Effects of Electric Current
Q1. Rewrite the following statements by choosing the correct option.
1. The quantity of heat generated in a conductor depends on ____________
a. Square of the current (I2)
b. Resistance of he conductor (R)
c. Time for which the current flows (t)
d. I2Rt.
2. A coil of many circular turns of wire wrapped in the space of a cylinder forms _______
a. torroid
b. solenoid
c. conductor
d. insulator
3. Telephone ear piece works on the principle of __________ effect of electric current.
a. magnetic
b. heating
c. chemical
d. optical
4. The relation between electricity and magnetism was fist established by __________
a. Joule
b. Ohm
c. Orested
d. Fleming
5. The magnetic field produced at the centre of circular wire is directly proportional to the ______
a. radius of the loop.
b. magnitude of the current passing through it.
c. resistance of the wire.
d. time for which the current passes through it.
Q2. Fill in the blanks by choosing the correct option from bracket.
(magnetic field, 50 cycles per seconds, Right hand rule, Newton
1. Heat energy produced is expressed in terms of calorie.
2. Orested proved that when current passes through a conducting wire it produces a magnetic field around it.
3. The direction of the magnetic field produced by a conductor can be determined by right hand rule.
4. In India
5. The magnetic field produced at the centre of circular wire depends inversely on the radius of the loop.
Q3. State whether the following statements are true or false.
- Like magnetic poles attract each other, unlike magnetic poles repel.
- The direct current always flows in one direction.
- Fuse is made of wire having high melting point.
- If there is fire caused by sparking, the main switch should be switched off immediately.
- Two magnetic field lines may intersect each other.
Q4. Identify the odd one of the following group.
- Geyser, Heater, Electric iron, Refrigerator.
Answer. Refrigerator: - In refrigerator magnets are used while in the rest there are no magnets.
- Magnet, Solenoid, Compass needle, Oven.
Answer. Oven: - It is based on the heating effect of electric current while the rest have magnetic properties.
- Telephone ear piece, Electric bell, Electromagnet,
Answer. Battery : - It is based on heating effect of electric current while the rest are based on magnetic effect of electric current.
Answer.
This does not follow mathematical for of Joule’s law while the rest are mathematical forms of Joule’s law.
This does not follow mathematical for of Joule’s law while the rest are mathematical forms of Joule’s law. Q 5. Give scientific reasons.
Textual Questions
1. Most of the electrical appliances need earthing.
Answer.
1. Connecting one end of a conducting wire to the metal body of the appliance and burying its other end in the earth is called “earthing”.
2. If an electrical appliance is faulty, the live wire comes in contact with the metal body of the appliance and the current flows in the body of that appliance.
3. If the appliance is properly earthed, the current flowing through the metal body passes to the earth.
4. Hence, there is no danger to the person handling that appliance.
2. Filament of incandescent lamp is made of tungsten.
Answer.
1. Tungsten has a very high resistance. Thus, when an electric current passes through a filament of Tungsten, it becomes extremely hot and emits light.
2. Also Tungsten has a very high melting point due to which it can withstand the extreme heat and does not melt.
3. Hence, due to its high resistance and high melting point, Tungsten filament is used in incandescent lamp.
3. Fuse is made of a material having low melting point.
Answer.
1. A fuse is a wire that is used as a safety device to protect electrical appliances from damages caused due to excessive flow of current.
2. It is made of an alloy of lead and tin has a low melting point.
3. thus, if the current exceeds a certain value, the fuse melts due to its low melting point and the circuit is broken immediately.
4. Thus in turn protects the appliances against any damages by limiting the current passing through it.
4. In rainy season, we should not touch the wall of a building.
Answer.
1. In the rainy season, the walls of the buildings become wet due to rain or leakage of water.
2. If a bare wire comes in contact with such a wet wall, the electric current begins to flow through the wall since water is a good conductor of electricity.
3. Hence, in the rainy season, we should not touch the wall of a building.
5. When a compass needle is kept near a wire conducting current, it is deflected.
Answer.
1. When an electric current passes through a conductor, a magnetic field is produced around it.
2. Hence, if a compass needle is kept near such a conducting wire, it gets deflected.
Non – Textual Questions
1. The temperature of a conducting wire rises when an electric current flows through it. OR
A metal wire becomes hot when a current passes through it.
Answer.
1. When a potential difference is applied between the two ends of a wire, the free electrons move thorough the conductor from the negative terminal to the positive terminal.
2. During their motion, the electrons collide with each other and also with the ions in the conductor.
3. These collisions slow down the electrons there by resulting in a loss of their kinetic energy.
4. This loss in kinetic energy appears in the form of heat.
5. Hence, the temperature of the conducting wire rises when a current flows through it.
2. A nichrome coil is used in an electric heater.
Answer.
1. Nichrome is an alloy that has a very high resistance and hence it produces a large amount of heat.
2. Thus a nichrome coil is used in an electric heater in order to generate excess heat.
3. Short circuit produces sparks.
Answer.
1. When the exposed parts of the live wire and neutral wire come in contact with each other, then short circuit occurs.
2. During a short circuit, the resistance of the live wire decreases to a large extent. This leads to an enormous increase in the current.
3. Due to the increase in the current, the live wire gets heated and sparks are produced at the point of the short circuit.
4. If there is a circular coil of ‘n’ turns, then the field produced due to current is ‘n’ times the field produced by a single turn.
Answer.
1. In a circular coil of ‘n’ turns, the current in each circular turn has the same direction.
2. Due to this the field due to each turn adds up.
3. Hence, if a circular coil has ‘n’ turns, then the field produced is ‘n’ times stronger than the field produced by a single turn.
5. Alternating current follows a cyclic behaviour.
Answer.
1. In an alternating current (A.C), the magnitude as well as the direction of the current changes continuously.
2. It rises from zero to maximum n one direction. It then falls back to zero and attains maximum value in the reverse direction and goes back to zero. This is known as an AC cycle.
3. Hence, an AC follows a cyclic behaviour.
6. A three pin plug should be used instead of a two pin plug.
Answer.
1. A two – pin plug consists of a ‘live’ wire and a ‘neutral’ wire. It does not have an earthing wire.
2. So a person using an electrical appliance like a geyser or electric iron may get a shock if live wire comes in contact with the metal body of the appliance.
3. A three – pin plug has a provision for earthing. The earthing wire carries away the current flowing through the metal body, into the earth. Thus the user remains safe.
4. Hence, a three – pin plug must be used instead of a two – pin plug.
7. Electric boards and sockets should be kept fixed at a height beyond the reach of children.
Answer.
Children have a tendency to insert pins and nails into the holes of electrical sockets.
If the switch is ‘on’, the child inserting the pin/nail into the socket may get a severe shock.
Hence, electric boards and sockets should be fixed at a height beyond the reach of children.
8. Our hands should be dry while handling electrical devices.
Answer.
1. Water is a good conductor of electricity.
2. Wet hands increase the chances of current flowing through the body of the user of the appliance.
3. Hence, while handling electric devices, our hands must be dry.
9. It is necessary to put on rubber soled foot wear while using electric devices.
Answer.
While handling electrical devices, there are chances of the current flowing through the body of the person handling that appliance. Thus the person may get a nasty shock.
Rubber is a bad conductor of electricity and by wearing rubber footwear, a large current is prevented from passing through the body of the user.
Hence, rubber – soled footwear must be used while handling electrical devices.
10. Artificial respiration is necessary for a person who gets a serve electric shock.
Answer.
1. If a person gets a serve electric shock, his breathing may stop.
2. In order to revive him quickly, artificial respiration must be given to him immediately.
Q6. Solve the following numerical problems.
Ex. 6.1
1. Calculate the hear generated in a coil of resistance 100 Ω in 6 minutes when P.D. of 10 V is applied across it.
2. The heat generated in a coil of resistance 100 Ω in 5 seconds is 522.2 cal. Calculate the P.D. applied across it to produce above heat.
3. Calculate the heat generated in a wire of resistance 500 Ω in 6 minutes when P.D. of 50V is applied across it.
4. Calculate the heat in joule, generated in a coil of resistance 200 Ω in 3 minutes when a P.D. of 100 V is applied across it.
5. Calculate the heat generated in calories when a current flows through a wire of resistance 100 Ω and P.D. 100V for 3 min 29 sec.
6. The heat generated in coil of resistance 4.18 Ω in 1 minute is 540 cal. Calculate the P.D. applied across it to produce above heat.
7. The heat generated in a coil of resistance 100 Ω in 2 minutes is 125.4 cal. Calculate the P.D. applied across it to produce the above heat.
Ex. 6.2
1. Find resistance of 10W, 240V bulb.
2. Find resistance of 100W, 240V bulb.
3. Find resistance of 60W, 220V bulb.
4. The current flowing through a 12V Tungsten filament is 3A. Find the power of the bulb, if the resistance of the filament is 4 Ω.
5. Calculate the current through a 60W bulb when a P.D. of 250V is applied across its filament.
Ex. 6.2
1. Calculate the heat generated in a coil of resistance 41.8 Ω in 5 min. When 0.5A current is passed across it.
2. 240J heat is generated in a conductor of resistance 50 Ω and 0.2A current is passed across it. Find how long current will pass through it.
3. Find the heat generated when a current of 5A flows through a wire of resistance 2.09 Ω for 4 minutes.
4. Calculate the heat generated in a coil of resistance of 8.36 Ω when a current of 1.5A flows through it for 2 minutes.
5. Calculate the heat generated in calories, when a current of 0.5A flows through a coil for 15 min. Resistance of the coil is 2.09 Ω.
6. Find the resistance of a wire when 0.5A current flows through it for 1 min, producing 1500 cal of heat.
7. An electric current of 0.5A flows through a wire of resistance 41.8 Ω. How long does the current flow through it to produce 300 calories of heat?
8. 300J heat is generated in a conductor of resistance 100 Ω and 0.5A current is passed across it. Find how long current will pass through it.
9. 540 cal of heat is generated in a conductor of resistance 62.7 Ω and 2A current is passed across it. Find how long current will pass through it.
10. The P.D. in an electrical circuit is 60V. An electric current of 0.3A flows through it. If 160J of heat is produced, find the time for which the current is passed.
11. 300 calories of heat are generated in a wire of resistance 41.8 ohms when a current of 0.5 A flows through it. Find the time in minutes for which the current is passes.
Q7. Answer the following questions.
Textual Questions
1. What is the function of earth wire? Why it is necessary to earth metallic appliances?
Answer.
1. Most of the electric appliances are provided with three wires connected to a three pin plug.
2. One of the wires is connected to live wire in the mains, the other to the neutral wire and the third one to the earthing wire.
3. The earth wire is a wire that carries away the current from the metal body of an appliance into the earth.
4. The end of earth wire is connected to a metallic body deep inside earth.
5. When plug of appliances is inserted in socket and current is switched on, the live wire and neutral wire come in contact with two wires of the appliance while the earth wire comes in contact with the metal body of the appliance.
6. If an electrical appliance is faulty, the live wire comes in contact with the metal body of the appliance and the current flows in the body of that appliance. Thus, if a person touches such an appliance, he will get a shock.
7. By earthing metal appliances, the current flows from the metal body into the earth and there is no danger to the person handling that appliance.
2. Mention any four applications of heating effect of electric current.
Answer. The four applications of heating effect of electric current are
1. Electric heater
2. Geyser
3. Electric iron
4. Electric oven
Non – Textual Questions
1. How does short circuit occur? What measures can be taken to avoid it?
Answer.
1. In house wiring when two wires, live and neutral come in contact with each other short circuit occurs.
2. This may happen either due to some damage to their insulation or due to a fault in electrical appliance.
3. In such a situation, resistance of the circuit decreases to a very small value, the current in the circuit increases enormously and as a result the liver wires are heated.
4. This produces spark at the place of short circuit which may even cause fire in the building.
5. To avoid such incidents:
i. A fuse must be included in the electric circuit.
ii. Insulated wires of the house must be replaced periodically and
iii. Bare or exposed wires should be replaced immediately.
2. Describe Orested’s experiment with the help of diagrams to prove that electric current produces a magnetic field around it.
Answer.
1. Take a wire AB and stretch it above a compass needle in a direction parallel to the needle, so that the wire is along the north - south direction. The wire should be held in this position by supporting it on the non - conducting stands. Connect the wire in series with cell E and tap key K.
2. Press the tap key so that electric current through the wire form A to B. You will notice that the needle is deflected. Release the tap key to cut off the current. You will notice that s soon as the current stops, the needle returns to its original position.
3. Interchange the connections of the terminals of the cell. Press the tap key. Now the direction of the current in the wire is reversed and the current flows from B to A. You will notice that in this case, the needle is deflected in the opposite direction. Release the key to stop the current. You will notice that the needle returns to the original direction.
Conclusion: - From the above experiment we conclude that a current carrying conductor produces a magnetic field around it.
Q8. State the following laws.
Textual Questions
1. Joule’s law of heating effect of electric current: -
Answer.
Joule’s Law: - The heat generated in a conductor is directly proportional to
- The square of the current (I) flowing through it.
- The resistance (R) of the conductor
- The time (t) for which current flows
2. Right hand rule
Answer. Imagine that you are holding a current carrying conductor in your right hand with the fingers curled around it and then 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.
Q9. Draw and label the diagram.
Textual Questions
1. Lines of magnetic field due to a straight conductor carrying current.
2. Lines of magnetic field around solenoid carrying current.
3. Heating effect of electric current verification of Joule’s law.
4. Orested’s experiment for magnetic effect of electric current.
5. Telephone ear piece.
Q10. Explain in detail the working of electric bell.
Answer.
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.
4. When the current stops the electromagnet losses its magnetisms and the soft iron rod goes to its original position.
5. 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.
Q11. Write any two precautions_
1. While using electrical appliance: - The two precautions to be taken while using electrical appliances are
1. All electrical appliances must be handled only with dry hands.
2. Always switch off the current and remove the plug from the socket before cleaning electric appliances like refrigerator.
2. Against bad insulations: - The two precautions to be taken against bad insulation are
- The live wire must always be insulated.
- The live wire – even if it is insulated – must be replaced periodically i.e. every 3-4 years. But if it has become exposed, then it must be replaced immediately.
3. Against improper earthing: -The two precautions to be taken against improper earthing are
- A three pin plug must always be used for all metal appliances.
- The earth wire in a 3 pin – plug protects a person from the danger of electric shock.
Q12. State general precaution for safety with respect to electricity.
Answer. The following are the general safety precautions that need to be observed while handling electrical appliances.
1. Electric boards and sockets should be fixed at a height beyond the reach of children.
2. The current must be switched off and the plug must be removed from the socket before cleaning electric appliances.
3. Electrical appliances must be only handled with dry hands.
4. Rubber soled footwear must be worn while using electrical appliances.
5. Do not touch with bare hands, a person who is stuck to a liver wire. Use a wooden object like a stick of a chair in order to save him.
Q13. Define the following.
1. Electric power: - Electric power is defined as the rate of doing work. OR Electric power is defined as the rate at which electrical energy is produced.
2. Alternating current (A.C): - The current which varies in magnitude continuously and changes direction after equal interval of time is called as alternating current. (A.C.)
3. Direct current (D.C):- The current that always flows in one direction and hence is non – oscillating is known as direct current.
Q14. Distinguish between.
1. Alternating current and Direct current
| Alternating current | Direct current. |
| 1. It is an oscillating current. 2. There is not much loss of energy when it is transmitted over long distances. | 1. It is a non oscillating current. 2. There is loss of energy when it is transmitted over long distances. |
Q15. Answer in 2 or 3 sentences.
1. What are the factors on which the heat produced due to an electric current depends?
Answer. The heat produced due to an electric current depends upon the following factors.
- The current passing through the wire.
- The resistance of the wire.
- The time for which the current is passed.
- The resistance of the wire.
2. What are the factors on which the magnetic field produced at the centre of a circular wire depends?
Answer. The strength of the magnetic field produced at the centre of a circular wire depends upon the following factors:
1. It is directly proportional to the magnitude of the current passing through it.
2. It is inversely proportional to the radius of the loop.
3. What are the characteristics of magnetic field due to current flowing in a solenoid?
Answer. The characteristics of the magnetic field due to a current flowing in a solenoid are as follows:
1. One end of the coil acts like a north pole, while the other end acts like a south pole.
2. The magnetic field inside the coil is strong and uniform while outside the coil, the field is weak.
3. The strength (magnitude) of the magnetic field produced by a solenoid depends upon
i. the current passing through it and
ii. the number of turns per unit length.
4. Mention any four applications of magnetic effects of electric current.
Answer. The four applications of magnetic effects of electric current are:
- Electric generators.
- Radio
- Galvanometer
- Ammeter
- Voltmeter
- Electromagnet.
5. What is an electromagnet? How is it made?
Answer.
- An electromagnet is a device that works on the magnetic effect of electric current.
- It is made by taking a soft iron rod and winding a large number of turns of an insulated copper wire around it.
- When a current passes through the wire, the rod becomes a magnet and attracts small pieces of iron.
6. Write any four applications of magnets.
Answer. Magnets are used in the following applications.
- They are used in refrigerator doors in order to close them.
- They are used in audio and video tapes.
- They are used in the hard disk and floppies of computers.
- They are used in children toys.
5. What are the characteristics of magnetic field due to current flowing in a solenoid?
Answer. The characteristics of the magnetic field due to a current flowing in a solenoid are as follows.
- The magnetic field inside the coil is strong and uniform while outside the coil, the field is weak.
- One end of the coil acts like a north pole, while the other end acts like a south pole.
- The strength (magnitude) of the magnetic field produced by a solenoid depends upon
- the current passing through it and
- the number of turns per unit length.
Q16. Write short notes on
1. Heating effect of electric current.
Answer.
- When a P.D. is applied between the two ends of a wire, the free electrons move through the conductor from the negative terminal to the positive terminal.
- During their motion, the electrons collide with each other and also with the ions in the conductor.
- These collisions slow down the electrons, thereby resulting in the loss of their kinetic energy.
- This loss of kinetic energy appears in the form of heat.
- Appliances like the geyser, heater, electric iron works on the heating effect of electric current.
2. Electric fuse.
Answer.
- An electric fuse is a safety device that works on the heating effect of electric current.
- The fuse wire is made up an alloy of lead and tin and it has a low melting point.
- It allows current upto a certain maximum value to flow through it. If the current exceeds this value, the wire melts and the circuit is immediately broken.
- In this way, the fuse protects our electrical appliances against damage, by limiting the current passing through it.
3. Magnetic effect of electric current.
Answer.
- When an electric current passes through a conductor, a magnetic field is produced around it. The magnetic field is represented by drawing lines called ‘magnetic field lines’.
- The strength of the magnetic field at a given point is directly proportional to the current passing through the wire.
- The strength of the magnetic field produced by a current in the wire decreases as we move away from the wire.
- The direction of the magnetic field around a straight conductor is obtained by the Right Hand Rule.
- The magnetic effect due to an electric current lasts only as long as the current flows through the conductor.
- Galvanometer, ammeter, voltmeter, electric bell are the application of the magnetic effect of electric current.
4. Alternating Current (A.C)\
Answer.
- The current which varies in magnitude continuously and changes direction after equal intervals of time is called as alternating current.
- The A.C. first rises from zero to a certain maximum value. It then falls back to zero and quickly inverse its direction. it attains a certain maximum value in the reverse direction and again falls back to zero.
- The current follows cycle behaviour and it oscillates sinusoidal with time.
- Alternating current can be transmitted over long distances without loss of energy.
5. Direct Current (D.C)
Answer.
- A direct current is a non – oscillating current and it flows only in one direction.
- Direct current is generated from a battery.
- When D.C. is transmitted over long distances there is a loss of energy due to the resistance of the conductor.
