Chapter 12
Electricity
1. Name a controllable form of energy.
A. Electricity.
2. How does switch helps the bulb to glow?
A. A switch makes a conducting link between the cell and the bulb.
3. What is an electric circuit?
A. A continuous and closed path of an electric current is called an electric circuit.
4. Why bulb does not glow when the switch is off?
A. When the switch is off the current stops flowing, so bulb does not glow.
5. How is electric current expressed?
A. Electric current is expressed by the amount of charge flowing through a particular
area in unit time.
I = Q/t
I-Current flowing through conductor, Q-amount of charge and t-time.
6. Write the path of the electric current [conventionally]?
A. Conventionally Electric current flows from positive terminal to negative terminal.
7. What is the SI unit of electric charge?
A. Coulomb (C).
8. What is the charge present on an electron?
A. 1.610-19 C.
9. How many electrons charge is equal to one electron?
A. 1.61018 C.
10. What is the SI unit of current?
A. Ampere (A).
11. Define one Ampere.
A. One ampere is the current flowing through a conductor when 1 coulomb of charge
passes through the cross-section in 1 second.
1A =1C/1s
12. Name the instrument, which measures electric current.
A. Ammeter.
13. Where is ammeter connected in a circuit?
A. Ammeter is always connected in series in a circuit.
14. How is potential difference generated across the terminals of cell?
A. The chemical action within a cell generates the potential difference across the
terminals of the cell.
15. What is potential difference?
A. Potential difference between two points in an electric circuit carry some current
as the work done to move a unit charge from one point to the other.
V=W/Q
V-Potential difference, W-work done and Q-charge.
16. What is the SI unit of electric potential difference?
A. Volt (V).
17. Define 1 volt?
A. One volt is the electric potential difference between two points in a current
carrying conductor when 1 joule of work is done to move a charge of 1 coulomb from
one point to the other.
1V=1J/1C
18. What are the constituents of nichrome?
A. Nichrome is an alloy of nickel, chromium, manganese and iron metals.
19. State Ohm’s law.
A. Ohm’s law states that electric current flowing through a metallic wire is directly
proportional to the potential difference (V), across its ends provided its temperature
remains the same.
VI
20. What is resistance?
A. The property of a conductor to resist the flow of charge through is called
resistance.
21. What is SI unit of resistance?
A. Ohm ().
22. Define 1 ohm.
A. 1 Ohm is resistance of the conductor if the potential difference across the two
ends of a conductor if 1V and the current through it is 1A.
1 =1V/1A
23. What is variable resistance?
A. A component used to regulate current without changing the voltage source is
called variable resistance.
24. What is the use of rheostat?
A. Rheostat is used to change the resistance in the circuit.
25. What is a good conductor?
A. A component of a given size that offers a low resistance is a good conductor.
26. What is an insulator?
A. A component of identical size that offers a higher resistance is called insulator.
27. What is resistor?
A. A conductor having some appreciable resistance is called resistor.
28. On what factors does the resistance of the conductor depend?
A. Resistance of the conductor depends on
a. Length of conductor (Rl).
b. Area of cross-section of conductor (R1/A).
c. The nature of the material.
R=pl/A
29. What is the SI unit of resistivity?
A. m.
30. Why alloys are used in electrical heating devices like electric iron, toasters etc?
A. Alloys resistivity is generally higher than that of its constituent metals. They do
not oxidize (burn) readily at high temperatures. So they are commonly used in electric
heating devices.
31. Which metals are used in transmission lines?
A. Copper and Aluminium.
32. What is present in the filaments of electric bulbs?
A. Tungsten.
33. When do we say that resistors are connected in parallel?
A. When the resistors are connected together between two points, then they are said
to be connected in parallel.
34. Is current in series combination depends on the ammeter position?
A. No.
35. Write the formula for total potential difference in series combination.
A. V=V1+V2+V3.
36. Write the formula for total resistance in series combination.
A. R=R1+R2+R3.
37. Is current in parallel combination same everywhere?
A. No.
38. What is the value of total resistance in parallel combination?
A. 1/R=1R1+1/R2+1/R3.
39. What is the value of total current in parallel combination?
A. I=I1+I2+I3.
40. What are the disadvantages of connecting appliances in series?
A. Disadvantages
1. Electric bulb and an electric heater are not connected in series because they
need currents of widely different values to operate properly.
2. When one component fails the circuit is broken and none of the component
works.
41. What are the advantages of connecting appliance in parallel?
A. Advantages
1. A parallel circuit divides the current through the electrical gadgets.
2. The total resistance in a parallel circuit is decreased. This is helpful
particularly when each gadget has different current to operate properly.
42. What is the formula for Joules law of heating?
A. H=I2Rt.
43. Name the devices, which are based on Joules heating.
A. Electric laundry iron, electric toaster, electric oven, electric kettle, electric heater
etc.
44. Which gases are filled in bulb to prolong the life of the filament?
A. Inactive nitrogen and argon.
45. How fuse protects when more current flows?
A. Fuse consists of a piece of wire made of a metal or an alloy of appropriate
melting point. If a current larger than the specified value flows through the circuit,
the temperature of the fuse wire increases. This melts the fuse wire and breaks the
circuit.
46. What is electric power?
A. The rate at which electric energy is consumed in an electric circuit is called
electric power.
47. What is the SI unit of electric power?
A. Watt (W).
48. Define one Watt.
A. One Watt is the power consumed by a device that carries 1A of current when
operated at a potential difference of 1V.
1W=1Volt1Ampere
=1VA
49. Express 1kiloWatt in Watts.
A. 1000W.
50. Define one-watt hour.
A. One Watt hour is the energy consumed when 1W of power is used in 1hour.
51.
A.
52.
A.
Express 1unit or 1kiloWatt hour in Joules.
3.6106Joule(J).
What is discovered first-electricity or electrons?
Electricity.
53. Write the other units of current.
A. Milliampere (1mA=10-3A) and Microampere (1A=10-6A).
54. Name the components present in typical electric circuit.
A. Cell, and electric bulb, an ammeter and a plug key.
55. To which country Volta belongs?
A. Italy.
56. What does the cell do to maintain the current in an electric circuit?
A. In order to maintain the current in an electric circuit, the cell expends the
chemical energy stored in it.
57. To which country Ampere belongs?
A. France.
58. Name the instrument used to measure potential difference.
A. Voltmeter.
59. How is voltmeter connected in circuit?
A. The voltmeter is always connected in parallel across the points between which the
potential difference is to be measured.
60. Who founded the relationship between the current and the potential difference?
A. Georg Simon Ohm.
61. To which country ohm belong?
A. Germany.
62. Through which symbol ohm is represented?
A.  (Greek letter).
63. Is current directly proportional to resistance?
A. No, current is inversely proportional to resistance.
64. Name the property of conductor, which retards the motion of electrons.
A. Resistance.
65. Why electrons not free to move within a conductor?
A. Electrons are restrained by the attraction of the atoms among which they move.
66. What is the range of resistivity of metals?
A. 10-8m to 10-6m.
67. What is the range of resistivity of insulators?
A. 1012m to 1017m
68. Do resistance and resistivity vary with temperature?
A. Yes.
69. Compare the total resistance to individual resistance in both series and parallel
combination.
A. In series combination, value total resistance is greater than values individual
resistances but in parallel combination value total resistance is lower than values
individual resistances.
70. Derive power input to the circuit.
a. Consider a current I flowing through a resistor of resistance R. Let the potential
difference across it be V. Let t be the time during which a charge Q flows across.
The work done in moving the charge Q through a potential difference V is VQ.
Therefore, the source must supply energy equal to VQ in time t. Hence, the power
input to the circuit by the source is
P = VQ/t =VI
71. Derive the formula for amount of heat produced.
A. The energy supplied to the circuit by the source in time t is Pt that is, VIt. The
energy expended by the source gets dissipated in the resistor as heat. Thus for a
steady current I, The amount of heat H produced in time t is
H = VIt
H = I2Rt
72. What is the melting point of tungsten?
A. 3380oC.
73. Why is tungsten used for making bulb filaments?
A. Tungsten can retain as much of the heat and emits light. Tungsten does not melt
at high temperature. So tungsten is used for making bulb filaments.
74. Where is fuse wire encased?
A. The fuse wire is usually encased in a cartridge of porcelain or material with metal
ends.
75. What fuses are generally used for domestic purposes?
A. 1A, 2A, 3A, 5A, 10A etc.
76. What fuse is used for an electric iron, which consumes 1kWh electric power
when operated at 220V?
A. 5A fuse is used.
77. What is power?
A. The rate of doing work is called power.
78. What is the commercial unit of electric energy?
A. The commercial unit of electric energy is kilowatt hour (kWh)
79.
A.
80.
A.
Write the common name of commercial unit of electric energy
Unit.
What is the shape of V-I graph?
V-I graph is a straight line that passes through the origin of the graph.
81. Do electrons have any difficulty in passing through a solid conductor since the
atoms are packed closely with very little spacing between them?
A. No electrons are able to travel through a perfect solid crystal smoothly and easily.