CHAPTER-10 –Electromagnetic Induction
1-Marks questions
Q.1. How can you produce electric current without an electric battery?
Ans: Electric current can be produced by electromagnetic induction which is a process in which induced current flows in a conductor whenever there is a relative motion between the conductor and external magnetic field.
Q.2. What do you mean by electromagnetic induction?
Ans: [See Q.1]
Q.3. State Flemings Right hand rule.
Ans: If we stretch the thumb, forefinger and middle finger of right hand so that they are perpendicular to each other, the forefinger indicates the direction of the magnetic field and the thumb shows the direction of motion of conductor. Then, the middle finger will show the direction of induced current.
Q.4. Write some applications of electromagnetic induction.
Ans: Electric generator, microphone, electric guitar etc.
Q.5. How frequently does the direction of current changes in one second if the AC source has frequency of 50Hz (or 50 c.p.s.)?
Ans: 2 x 50 =100 times.
Q.6. Describe/Give the main difference between AC generator and DC generator in construction.
Ans: The output current of the AC generator is obtained through a slip ring and brush assembly. Whereas for DC generator the output is obtained through split ring and brush assembly.
Q.7. Write the colour convention of household electrical wiring.
Ans: Live wire has red insulation, neutral wire has black insulation and earth/ground wire has green insulation. [But, black, white and green colour re used for live, neutral and ground wire in some countries]
Q.8. What is earth wire?
Ans: Earth wire is the wire connects the metallic body of electrical appliances with the ground or earth such that the metallic body is always at zero potential.
Q.9. What are the light power appliances normally used in our home?
Ans: Electric bulb, Fan, TV sets, radio, etc.
[Heater, cooker, refrigerator etc. are categorized as heavy power appliances]
Q.10. What are plug and socket in electric circuit?
Ans: Plug and socket are the electrical arrangements which make an appliance like an electric iron, TV set etc. connect to the electric supply/circuit temporarily.
Q.11. Name two safety measures used in domestic electric circuits.
Ans: Use of fuse and earthing of metallic body of appliances.
Q.12. Name some materials used for fuse wire.
Ans: Pure tin, alloy of tin and copper etc.
Q.13. What is a Genset?
Ans: Genset is an electric generator in which the alternator is driven by an oi-engine.
Q.14. Is it possible to step up DC voltage using transformer? Why?
Ans: No, because transformer work on the principle of mutual inductance, i.e. It is work only with AC.
Q.15. What is the use of a commutator in agenerator?
Ans: A commutator makes the current to flow in a fixed direction in a generator.
Q.16. which device is used to change the AC voltage?
Ans: A transformer.
Q.17. what is the basic difference between thermal power plant and Hydel power plant.
Ans: In thermal power plant, the turbine is driven by high pressure steam and in case of Hydel power plant the turbine is driven by kinetic energy of running water.
Q.18. What is short circuiting?
Ans: Short circuiting is a condition in which the live wire is in direct contact with the neutral wire < or direct contact between +ve and –ve terminal>.
Q.19. What is meant by overloading in an electric supply?
Ans: overloading is condition in which larger current, crossing the permissible level flows in a circuit.
2-Marks Questions.
Q.1. State Flemings Right hand rule. Give some application of electromagnetic induction.
Ans: [For 1st part see, Q.3 of 1 mark type]
[For 2nd part see, Q.4. of 1 mark type]
Q.2. Write the advantages of AC generator over a DC generator.
Ans: i) Energy loss in long distance transmission can be minimized by stepping up the output voltage of AC generator by using a transformer. This is not possible for DC voltage produced by DC generator.
ii) AC voltage can be transformed easily to desired AC voltages where as transformation of DC voltage is complex process.
iii) Conversion of AC to DC is simple in caparison to the conversion of DC to AC.
[the above answer can be use for question like advantages of AC voltage over DC voltage]
Q.3. Explain the function of commutator in DC generator.
Ans: Commutator in DC generator consists of a split ring and brush arrangement. While rotating the armature coil in the external magnetic field, the direction of current in the coil changes twice in a complete rotation. This change in the direction of current is made to flow in a fixed direction in the external circuit by the commutator as in figure (i) and (ii).
Q.4. Describe the earth wire in household electric wiring.
Ans: The earth wire is connected to a large metal plate which is embedded deep in the earth in a moist region near the house. The earth wire carries no electricity as t as at zero potential but it is connected to the metallic body or frame of an electric circuit or appliance to maintain it at zero potential.
Q.5. How do you choose the right wire for an electric circuit?
And: First of all maximum power that will be dissipated in the circuit is calculated to get the maximum operating current value in the circuit. Then the size of wire that could with stand the current is selected for the circuit.
Q.6. What is the purpose of earthing the electrical appliance? Or Many electrical appliances are earthed. What is the reason?
Ans: Earthing the metallic frame or body of an electrical appliance make it at zero potential. So, in case of a line wire touches/current leakage to the frame or body, the current will flows towards the earth and electric shock to anyone who touches the frame or body is prevented.
Q.7. What precautions should be taken to avoid overloading of domestic circuit?
Ans: Overloading occurred in a circuit when excessive amount of current flows in the circuit due to some reason like short circuiting or connection of appliances of power dissipation crossing the permissible value. So, in order to prevent the overloading current in the circuit a fuse is usually connected in series. Right wire size and no of appliances are also other precautions.
Q.8. List the advantages of parallel connection of the domestic appliances in household electric wiring.
Ans: i) The appliances can produce the rated power as they are connected to the same voltage source provided the voltage source produces the rated voltage.
ii) When an appliance is out of order, the other appliances will not be disturbed provided there is no short circuiting condition.
iii) location of fault in the circuit is easier.
Q.9. For domestic wiring give reasons why section are separated.
Ans: Having separate sections in households wiring gives the following advantages:
i)Easy fault finding.
ii)No disturbance to other sections while there is any disruption in a section due torepairing, up gradation or faulty condition.
iii)Prevents overloading s current in distributed according to need.
Q.10. Briefly explain how a nuclear power plant produces electricity.
Ans: In a nuclear power plant, the heat generated in the reactor during reaction is used to produce high pressure steam. This steam drives the turbine. As the turbine connects to the electric generator, the generator produces electricity.
Q.11. A galvanometer is connected to the terminal of a metal rod. The rod falls with its length horizontally extending East-West. What changes will you observe in the galvanometer? Explain the reason.
Ans: A deflection of the galvonometer will be observed. This is because on induced e.m.f. is developed across the conductor, as it falls it cut through the earth magnetic field which is in the south to north direction i.e. changing magnetic filed linkage with the conductor.
3-Marks Questions
Q.1. Write the three results of Faraday’s experiment an electromagnetic induction.
Ans: i) Whenever there is a relative motion between a coil and a magnet an induced current flows in the coil.
ii) Induced current in the coil lasts only as long as there is change in number of magnetic lines of force linking with the coil.
iii) The strength of induced e.m.f is directly proportional to the rate of change of magnetic liens of forces linked with the coil.
Q.2. Write the difference between AC and DC generators.
Ans: i) In AC generator the electric current reverses its directions periodically. But in DC generator the current flows in a fixed direction.
ii) Slip ring and brush arrangement is used for the output AC in the AC generator while split ring and brush is used for the same in case of DC generator.
iv)The brushes are in contact with the two slip rings separately in AC generator. But, in DC generator the brushes are in contact alternately with the split ring which is a single ring consist of split metal plates connected to the terminals of the armature coil.
Q.3. Describe how electricity is distributed from power station to our house.
Ans: the electricity from a power station is transmitted though a long distance at high voltage. It is to minimized the loss of energy in the transmission line in the form of heat. The high voltage is reduced using a step down transformer at our locality. In our house we receive, the electric power at 220V. it is to suit with the domestic appliances commonly used.
Q.4. Describe the colourconvention of household wiring. Explain why the electric appliances are connected in parallel.
Ans: [for 1st part see Q.7 of 1-Mark type; for 2nd part see Q.8. of 2-mark type of question]
Q.5. How a switch is connected in the household electric circuit. Describe the connection of a three pin plug.
Ans: A switch is connected in series with an appliance along the live wire to the appliance. A three pin plug consists of three pins corresponding to a socket. The two small pin is for the connection to the live and neutral wire and the 3rd bigger pin is to connect to the earth wire. The reason for the bigger 3rd pin is not to insult to any of the socket holes for the live and neutral wire.
Q.6. Write about the role of fuse as a safety device.
Ans: The fuse is a piece of wire mode of material with a low melting point. It is connected in series along the live wire. Whenever a high current flows in the circuit due to overloading or short circuiting, the fuse gets heated then melts and disconnected the circuit from the main supply lines. Hence for the damages or other related accidents/hazards are prevented.
Q.7. Whatis meant by overloading in an electric supply? How can it be prevented?
Ans: [See Q.19 of 1-mark type for the 1st part]
[See Q.7. of 2-mark type for the 2nd part]
Q.8. What is short circuiting? Mention the possible consequences of short circuiting.
Ans: [See Q.18 of 1-mark question for 1st part]
Consequences of short circuiting:
i)Overloading current in the circuit.
ii)Large amount of heat produces in the circuit resulting a possible fire accident and complete damage to the circuit.
Q.9. Explain two safety measures commonly used in electric circuit and appliances.
Ans: Earthing and safety fuse are two commonly used safety measures in the electric circuits and appliances.
i)Earthing is to make metallic frames or bodies of circuits or appliances at zero potential such that any leakage current will flow toward the earth preventing possible electric socks.
ii)Safety fuse is to prevent any overloading current by disconnected from the main supply in case there is overloading happens.
Q.10. Write a note on Hydel power plant.
Ans: Hydel power plant is power plant where the kinetic energy of running water is converted into electric energy. Water at the higher level is allowed to fall on the blades of the turbine. The turbine is rotated due to the pressure and the armature coil of the generator rotates at high speed producing the electric energy. [Read other power plant types also]
5-Marks Questions.
Q.1. Explain with labeled diagram, the principal, construction and working of an electric generator.
Ans: Principle: An electric generator is based on electromagnetic induction under this principle rotating coil in magnetic field produces electricity.
Construction: A bases electric generator consist of a rotating rectangular coil ABCB, called armature placed in between the two poles of a magnet (may be electromagnet or permanent magnet. The two ends of the coil are connected to split ring R1,R2 in case of AC generator or connected to split ring S1 x S2 in case of DC generator. The coil and the rings are fixed to a rotatable shaft or axle perpendicular to the magnet field. In order to obtain the out put conducting brushes are made to slide over the ring (S) during the rotation of the coil. The figure shows the basic construction of the generator.
Working: Consider the coil rotate in the clockwise direction. Then by Fleming’s right hand rule the induced current flows along the direction ABCD as in figure. After a half rotation the position of the part AB and CD interchanges and due to the some rule, the current starts to flow in the direction DCBA, i.e opposite to the direction during the 1st half rotation. Thus, there will be two changes in the direction of flow of current in a complete rotation. So, AC will be obtained as output if slip rings and brushes are used and DC will be obtained if split ring and brushes are used.
Q.2. Write a note on Hazard’s of electricity and the precautions.
Ans: Hazard of electricity induces dangerous situation or accident related to electricity, like fire accident du to short circuiting or overloading, electric shocks due to expose circuit, defective switches, damaged wire etc.
In order to avoid such situations and accidents we can do the following precautions:
i)Earthing/grounding of metallic bodies or frames of appliances or circuits.
ii)Use of correct rating fuses.
iii)Use of good quality and properly insulated wires, appliances and tools.
iv)Use of correct current rating wires.
v)Use of rubber gloves while handing or repairing electric circuits.
vi)Not to touch electric circuit or appliances with wet skin or bare footed.
vii)In case of any accident related to electricity, the main switch must be immediately switched off.
viii)Proper knowledge should be acquired before handling electric circuits and appliances.