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# Electric Power:
The rate of doing work by the electrical device is known as electric power. The electric power (P) of an electrical device is equal to the product of the current (I) flowing through the device and potential difference (V) across its two ends i.e.
Using Ohm’s laws it can be expressed as:
P = IV
P = I²R
One Kilowatt-hour is the energy supplied when one kilowatt power is used for one hour. One kilowatt hour (KWh) is also called one unit.
Energy = Power × time
1kWh = (1kW) (1h)
=1000W×3600s
∴ 1kWh = 3.6 × 10⁶ J
Total cost of electricity = total electrical energy in kWh unit cost rate
# Direct Current:
If the polarity of electrical sources does not change with time, it is called direct current (DC).A dry cell and DC generators produce this current.
# Alternating Current:
If the polarity of electrical sources changes continuously with the time, it is called alternating current (AC).AC generators produces it.
# Differences between AC and DC current
AC
1. The polarity changes continuously with time in it.
2. It is produced generators or dynamos.
3. Its frequency is 50 Hz (in Nepal)
DC
1. The polarity does not change with time in DC, i.e., it always l flows in one direction.
2. It is produced by dry cells or batteries and DC generators.
3. It has no frequency.
# Fuse:
A fuse is a safety device having short length and thin wire of low melting point used for protecting electric circuit. It works s on heating effect of current. A fuse wire is connected in series in the circuit. The maximum amount of current which can pass through a fuse without breaking the circuit called fuse rating.
Q] The fuse is connected to live wire, why?
Ans] The fuse is connected to live wire. When large current flows through the circuit, the fuse wire gets heated because of its low melting point. As a result of this the circuits is broken, and hence no current flows through the electrical device and it remains safe.
Q] A copper wire cannot be used as a fuse, why?
Ans] A copper wire cannot be used as a fuse because it has high melting point. Due to this reason it does not melt easily when current overflows in it.
The fuses normally available for household purposes are 3A, 5A, 10A, 13A, 15A, etc.
Q] What is the size of fuse for an electric device of power 1 KW on 220V?
Solution Here, Power (P) = 1 KW= 1000W
Voltage (V) =220V
Current (I) =?
We know, P = VI
Or,
The current is 4.54A.
Therefore, the capacity of the fuse should be 5A. (Ans)
# Domestic Wiring:
Electricity generated form a power station is fed into house by two insulated wires, the live wire (L) and the neutral wire (N). The live wire ‘L’ is at high voltage 220V and the neutral wire (N) is at the ground of potential IV. The wire enters the box which contains (M) main fuse in the live wire and meter. The two wires form the meter box enters. Main switch box is that connected to the distribution board through a main switch box. Appliances like electric bulb, fans, and three pin sockets are connected from the distribution box as shown in the figure.
The fuse in the meter box is called electricity authority’s main fuse and the fuse after the main switch box is called consumer’s fuse. The live terminal of each socket is connected to the live wire, earth wire. A switch is kept in the live wire of three pin socket.
# Some important rules for household wiring are:
* The fuse and all the switches should be connected in live wire in order to break the power supply in the circuit when the fuse is broken or the switch is off.
* The fuse wire should be of proper rating and proper material.
* There should be separate fuses in each room or floor.
* There should be separate circuits. They are light circuit and power circuit. In case fuse of power circuit is blown, the light circuit still works.
* Wiring should not be done in a damp area or in the region where water leaks on the wire. It prevents leakage of electricity and the chance of electric shocks.
# Hazards or dangers of electricity are:
* If a person touches a live wire, he/she gets a severe electric shock.
* Short-circuit due to damaged wiring or overloading of the circuit may cause electrical fires in a building.
* The defects in the household wiring like loose connections and defective switches, sockets and plugs can cause sparkling and leads to fires.
* The phase wire is a live wire. In the live wire the indicator of the line tester glows, while in other wires the line tester does not glow.
The colour codes used in household wiring is given below:
Wire
(Colour codes)
Earthing
(Green, Green with yellow strip)
# Grounding /Earthing
Grounding /Earthing is the process of connecting earth- wire to the metal body or frame of electrical appliance. It is a safety device for the consumers, which protect them from electric shocks.
Q] A person wearing shoes with rubber sole does not get electric shock even when he touches the live wire, why?
Ans] A person wearing shoes with rubber sole does not get electric shock even when he touches the live wire, because the current will not pass through the body due to insulation between the body and the ground. But if the person is on the ground (earth) or touching the neutral line, the person gets and electric shock because the earth and our body both are conductors of electricity.
# Effects of Electricity
Current electricity has following effects
Heat effect
Light effect
Magnetic effect and
Chemical effect
1. Heating Effect:
When electric current is passed through a conductor, it gets heated up. This is called heating effect of electric current.
Nichrome, a heating element, is an alloy of nickel and chromium. Its resistance is very high and it does not oxidize even at very high temperature, therefore it is used as a heating element. Its melting point is also very high.
2. Lighting effect:
When electric current is passed through a conductor, it becomes very hot due t which it emits light. This is called lighting effect of electric current, e.g., electric bulbs, fluorescent lamps, other lighting devices, etc.
a. Filament lamp:
The structure of a filament lamp is as shown in the diagram. When electric current is passed in it, due to more resistance the tungsten filament gets heated to white and emits light. Its working capacity is about 1000 hours. It converts about 10% of electrical energy into light energy. The rest changes into heat energy.
b. Fluorescent lamp:
The structure of fluorescent lamp is shown in the diagram. When electric current is passed in it, the choke magnifies the voltage and the starter provides the complete circuit before the mercury is ionized. After the ionization of mercury, the starter disconnects the circuit and the ionized mercury completes the circuit. It also produces ultraviolet rays. The rays make the fluorescent powder bright, which emits light. It changes more parts of electrical energy into high energy and its working capacity is about 3,000 hours only.
Q] Tungsten filament is used in an electric bulb, why?
Ans] Tungsten filament is used in an electric bulb because of its high resistance. When electric current is passed through the filament of t tungsten, it attains high temperature and emits light. Its melting point is also very high (3400°C).
Q] Electric bulbs (or filament lamps) are filled with inert gases like nitrogen or argon, why?
Ans] Electric bulbs (or filament lamps) are filled with inert gases like nitrogen or argon in order to prevent oxidation at high temperature due to which the filament remains safe.
# Differences between filament lamps and fluorescent lamp
Filament lamps
1. It is an electric lamp made up of a glass bulb, inert gas, filament, etc.
2. It converts 10% light and 90% heat of total electrical energy.
3. Its average life is shorter, i.e. about 1000 hours only.
4. It is less efficient and cheap.
5. Tungsten filament gets heated and emits light.
Fluorescent lamps
1. It is an electric lamp made up of fluorescent powder, mercury vapour, glass tube, electrodes, etc.
2. It can convert 30% light of total electrical energy.
3. Its average life is longer, i.e. about 3000 hours
4. It is more efficient and expensive.
5. Fluorescent powder absorbs UV rays obtained from Hg and emits light.
# Magnetic Effect:
When an electric current is passed through a conductor, it behaves like a magnet. This is called magnetic effect of current.
Magnetic effect of electricity was discovered by Hans Christian Oersted in 1820 AD.While preparing for an evening lecture on 21 April 1820, Hans Christian Ørsted made a surprising observation. As he was setting up his materials, he noticed a compass needle deflected away from magnetic north when the electric current from the battery he was using was switched on and off This deflection convinced him that magnetic fields radiatefrom all sides of a wire carrying an electric current, just as light and heat do, and that it confirmed a direct relationship between electricity and magnetism.
At the time of discovery, Ørsted did not suggest any satisfactory explanation of the phenomenon, nor did he try to represent the phenomenon in a mathematical framework. However, three months later he began more intensive investigations Soon thereafter he published his findings, proving that an electric current produces a magnetic field as it flows through a wire. The CGS unit of magnetic induction ( oersted ) is named in honor of his contributions to the field of electromagnetism.
Magnetic field around a straight wire is found in the form of rings. The direction of lines of force is determined by using magnetic compass or by Maxwell’s screw rule or by right hand thumb rule.
# Electromagnet:
When an iron bar is wrapped with an insulated wire and the current is passed through it, the iron bar changes into a magnet. The temporary magnet so formed is called an electromagnet.
The strength of an electromagnet can be increased by:
* Increasing the number of turns in the coil.
* Increasing the amount of current passing through the wire.
* Decreasing the length of air gap between the poles of a ‘U’ shaped magnet.
* Increasing the relative density of magnet and coil.
Electromagnets are used in electric bells, electric motors, loudspeakers, the receivers of telephones etc.
Advantages of an electromagnet over a permanent magnet are in
* It is temporary.
* Its strength can be altered.
* It can be modified into any shape.
# Electric Bell:
When electric current is passed in an electric bell the electromagnet pulls the hammer towards the gong and the bell rings. On this condition, the circuit gets disconnected at point of contact and the electromagnet gets demagnetized. The soft magnetic plate touches at point of contact again and the circuit becomes closed. The process repeats again and again till the switch is on. It causes the bell to ring.
Working principle: A simple electric bell is shown in the diagram - it works like this. When the switch S is closed current flows round thecircuit, through the electromagnet - along the spring - through the contacts - through the adjusting screw and back to the battery again.
The electromagnet therefore attracts the soft iron armature and the striker hits the gong once. As soon as the armature is attracted the contacts are opened and so the current stops flowing.
This switches off the electromagnet, the spring pulls the armature away — the contacts meet again and so the current starts flowing again. This is repeated over and over again and so the bell rings.
# Chemical Effect:
When current is passed through a liquid and it under goes chemical, the effect is called the chemical effect of current.
# Electromagnetic Induction:
The phenomenon by which the electric current is produced in a closed coil due to relative motion between the coil and a magnet is called electromagnetic induction. The electric current so produced is called induced current. The phenomenon of electromagnetic induction was discovered by Michael Faraday in 1831.
# Faraday’s Laws of Electromagnetic induction:
a. Whenever the magnetic flux linked with a closed circuit changes, an induced emf is produced in the circuit.
b. The induced emf lasts as long as the change in magnetic flux continues in the circuit.
c. The induced emf is directly proportional to the rate of change of magnetic flux in the circuit.
# Magnetic flux:
Magnetic flux through a surface is the number of magnetic lines of force crossing through the surface of coil placed perpendicular to the line of forces.
Fleming right hand rule :
Fleming’s right hand rule states that “If the first three fingers of the right hand are held mutually perpendicular to each other, with the index finger in the direction of the magnetic flux and the thumb in the direction of motion of coil, the middle finger points to the direction of the
induced current.”
Fleming’s right hand rule is used to determine the direction of the induced current in a generator.
When a conductor moves across a magnetic field, it cuts the lines of force and an emf is induced current is given by Fleming’s right hand rule. The rule states that if the first three fingers of the right hand are held mutually perpendicular to each other, the fore finger, the thumb and the central finger give the direction of the magnetic field, motion of the coil and the induced current respectively.
# Electrical generator or dynamo:
The electrical device to produce electric current or electricity is known as electrical generator or dynamo. It converts mechanical energy into electrical energy.
A device which generated small amount of AC is known as dynamo and the device, which generates large amount of AC, is known as generator.
# Ways of increasing emf or voltage in a generator or dynamo:
The induced emf (or voltage) of generator or dynamo can be increased by:
a. Increasing the number of turns in the coil.
b. Increasing strength of magnetic field.
c. Wounding coil on a soft iron core.
d. Increasing speed of rotation of the magnet near the coil
e. Decreasing the distance between the coil and the magnet.
# Electric motor:
An electric motor is a device which converts electrical energy to mechanical energy. It is based on the principle of motor effect. When a current carrying conductor is kept in a magnetic field, the conductor moves. This is called motor effect.
Know more about electric motor
# Differences between electric motor and generator
Electric Motor
a. It converts electrical energy into mechanical energy.
b. It is based on the principle motor effect.
c. Fleming’s left hand rule gives the direction of motions of the conduction.
d. It is used in fans, cars etc.
Generator
a. It converts mechanical energy into electrical energy.
b. It is based on the principle of electromagnetic induction
c. Fleming’s right hand rule gives the direction of induced current through the conductor.
d. It is used in bicycles, power plants etc.
# Fleming’s left hand rule:
The motion of an electric motor is given by Fleming’s left hand rule. The rule states that if the first three fingers are mutually perpendicular to each other, the index finger, the middle finger and the thumbs represent the direction of magnetic field, current and the motion of the conductor respectively.
A transformer is a device which converts low alternating voltage at high current into high alternating voltage at low current and vice versa. In practice, it is used to increase or decrease alternating emf.
The transformer which changes high voltage alternating current to low voltage alternating current is called step down transformer. They are used at power substations and in streets to lower the supplied AC voltage.
The transformer which changes low voltage alternating current to high voltage alternating current is called step up transformer. They are used at power stations.
A transformer works on the principle of mutual induction i.e., when a change in current or magnetic flux takes place in one coil, induced emf is produced in the other coil.
The coil connected to the a.c., source is called primary coil and the coil connected to the output is called secondary coil.
The current in the primary coil is called input current and the voltage across the primary coil is called the input voltage or primary voltage VP.
The current in the secondary coil is called output current and the voltage across the secondary coil is called the output voltage or secondary voltage VS.
The relation between primary and secondary voltage in a transformer is given by:
Vₛ/Vₚ=Nₛ/Nₚ
Losses In Transformer: In any electrical machine, 'loss' can be defined as the difference between input power and output power. An electrical transformer is an static device, hence mechanical losses (like windage or friction losses) are absent in it. A transformer only consists of electrical losses (iron losses and copper losses). Transformer losses are similar to losses in a DC machine, except that transformers do not have mechanical losses. Losses in transformer are explained below -
(i) Core Losses Or Iron Losses: Eddy current loss and hysteresis loss depend upon the magnetic properties of the material used for the construction of core. Hence these losses are also known as core losses or iron losses .
Hysteresis loss in transformer: Hysteresis loss is due to reversal of magnetization in the transformer core.
Eddy current loss in transformer : In transformer, AC current is supplied to the primary winding which sets up alternating magnetizing flux. When this flux links with secondary winding, it produces induced emf in it. But some part of this flux also gets linked with other conducting parts like steel core or iron body or the transformer, which will result in induced emf in those parts, causing small circulating current in them. This current is called as eddy current. Due to these eddy currents, some energy will be dissipated in the form of heat.
(ii) Copper Loss In Transformer: Copper loss is due to ohmic resistance of the transformer windings. Copper loss for the primary winding is I₁²R₁ and for secondary winding is I₂²R₂ . Where, I₁ and I₂ are current in primary and secondary winding respectively, R₁ and R₂ are the resistances of primary and secondary winding respectively. It is clear that Cu loss is proportional to square of the current, and current depends on the load. Hence copper loss in transformer varies with the load.
Q] The core of a transformer is laminated, why?
Ans] The core of a transformer is laminated in order to reduce the eddy-current loss to prevent heating of transformer due to which out put is increased.
# Eddy current:
Eddy current is the induced current set up in a conductor by a changing magnetic field.
# Differences between step up & step-down transformer
Step up Transformer
1. It changes low voltage ac into high voltage
2. The number of turns in the secondary coil is more than that in the primary coil
3. It is used between electric tower and power generating station
Step down transformer
1. It changes high voltage ac into low voltage
2. The number of turns in the secondary coil is less than that in the primary coil
3. It is used between the high power transmission line houses
Principle of transformer:
When an alternating emf is applied to the
primary coil, a changing current flowing in it produces an alternating magnetic flux in it. This causes to change the magnetic flux linked with the secondary coil. An alternating emf is then induced in the secondary coil. It is called the principle of mutual inductance on which transformers are based.
Uses :
Transformers are used in computer, television, air condition, doorbells etc.
Ac and Dc:
Alternating current is one which changes its magnitude continuously and reverses its
direction periodically and if the polarity of an electrical source does not change with time it is called direct current.
Graph of ac and dc circuit
Some important questions for examination.
1. What is heating effect of current?
Ans: The production of heat by passing an electric current through a wire of high resistance is called the heating effect of current.
2. Calculate the amount of electrical energy consumed in kWh and joule by primary winding of a transformer in a day if it is supplied with 6A, 220 V mains. Also calculate the current in secondary winding if it produces 110 V potential differences.
Ans: 31.68 kWh, 114.048 × 10⁶ Joules, 12A
3. Calculate the number of turns in the secondary coil in the given diagram if number of turns in primary coil is 1000.
4. In the given diagram, if 1000 V line is connected to the transformer, find number of turns in secondary coil?
5. Discuss the structure and working of transformer. Why is the core of a transformer laminated?
Ans: Transformer is a device used to alter the voltage of alternating current.
It is a simple device. It contains a rectangular iron core and two coils of copper.
The core contains several thin sheets of iron. The iron sheets are laminated with varnish or shellac so that the sheets do not touch to one another directly.
This helps to reduce the heating effect on the core when current is supplied.
There are two coils of copper wire. The first is primary coil and the second on is the secondary coil.
Primary coil is connected to the input current. The secondary coil gives the output.
The primary and the secondary coil have no direct connection.
When we pass the input current in the primary coil, the current produces a magnetic field around the coil due to the magnetic effect of the current. We have to supply an alternating current to produce a variable magnetic field.
The variable magnetic field produced in the primary coil runs upto the secondary coil. It causes a change in the magnetic flux in the secondary coil. The change in the magnetic flux creates a current in the secondary coil by electromagnetic induction. This current is the output of the transformer.
The voltage output current depends upon the ration of the number of turns in the primary and the secondary coils.
If the number of turns in the secondary coil is more, output voltage will be greater than the input voltage. This type of transformer is called as step up transformer. It is used to increase the voltage.
If the number of turns in the secondary coil is less, output voltage will be lesser than the input voltage. This type of transformer is called as step down transformer. It is used to decrease the voltage.
When current is passed to the primary coil of the transformer, it causes heating effect in the core. If the core is made of a single thick iron frame, there will be more heating effect. It reduces the efficiency of the transformer and also reduces the life of the transformer.
So, the core is made by joining different thin sheets of iron frames. The sheets are laminated to prevent them to join directly and act as a single thick core. This helps to reduce the heating effect and increase the efficiency as well as the life of the transformer.
6. What is transformer?
Ans: A transformer is a device which is used to increase or decrease the voltage of an alternating current.
7. Write down the two causes of energy losses in a transformer.
Ans: The causes of energy losses in a transformer are as followings:
I. Heat loss
Heat loss is the most common form of energy loss in a transformer. Since there is an electrical current in both the primary and secondary coils of the transformer, energy will be used to overcome resistance in the wires. This energy is transformed into heat which escapes from the coils resulting in the energy loss.
II. Eddy currents
The changing magnetic field not only induces currents in the secondary coil but also currents in the iron core itself. These currents flow in little circles in the iron core and are called eddy currents. The eddy currents cause heat loss.
8. A transformer is of 220 V primary voltage and 770 turns of the primary coil. How many of turns of secondary coil will be necessary in order to produce 120 volts from that transformer?
Ans: 420
9. Write any two uses of a transformer.
Ans: Uses of transformers are as followings:
I. Transformers are used in voltage regulators for computer, television, air conditioner, record player, trolley buses, etc.
II. They are used for doorbells, welding purposes and in electrical furnaces.
10. Give two reasons to use the soft iron core in a transformer.
Ans: Due to the following reasons, the soft iron core is used in a transformer:
I. The soft iron core can convert magnetic energy into electrical energy easily.
II. Soft iron core increases the magnitude of induced e.m.f. in the secondary coil of a transformer.
11. How is the electric current induced in the secondary coil of a transformer? Describe.
Ans: A transformer is a device to convert low a.c. voltage into high a.c. voltage and vice versa.
It consists of a rectangular soft iron core made of laminated sheets. There are two coils which are not connected to one another in any way. These coils are wound on the core as shown in the given figure. One of the coils may be connected to a source of a.c. and termed as the primary coil. The other coil is termed as the secondary coil. The e.m.f. of a.c. source applied across the primary coil is called input voltage and the e.m.f induced across the secondary coil is called secondary voltage.
When an alternating e.m.f. is applied to the primary coil, a changing current flowing in it produces an alternating magnetic flux in it. This causes to change the magnetic flux linked with the coil. An alternating e.m.f. is then induced in the secondary coil. It is called 'principle of mutual induction on which transformers are based.
12. "The use of alternating current would be limited, if transformer was not invented." Prove the statement with two clues.
Ans: The use of alternating current would be limited if the transformer was not invented due to the following reasons:
I. If transformer were not invented, then sending current over a long distance while keeping constant voltage would cause loss of power in form of heat.
II. To use alternating current, there would be need of generation/power plant everywhere. But due to the invention of a transformer, to there is no need of establishing power plant everywhere.
13. Give two reasons to use soft iron core in a transformer.
Ans: Due to the following reasons soft iron core is used in a transformer:
I. Soft iron core can convert magnetic energy into electrical energy easily.
II. Soft iron core increases the magnitude of induced e.m.f. in the secondary coil of transformer.
14. Write two differences between generator and motor.
Ans: The differences between generator and motor are:
Generator
1. It is a device which converts mechanical energy into electrical energy.
2. An electromagnet is used in the generator.
Motor
1. It is a device which converts electrical energy into mechanical energy.
2. A permanent magnet is used in the motor.
15. What steps should be taken to increase the amount of current produced by the generator? Write any four ways.
Ans: In a generator, amount of current produced by a generator can be increased by following ways:
I. By increasing the number of turns in the coil.
II. By increasing the strength of magnetic field.
III. By increasing the speed of rotation of the coil in the magnetic field or by increasing the speed of rotation of magnet near the coil.
IV. By decreasing the distance between the coil and the magnet.
16. Calculate the resistance of a device if 0.3 A of current flows through it when the potential difference across it is 9 V.
Ans: 30 ohm
17. What is induced current?
Ans: The electric current produced due to the change of magnetic lines of force in a closed circuit is called the induced current.
18. What is eddy current?
Ans: The circulating current produced within any metal when magnetic flux linked with it is changed is called eddy current.
19. What is direct current?
Ans: The current which always flows in the same direction is called direct current.
20. What is alternating current?
Ans: The current which changes its direction a number of times in one second is called alternating current.
21. What is mutual induction?
Ans: The process of induction of an e.m.f. and current in a coil when the current in a neighbouring coil is switched on or off or changed is called mutual induction.
22. What is solenoid?
Ans: The insulated conductor wire wound in the form of a cylinder is known as a solenoid.
23. Write any four safety measures in using
electricity.
Ans: Safety measures we should follow while using electricity are as following:
I. Avoid water at all times when working with electricity. Never touch or try repairing any electrical equipment or circuits with wet hands. It increases the conductivity of electric current.
II. When working with electricity do not stand on metal, wet concrete or wet ground. It is wiser to stand on a rubber-mat or a dry wooden platform.
III. Children should be given proper training in the handling of electrical appliances.
IV. Faulty electrical appliances and equipment must be properly handled and repaired promptly.
24. What are live wires?
Ans: The wires that deliver the electricity to the electric load connected in the circuit are called live wires.
25. What is heating element?
Ans: A heating element is a coil having a very high resistance which is used to convert electrical energy into heat energy.
26. When electric current is passed through a nichrome wire, it becomes hot, but if same current is passed through copper wire it does not become hot, why?
Ans: Potential difference is a measure of work done in moving a unit charge across a circuit. Current in a circuit is equal to the amount of charge flowing in one second.
Therefore, the work done in moving 'Q' charges through a potential difference 'V' in a time't' is given by:
Work done = potential difference x current x time
W = VIt
The same can be expressed differently using ohm's law.
According to ohm's law V = IR
Therefore work can be expressed as
W = VIt
Or, W = (IR) It = I²Rt
W = V( V/R)T = ( V² / r) / T
Thus, heat produced is directly proportional to the resistance, to the time and to the square of the current.
As we know that, heat produced is directly proportional to the resistance generated in the wire in an electrical circuit, to the time and to the square of the current.
Nichrome wire has very high resistance as compared to copper wire. Due to the high resistance present in nichrome wire, large amount of current is converted into heat energy. Hence, when electric current is passed through a nichrome wire, it becomes hot, but if same current is passed through copper wire it does not become hot.
27. Write any two differences between heating effect and lighting effect of electricity.
Ans: Differences between heating effect and lighting effect of electricity are as followings:
Heating effect
1. When electric current passes through some electric appliances, they change electrical energy into heat energy. This is called heating effect of electricity.
2. Electrical appliances having heating effect have a coil of wire that converts electrical energy into heat energy. Such coil is termed as heating element.
Lighting Effect
1. When electric current is passed through some electrical appliances they change electrical energy into light energy. This is called lighting effect of electricity.
2. In filament lamp tungsten coil is used, which is termed as filament.
28. What is heating device? Give two examples of this.
Ans: When electric current passes through some electric appliances, they change electrical energy into heat energy. Those electrical appliances are termed as heating device. Heating devices have a coil of wire that converts electrical energy into heat energy. Such coil is called heating element, heating element is usually made of nichrome.
Examples of heating device-
I. Electric iron
II. Toaster
29. What is an electric motor?
Ans: An electric motor is a device which converts electrical energy into mechanical energy.
30. Ten electric bulbs of 100 W each and two electric heaters of 1000 W each are used for 6 hours continuously. Calculate the unit of electricity consumed.
Ans: 18 unit
31. If two irons with 750 W each are used for eight hours a month, how much tariff should be paid if the cost of 1 unit electricity is Rs. 7.
Ans: Rs. 84
32. 10 bulbs of 60 watts each two heaters of 1500 watt are used two hours daily. Find the units of electricity consumed in 30 days.
Ans: 216 units
33. 10 electric bulbs of 100 watts each are used for 6 hrs. and 2 heaters of 2 kW are used for 4 hrs. daily, calculate the consumptions of electricity in one day.
Ans: 22 units
34. If three bulbs of 40 W each are used 5 hours daily and a heater of 1500 W is used at the rate of 2 hours daily. How much electricity is consumed in one month?
Ans: 108 units
35. 10 electric bulbs of 100 watt each are used for 6 hrs and 2 heaters of 2 kW are used for 4 hrs daily, calculate the consumptions of electricity in one day.
Ans: 22 units
36. If three bulbs of 40 W each are used 5 hours daily and a heater of 1500 W is used at the rate of 2 hours daily. How much electricity is consumed in one month?
Ans: 108 units
37. What is electromagnetic induction?
Ans: The method of production of the current by changing the magnetic flux linked with coil or conductor is called electromagnetic induction.
38. State Faraday’s first law of electromagnetic induction.
Ans: Faraday’s first law of electromagnetic induction states that whenever the magnetic flux linked with a closed circuit changes, an e.m.f. is induced in the circuit.
39. State Faraday’s second law of electromagnetic induction.
Ans: Faraday’s second law of electromagnetic induction states that the strength of the induced e.m.f. is directly proportional to the rate of change of the magnetic flux in that circuit.
40. How does the electric bell work? Explain.
Ans: An electric bell is a source of sound that converts electrical energy into kinetic energy and sound energy.
An electric bell consists of the U-shaped soft iron core around which an insulated wire is wound to make it an electromagnet. A soft iron armature is adjusted in front of the electromagnet. Its one end is fixed, while the other end is connected with a hammer which is free to move. Nearby the hammer, a gong is adjusted in such a way that the hammer can beat on the gong. The armature is connected with a screw at the point x.
When the key is closed, the electromagnet attracts the armature. Due to this, the hammer strikes the gong and the bell rings. As soon as the armature is pulled by the electromagnet, current in the circuit is automatically switched off. This is because the circuit gets broken at point x. In this situation, the electromagnet loses its magnetism and it no longer attracts the armature thus the armature returns to its original position. The cycle is completed and current flows in the circuit. The cycle is repeated again and again to ring the bell until the switch is on.
41. A fuse of what capacity is needed to be put in the connecting wire of an electric iron with 1000 W and 220 V.
And: 5A
42. A circuit with 220 V is supplied with a fuse of 5 A. How many bulbs of 100 W can be safely used in the circuit?
Ans: 10 bulbs
43. What is a fuse?
Ans: A piece of wire made of a material of high resistance and a very low melting point is called Fuse.
44. Calculate the power of the bulb if 0.2 A current flows through the given circuit.
Ans: 0.6W
45. What is the amount of current needed for a fluorescent lamp of 40 watt connected to a source of 220 V?
Ans: 0.1818A
46. Write differences between filament lamp and florescent lamp.
Ans: Differences between filament lamp and fluorescent lamp are as followings:
Filament lamp
1. It converts 10% of total energy into light and remaining into heat.
2. It is filled with nitrogen or inert gases like argon or neon to prevent oxidation and evaporation of tungsten metal at high temperature.
3. Its average life is about 1000 hours.
Florescent Lamp
1. It converts 30% of electrical energy into light and remaining into heat.
2. It is filled with mercury vapour, which produces ultraviolet radiation when the current passes through it.
3. Its average life is about 3000 hours. 4. Tungsten filament emits lights.4. Fluorescent powder emits light.
47. What is electroplating?
Ans: The process in which a layer of a particular element is coated on the surface of another conductor by the method of electrolysis is called electroplating.
48. What is electromagnet?
Ans: A temporary magnet formed by passing current through a coil of wire wound around a piece of soft iron is called electromagnet.
49. Fluorescent lamps are filled with mercury vapour, why?
Ans: Fluorescent lamps are filled with mercury vapour in order to produce ultraviolet rays, which are emitted due to the flow of electrons through the mercury vapour. The fluorescent powder absorbs the ultraviolet rays and emits visible light.
50. When electric current is passed in a filament lamp, the filament is heated to bright but not the other wires, why?
Ans: When electric current is passed in a filament lamp, the filament is heated to bright but not the other wires because the filament is very thin and has high resistance. Due to high resistance the filament converts electrical energy into heat and light but not the other wires.
