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ELECTROMAGNETIC INDUCTION (Part 2)Notes

1. What is mutual induction?Consider two insulated coils of wire kept side by side. When the direction of the current in the primary coil changes, the magnetic flux around it changes. As a result, an emf is induced in the secondary coil. This phenomenon is the mutual induction.

2. Give an experiment for showing mutual induction.As shown in Fig., a few turns of finely insulated copper wire are wound at each end of a soft iron core (approx. 500 turns). The ends of one of the insulated copper wire are connected to a cell and a switch. The insulated copper wire which is wound on the other end of the soft iron rod is connected to a bulb.Primary coil Secondary coil

When the switch is turned on the bulb starts to glow. Magnetic flux is formed around soft iron core when current is passed through the coil. The coil into which we give current for the production of magnetic field is the primary coil and the coil in which induced emf is generated is the secondary coil.When an AC passes through the primary, a varying magnetic field is formed in and around the soft iron core. The secondary is situated in this varying magnetic field. The flux change produced in the secondary coil induces an emf in it. This phenomenon is the mutual induction.

3. What is a transformer?Transformer is a device for increasing or decreasing the voltage of AC by the principle of mutual induction (Electromagnetic induction) without any change in the electric power.

4. Which are the two types of transformers?Step up and step down transformers.

5. What is the working principle of transformer?Mutual induction. When there are two nearby coils the strength or direction of the current in one coil changes, the magnetic flux around it changes. As a result, an emf is induced in the secondary coil. This phenomenon is the mutual induction.

6. Distinguish between step up transformer and step down transformer with the help of diagrams. step up transformer step down transformer

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7. Write the mathematical expression relating number of turns, voltages and current in

transformer.If Vs is the secondary voltage, Vp is the primary voltage, NS is the number of turns in the secondary and NP the number of turns in the primary in a transformer then Vs/Vp = Ns/Np

In a transformer, if the voltage of the primary is VP and the current in the primary is IP, voltage in the secondary is Vs and the current Is, then the formula connecting them is Vs/Vp = Ip/Is

8. Do the problems given in Page 57 & 58 of TB.9.

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10. What is meant by self-induction?The change in magnetic flux due to the flow of an AC in a solenoid will generate a back emf in the same solenoid in a direction opposite to the applied emf. This phenomenon is known as the self-induction.

11. Give an experiment to show self-induction.

To Demonstrate Self-Induction (Back emf):

Apparatus:6 V AC power supply, coil of wire with 1200 turns, soft iron core, 6 V filament lamp.

Procedure:1. Connect the bulb, coil and AC supply in series.2. Switch on the power supply. The lamp lights.3. Insert the iron core into the coil. The lamp becomes dimmer.

Explanation:The a.c. produces a changing magnetic field in the coil. This induces a back emf that opposes the applied current. The iron core increases the magnetic flux and hence the back emf is increased. The resultant current in the circuit is reduced and the bulb becomes dimmer.

Note:

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If this circuit is set up using a D.C. power supply, no dimming occurs with the core in the coil as there is no changing magnetic field. So back emf is not induced.

12. What are inductors?Inductors are coils used to oppose the changes in electric current in a circuit. They are used to reduce current in a circuit to the desired value without loss of power.

13. Inductors are widely used in AC circuits. Why?Current can be reduced without any loss of electrical energy in the form of heat.

14. If resistors are used instead of inductors, what will be the disadvantage?Electrical energy will be wasted in the form of heat.

15. Inductors are not used in DC circuits. Find out the reason.As the current is unidirectional there is no changing magnetic field. So back emf is not induced.

16. What is a moving coil microphone?A device that changes sound energy into electrical energy by using the principle of electromagnetic

induction.

17. Explain the working of a moving coil microphone.The voice coil is situated in a magnetic field. The diaphragm connected to the voice coil vibrates in

accordance with the sound waves falling on it. As a result, electrical signals corresponding to the sound waves are generated in the voice coil. In the microphone, mechanical energy is converted into electrical energy

18. What is a power station?Place where electricity if produced on a large scale for the purpose of distribution.

19. Distinguish between single phase AC generator and three phase AC generator.Single Phase Generators have only one set of coil in between the poles of a field magnet. Three Phase Generators are used for the large scale production of electricity.In three phase generators, three armature coils are arranged around the field magnet at an angular

separation of 120o. When the field magnet rotates, three alternating currents of different phases are generated simultaneously in the three armatures. In each armature, maximum and minimum emfs are generated at different instances. Such generators are three phase generators.

20. What is transmission loss?When electricity is transmitted to distant places there is loss of energy in the conductors in the form of

heat. This is known as transmission loss.21. Explain the problems of long distance transmission of power and their remedies.As electricity is transmitted over long distances, there is inherent energy loss along the way. Energy loss:Reasons: i) Intensity of current. ii) Resistance of the transmission lines

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High voltage transmission minimizes the amount of power lost as electricity flows from one location to the next. The higher the voltage, the lower the current. The lower the current, the lower the production of heat in the conductors (H=I2RT).

Select thick wires of low resistance so that conversation of power as heat will be reduced.

22. Briefly describe the process of power transmission from the power house to distant places where it is consumed.

The voltage at the power station initially is 11000V (11KV). The voltage is increased up to 220 KV at the power station itself by using step up transformer. Then reduced into 110KV and 66 KV at different substations. As a result the current and loss of energy in the form of heat decreases. Later the voltage is lowered at different stages of power transmission and electricity is made available to the distribution transformer at 11 kV.

230 V required for house hold purposes is made available by distribution transformers. 400 V needed for industrial purposes are also obtained from distribution transformers.

23. If a person standing on the earth touches a phase line, will she get an electric shock? Why?The live wire (phase line) is at a high potential but it is not at zero potential.On the other hand the neutral wire is at zero potential and always remains at that potential.Also remember that your feet are touching the ground (earth) which is also at zero potential.Now when you touch the live wire one end of your body is at a non-zero potential while the other end is at zero potential this causes electrical current to flow through your body giving you a shock whereas when you touch a neutral wire almost no current flows through your body hence you do not get a shock.

24. Copper wires of the same length and thickness are connected to points A and B in all the three circuits. In circuit (a) copper wire is not coiled. In circuits (b) and (c), the copper wire is coiled.Observe the circuits and answer the following questions.

a) When circuit (a) is on, what do you observe?b) When circuit (b) is switched on, what change do you observe in the intensity of light? Justify

your answer.c) When circuit (c) is switched on, what change do you observe in the intensity of light? Justify your

answer.Ans a)The bulb glows.b) There is no change in the intensity of light because here cell is used and DC is passed through

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the inductor. There is no flux variation and back emf.c) The intensity of light decreases. Here AC is used. There is a variation of magneticflux linked with the coil. As a result back emf is produced.

25. Thick insulated copper wires are used in the primary coil of a step up transformer and in the secondary of a step down transformer. What is the necessity of this?

Ans: The power in the primary and secondary coils of a transformer is the same. So more current flows in the primary Coil of a step up transformer and secondary coil of a step down transformer. So to withstand the heat thick wires are used.

26. Which situation causes short circuit?Ans: Touching of the live wire and neutral wire directly is known as short circuit. (In power

transmission it is the touching of two phase lines or one phase and neutral line.)27. What is the role of earthing wire in a household circuit?Ans: The earthing of electrical appliance is used as a safety measure. If by chance the live wire

touches the metal case of the electric appliance having metal body which has been earthed, then the current passes directly to the earth through the earth wire. It does not need our body topass the current and, therefore, we do not get an electric shock. Actually, very heavy current flows through the earth wire and the fuse of household wiring blows out or melts. And it cuts off the power supply, In this way, earthing saves the electrical appliance and us from damage due to excessive current.

28. Why do we say that metallic devices should be earthed?Ans: We have receive electric shock from metallic electrical devices. To avoid therisk of electric shocks, the metal body of the electrical appliance is earthed. Large appliances like motor are earthed directly.

29. An electric heater calibrated 1.5 kW, 230 V is connected to a house hold branch circuit having 5A fuse wire and is made to work. What will happen?Ans: The current flows through the circuit due to the working of this appliance, I = P/V = 1500/230=6.52 A. So the fuse melts.

30. Which are the devices connected in series in a household circuit? energy in schools and houses?Ans: In household circuit fan, switch, fan regulator etc. are connected in series

31. What can be done to save electricalAns: Switch off lights, fans, TV and other such electrical appliances when not need-ed. Does not open and close the refrigerator door frequently. Close the tap after use.Use LED instead of incandescent and CFL lamp.

32. Why do some mobile phones use three pin plugs?Ans: In mobile phone charger there IS a plastic pin in place of the earth pin. For safety measures, phase and neutral sockets has lids. Only when the pin enters into the earth socket, the other sockets will be opened. It is difficult to connect two pin in these sockets. To solve the problemmobile phones are use three pin plugs.

33. Briefly explain about MCB and ELCB.

MCB (Miniature Circuit Breaker), ELCB (Earth Leakage Circuit Breaker)

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MCB is a device that is used in the place of a fuse wire branch circuits. MCB automatically breaks the circuit whenever there is an excess flow of current due to short circuit or overloading. After rectifying the circuit we can switch on the MCB and make the circuit as it was. MCB works making use of heating and magnetic effects of electricity.

ELCB helps to break the circuit automatically whenever there is a current leak due to insulation failure or any other reason. Hence a person touching the electric Circuit or a device does not get an electric shock. Nowadays RCCB, which ensures more safety than ELCB is made use of.

34. What are the differences between ordinary fuse and MCB?

In ordinary fuse, when the current in the electric circuit rises too much then the fusewire melts and breaks. MCB automatically breaks the circuit whenever there is an excess flow of current due to short circuit or overloading. After rectifying the circuit we can switch on the MCB.35. What is the function of ELCB/RCCB (residual current circuit breaker) in the circuit?

To protect household wiring from the excessive flow of electric current through it.

36. How many joules make one kilo watt hour?

1 KWh = 3600000 J

37. Write the equation to calculate the electrical energy in kilo watt hour.

E = (P T) /1000

38. 4 four 100 W bulb are used for 5 hours and an iron box of 1000 W used for one hour in a house. Calculate the electrical energy.

Electrical energy = (P T) / 1000 = (100x4x5+1000x 1x1) / 1000 = (2000+1000)/1000 = 3 kWh

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