Electricity and magnetism

 

UNIT 11: ELECTRICITY AND MAGNETISM

Multiple Choice Questions: [1 Mark Each]

1.      What is the conducting path in which current can flow?

a)      Conductor             b) Conducting path                 c) Electric circuit         d) Electric current

2.      What kind of electricity has zero frequency?

a) direct current           b) alternating current               c) lightening                d) Induced current

3.      Write the name of the device shown in the figure.

a) generator                 b) dynamo       c) motor                       d) electric turbine

4.      Who discovered the magnetic effect of electric current?

a) Fleming                   b) Dalton                     c) Orsted         d) Faraday                  

5.      Which instrument change the low voltage AC to high Voltage AC or vice-versa?

a) dynamo                   b) transformer             c) inverter        d) adaptor

6.      Who discovered that if there is change in magnetic field linked with a closed coil, an emf is induced in the wire of the coil?

a) Ampere                   b) Maxwell                  c) Oersted                   d) Faraday

7.      Which of the following power plant use fossil fuel to generate electricity?

a) solar power              b) thermal power         c) wind power             d) hydropower

8.      What is the purpose to use right hand grip rule?

a) to determine magnetic poles of an electromagnet

b) to determine magnetic field of an electromagnet

c) to determine magnetic field of solenoid

d) to determine magnetic field of a circular wire

9.      Which instrument converts electrical energy into kinetic energy?

a) generator     b) electromagnet         c) bicycle dynamo       d) electric motor

10.   Answers: 1(c), 2(a), 3(a), 4(c), 5(b), 6(d), 7(b), 8(c), 9(d)

Very Short Answer Question [1 mark each]

1.      Define electric current.

Flow of electrons in a definite direction is called electric current.

2.      Define magnetic flux.

The number of magnetic lines of forces passing perpendicular through a conductor is called magnetic flux.

3.      Who propounded the law of electromagnetic induction?

Michael Faraday propounded the law of electromagnetic induction.

4.      What is motor effect?

When electric current is passed through a conductor carrying electric current is placed in magnetic field, motion develops in conductor. This effect of electric current is called motor effect.

5.      What is electric motor?

Electric motor is a device that converts electric energy into kinetic energy.

6.      What is solenoid?

A spirally coiled insulated copper wire closely wrapped in the shape of a cylinder is called solenoid.

7.      State the right hand’s thumb rule.

The right hand’s thumb rule states that if a solenoid is gripped by right hand in such a way that the fingers are in the direction of the current flowing through the wire, the thumb will point towards the North Pole.

8.      What is the working principle of transformer?

The working principle of transformer is mutual induction which states that when a changing magnetic field in one circuit induces an electromotive force (EMF) or voltage in another nearby circuit.

9.      Mention the major use of transformer.

Transformers are used in voltage regulators of computer, television, air conditioners, radios, trolley buses, television, etc. They are used to increase the voltage in power plants and decrease the voltage at sub-power stations.

10. Which instrument can converts kinetic energy into electrical energy?

A generator or dynamo converts kinetic energy into electrical energy.

11. Define electromagnetic induction.

The process of inducing the electric current or e.m.f. in a closed coil due to the relative motion between the magnet and the coil of conductor is called electromagnetic induction.

12. State the principle of mutual induction.

The principle of mutual induction states that ‘when the alternating current is passed through a coil, an induced e.m.f. is produced in the neighbouring coil’.

13. Which type of transformer is used in power stations?

Step up transformer is used in power stations.

14. Write the two laws of transformer.

15. The two laws of transformer are:

a) Input power = Output Power i.e. I₁V₁ =I₂V₂

b) EMF is directly proportional to the number of turns in coil

16. Name the transformer which converts high voltage a.c. into low voltage a.c.

Step down transformer converts high voltage ac into low voltage ac.

17. What is the commercial unit of electricity?

The commercial unit of electricity is Kilowatt hour.

18. The frequency of the power supply in the domestic circuit is 50 Hz. What does it mean?

19. The frequency of power supply in the domestic circuit is 50 Hz. It means that the polarity of alternating current in the domestic changes 50 times per second.

20. What is alternating current?

The electric current which changes magnitude and polarity is called alternating current (AC).

21. Name any two sources of alternating current.

Two sources of alternating current are power plants and generators.

22. Name any two devices that use AC to work.

Two devices that use AC to work are electric motor and transformer.

23. Name two devices that uses DC to work.

Two devices that uses DC to work are: LED lamp and electric vehicles.

24. Define step up transformer.

A transformer which changes low voltage AC to high voltage AC is called step up transformer.

25. Name the substance used to laminate the core of transformer.

The substance used to laminate the core of transformer is varnish or salac.

Short Answer type questions: [2 marks]

1.      A hydropower plant is better than thermal power plant. Give reason.

A hydropower plant is better than thermal power plant because of the following reasons:

a)      A hydropower generates electricity using water where as a thermal power plant consumes fossil fuel to produce electric current.

b)      The operating cost of hydropower plant is low where as operating cost of thermal power plant is very high as it need fossil fuel which has to be purchased at high cost.

c)      Hydropower plants are more reliable as they are not affected by change in weather condition or price of fossil fuel.

2.      Why is the core of transformer made up of a number of thin iron plates?

The electric current in the core of the transformer moves in spiral way as a result there is more chance of production of heat due to the formation of eddy current. Eddy current opposes the change in magnetic field. So in order to reduce heat, the core of transformer is made up of number of thin iron plates

3.      The core of transformer is laminated with varnish, why?

The core of transformer is laminated with varnish to prevent eddy currents from flowing between the lamination, which would cause heat loss. Varnis also prevent rust and corrosion and prevent lamination from deforming.

4.      Step up transformer are used in power stations, why?

The electricity produced at the power station is 19500 V. It needs to be transmitted over a very long distance where there is loss of electric potential. It needs to up raise up using step up transformer up to 33000V-132000 V to transmit to very long distance.

5.      Step down transformer are used in streets. Give reason.

The electricity supplied from the sub-power station is 11,000 V which needs to be lowered to 220V for the domestic and industrial purpose. In order to lower the voltage step down transformers are used in streets.

6.      Electric motor and generator are opposite to each other. Give suitable reason.

Electric motor and generator are opposite to each other because of the following reasons:

a)      Electric motor works on the principle of motor effect while generator works on the principle of electromagnetic induction.

b)      Electric motor converts electrical energy into kinetic energy where as generator converts kinetic energy into electrical energy.

7.      Why do the number of turns in primary coil and secondary coil never be equal?

A transformer is a device which is used to change the AC voltage. As the magnitude of the induced EMF (voltage) in secondary coil is directly proportional to the ratio of secondary turns to that of the primary turns. If the ratio of number of turns in primary and secondary coil are made equal, there won’t be change in induced EMF. So the number of turns in primary and secondary coil are never made equal. 

8.      A transformer cannot alter DC, why?

A transformer works on the change in magnetic flux through primary coiling. There is a continuous change in the magnetic flux in AC but DC cannot produce the varying magnetic flux and hence there is no production of induced EMF in secondary coil. So, a transformer cannot alter DC. 

9.      The use of alternating current would be limited if transformer was not invented. Give reason.

Transformer is a device which converts high voltage AC to low voltage AC and vice-versa. The use of alternating current would be limited, if the transformer was not invented because of the following reasons.

a)      The conversion of low voltage AC into high voltage AC in the power station would not be possible. Therefore, the electricity would be available at the area near by the power plants.

b)      It would be impossible to have different electrical appliance that runs using varying voltage.

10.  The direction of magnetic lines of forces changes when polarity is changed. Give reason.

The direction of magnetic lines of forces changes when polarity is changed. This is because the magnetic field lines are created by the moving charges in the conductor. When the direction of current is reversed, the direction of motion of the charges is also reversed, which in turn reverses the direction of the magnetic field lines.

11.  Slip rings are used in AC generator but split rings are used in DC generator, why?

Slip rings are used in AC generators to allow the output voltage to be varied. The slip rings are made of a number of metal rings that are connected to the rotating shaft of the generator. The output voltage is applied to the slip rings, and the brushes are used to make contact with the rings. As the shaft rotates, the brushes make contact with different rings, which in turn varies the output voltage. Split rings are used in DC generators to convert the output voltage from AC to DC. The split rings are made of two metal rings that are connected to the rotating shaft of the generator. The output voltage is applied to the split rings, and the brushes are used to make contact with the rings. As the shaft rotates, the brushes make contact with the different rings, which in turn reverses the polarity of the output voltage. This results in a DC output voltage.

12.  Magnetic lines of forces do not intersect each other. Why?

Magnetic lines of force do not intersect each other because magnetic field lines represent the direction of the magnetic field at a point. If two magnetic field lines intersected, then at the point of intersection, the magnetic field would have two directions, which is not possible.

13.  A magnetic needle show deflection when brought near an electric wire carrying electric current. Give reason.

An electric wire carrying electric current develops magnetic field around the wire. The magnetic field of the wire interacts with the magnetic field of the needle, causing the needle to deflect. So, a magnetic needle show deflection when brought near an electric wire carrying electric current.

14.  A stationary magnet does not induce a current in a coil. Why?

An induced EMF is generated in the coil of wire only when there is change in the magnetic flux. When a magnet is kept stationary, there is no change in the magnetic flux, as a result no induced EMF in the conductor.

15.  The brightness of the bicycle dynamo bulb increases when the speed of the bicycle increases. Suggest reason.

When the speed of the wheels of the bicycle increases, then the amount of induced current (e.m.f.) is increased. So, the brightness of the bicycle dynamo bulb increases. It is because of the amount of induced current is directly proportional to the rate of change in magnetic flux linked with the closed circuit which occur in bicycle dynamo by the rotation of strong magnet inside the coil of wire.

16.  Differentiate between Electric motor and generator.

Differences between electric motor and generator are:

Electric motor	Generator
1. It changes electrical energy into mechanical energy.	It changes mechanical energy into electrical energy.
2. It works on the principle of motor effect.	It works on the principle of electromagnetic induction.
3. The direction of the motion of conductor carrying current is given by Fleming's Left hand rule.	The direction of the motion of conductor carrying current is given by Fleming's Right hand rule.
4. It consumes electricity.	It generates electricity.

 

17.  Mention the differences between step-up transformer and step down transformer.

Differences between step up and step down transformer are:

Step up Transformer	Step down transformer
1. It increases the Voltage of AC	It decreases the voltage of AC
2. The number of turns in primary coil is lesser than secondary coil.	The number of turns in primary coil is more than secondary coil.
3. It is used in electric power station or grids to save electrical energy during power transmission through overhead wire.	It is used at power sub-station to step down the voltage before its distribution to the consumers.

 

18.  Write the differences between dynamo and generator.

The differences between dynamo and generator are:

Dynamo	Generator
1. It produces electricity on small scale.	1. It produces electricity on large scale
2. It contains permanent magnet.	2. It contains electromagnet.
3. Magnet rotates near the coil.	3. Coil rotates in a magnetic field.