Current Electricity

S.S.C CURRENT ELCTRICITY

Omtex – classes “THE HOME OF TEXT”

S.S.C NOTES

QI. Define the following. 1. One coulomb : It is the unit of electric charge. If two like point charges of equal

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magnitude are placed in a vacuum at a distance of one meter from each other and if they repel each other with a force of 9 x 109 Newton, then each charge is called one coulomb. One Volt : It is the unit of potential difference. The potential difference between two points is said to be one volt if one joule of work is done to carry a charge of one coulomb from one point to another. One Ampere : It is the unit of electric current. If a charge of one coulomb passes across any section of the conductor in one second, then the current in the conductor is said to be one ampere. Electromotive force (E.M.F) : The work done by a cell to send a unit charge once around the complete circuit is called a electric motive force. Potential difference : The work done in sending a unit charge through the external resistance is called the potential difference. Right hand rule: Imagine the conductor to be held in the right hand with the fingers curled round it and the thumb stretched along its length. If the thumb points in the direction of the current then the curled fingers denote the direction of the magnetic field.

QII. Give Scientific reasons: 1. Metals are good conductors of electricity. Or An electric current can flow through silver wire. Ans. 1. Metals contain a large number of free electrons. 2. When potential difference is applied to the ends of the wire the free electrons move from low potential to high potential. 3. This gives rise to a flow of electric current in the wire. Hence metals are good conductor of electricity. 2. When electric current is passed through a wore, it gets heated after some times. Ans. 1. When Potential difference is applied to ends of wire the electrons start moving. 2. The electrons collide with atoms of the wire. 3. Due to these collisions the kinetic energy of atoms increase and the thermal energy also increases. Hence, the wire gets heated after some times. 3In an electric bulb, a thin filament of tungsten is used. Ans. 1. Tungsten filament has a high melting point. 2. When current passed through the filament, it gets heated to a high temperature and emits light. Hence, thin filament of tungsten is used in an electric bulb. 4. Water must not be used to extinguish the electric fire. Ans. 1. Tap water is impure. 2. It conducts electricity. 1

S.S.C CURRENT ELCTRICITY

Omtex – classes “THE HOME OF TEXT”

S.S.C NOTES

3. If water is used to extinguish the fire then electric current will flow. Therefore, Water must not be used to extinguish the electric fire. 5. Wire carrying current is always insulated. Ans. 1. If a metal wire carrying current is not insulated, it may cause a fatal shock to the person toughing it. 2. When live and neutral wire come in contact with each other, it may cause fire. 3. When a bare wire carrying a current comes in contact with wet wall, the current starts flowing through the wall. If a person touches the wall, he gets a severe shock, hence, wire carrying current is always insulated. 6. Three Pin plug is preferred in place of two pin plug. Ans. 1. In two pin plug, there is no provision for earthing. Hence, the person using electrical appliances like an electric iron, heater, etc. may get shock if the body of the appliance becomes live due to the leakage of electric current. 2. In a three pin plug, the third wire is connected to the metal body of the appliance and to the earthing wire with the help of the socket. 3. Hence, the person handling the appliance does not get shock even if the live wire comes in contact with the metal body of the appliance. Therefore, a three pin plug should be used instead of two pin plug. 7. Ans. 1. 2. 3.

An electric fuse is used in domestic supply.

An electric fuse is used to protect electrical appliances and instruments from damage. An electric current up to a certain maximum value can pass through it. If the current exceeds this value, the wire melts due to the heat produced in it by the current. As soon as the wire melts the circuit is broken and hence the current stops flowing. 4. An excessive current does not flows through the electrical appliances connected in the circuit. Hence, an electric fuse is used in domestic supply.

8. The potential difference of cell is always less than the E.M.F of the cells. Ans. 1. E.M.F of a cell is the total energy required to over come both the internal resistance of the cell and the external resistance in the circuit. 2. Potential difference is the energy required to over come only the external resistance in the circuit. 3. Hence, the potential difference of cell is always less than the E.M.F of the cells. 9. What are the general precautions while handling electrical devices? Ans. 1. Keep your hands dry while handling electrical devices 2. Rubber soled footwear should be weared while handling electrical device. 3. Before cleaning an electrical device, switch off the current and remove the plug from the socket. 4. Beware of live wire whether bare or insulated. 5. Do not remove the plug by pulling the cord. 6. Incase of a fire caused by electric sparks, switch off the mains and use a fire extinguisher like Co of dry sand. Do not use water to extinguish electric current. 2

S.S.C CURRENT ELCTRICITY

Omtex – classes “THE HOME OF TEXT”

S.S.C NOTES

10. Describe the working of an electric bell with a neat labeled diagram. Ans. 1. The electric bell consists of an electromagnet gong, armature (i.e.) soft iron rod, contact screw and spring. 2. When current is passed through the circuit, the Electromagnet acquires magnetism and attracts the iron rod as a result of which the gong is struck by the hammer and the bell rings. 3. At the same time the contact between the contact screw and iron rod is lost and circuit is broken. When the current stops the electromagnet losses its magnetisms and the soft iron rod goes to its original position. 4. As the iron rod touches the contact screw, the circuit is complete the procedure is repeated and the gong is struck again. The bell goes on ringing as long as the switch is closed. 11. What accidents can occur due to bad insulation? Ans. 1. A person may touch the bare wire and get a shock. 2. If live and neutral wire comes in contact, it may give rise to sparks causing fire. 3. If the exposed part of a wire comes in contact with a wet wall, electric current begins, to flow through the wall. The person touching such a wall may get severe shock. 12. Write about Electromagnet? Ans. 1. When an electric current flows through a conductor, magnetic field is produced around the conductor. 2. This magnetic field is present only as long as the current flows through the wire. 3.In this case, large number of turns or insulated copper wire are wounded around a rod of soft iron. 4. When two ends of a wire are connected to a cell, the rod becomes magnet, which is called as electromagnet. 5. If the ends of the magnet is dipped into iron filling it clings (attached) to magnet. If the current is stopped 3

S.S.C

Omtex – classes

CURRENT ELCTRICITY

“THE HOME OF TEXT”

S.S.C NOTES

electromagnet losses magnetism. Iron fillings drop the magnet. Q. III. Distinguish between 1. Series combination and Parallel combination Series combination 1. In series combination number of resistance are connected one after another. 2. The same current flow through each resistance. 3. Potential difference across each resistance is different. 4. The equivalent resistance of the series combination is equal to the sum of the individual resistance. 5. This combination is used to increase the overall resistance of the circuit. Ammeter 1. An ammeter is used to measure the current flowing through a circuit. 2. An ammeter is connected in series with the conductor through which the current flowing is to be measured.

Parallel combination 1. In parallel combination number of resistance are connected between two points. 2. The current passing through each resistance is different. 3. Potential difference across each resistance is same. 4. The reciprocal of the equivalent resistance of the parallel combination is equal to the sum of the reciprocal of individual resistance. 5. This combination is used to decrease the overall resistance of the circuit. Voltmeter 1. A voltmeter is used to measure the potential difference between two points. 2. A voltmeter is connected in parallel to the conductor across which the potential difference is to be measured.

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