductor is reduced, the eddy currents set up are greatly reduced.