Quick Views - CH-12 Electricity

Quick View

CH- 12 Electricity

Class X

Syllabus of Electric current -Potential difference and Electric current. Ω's law; Resistance, Resistivity, Factors on which the resistance of a conductor depends. Series combination of resistors, a parallel combination of resistors and its applications in daily life. Heating effect of electric current and its applications in daily life. Electric power, Interrelation between P, V, I and R.

1. Electric current is a stream of moving electrons through a conductor and it is expressed by the amount of charge flowing through a particular area in unit time. In other words, it is the rate of flow of electric charges.
2. Electric circuit is a continuous and closed path of an electric current.
3. The SI unit of electric current is ampere (A).
4. If a net charge Q, flows across any cross-section of a conductor in time t, then the current I, through the cross-section is I = Q/ t.
5. The SI unit of an electric charge is the coulomb (C), which is equivalent to the charge contained in nearly 6 × 10¹⁸ electrons.
6. We use a cell or a battery to set the electrons in motion in an electric circuit. A cell generates a potential difference across its two terminals. It is measured in volts (V).
7. An electron possesses a negative charge of 1.6 × 10^–19 C.
8. One ampere is constituted by the flow of one coulomb of charge per second, that is,
   1 A = 1 C/1 s.
   1 mA = 10^–3 A or in microampere 1 µA = 10^–6 A.
9. Ammeter- An ammeter is an instrument that measures electric current in a circuit. It is always connected in series in a circuit through which the current is to be measured.
10. Conventionally, the direction of the current is taken opposite to the direction of the flow of electrons. 
11. The electric current flows in the circuit from the positive terminal of the cell to the negative a terminal of the cell through the bulb and ammeter.
12. The electric potential difference expressed as the work is done to move a unit charge from one point to the other between two points in an electric circuit carrying current.
13. Potential difference (V) between two points = Work done (W) / Charge (Q)
    V = W/Q .
14. The SI unit of electric potential difference is volt (V).
15. One volt is the 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. Therefore, 1 volt = 1 joule /1 coulomb
    V = 1 J/ C
16. Ohm’s law: The electric current flowing through a metallic wire is directly proportional to the potential difference V, across its ends when its temperature remains the same. This is called Ohm’s law.  Therefor V ∝ I or V/I = R (where R is constant) or V = IR.
17. R is a constant for the given metallic wire at a given temperature and is called its resistance.
18. SI unit of resistance is the ohm, represented by the Greek letter Ω. According to Ohm’s law, R = V/I
19. 1 Ohm- If the potential difference across the two ends of a conductor is 1 V and the current through it is 1 A, then the resistance R, of the conductor is 1 Ω. That is, 1 ohm = 1 volt /1 ampere.
20. The SI unit of electric power is the watt (W).
21. The current through a resistor is inversely proportional to its resistance. If the resistance is doubled the current gets halved.
22. Voltmeter - The potential difference is measured by means of an instrument called the voltmeter. The voltmeter is always connected in parallel across the points between which the potential difference is to be measured.
23. Resistance is a property that resists the flow of electrons in a conductor. It controls the magnitude of the current. The SI unit of resistance is the ohm (Ω).
24. Factors of resistance - The resistance of a conductor depends directly on its length, inversely on its area of cross-section, and also on the material of the conductor.
25. A component used to regulate current without changing the voltage source is called variable resistance.
26. The equivalent the resistance of several resistors in series is equal to the sum of their individual resistances.
27. Rheostat is a device in an electric circuit often used to change the resistance in the circuit.
28. Resistor - A conductor having some appreciable resistance is called a resistor.
29. A component of the identical size that offers a higher resistance is a poor conductor.
30. An insulator of the same size offers even higher resistance.
31. Variable resistance - A component used to regulate current without changing the voltage source is called variable resistance.
32. The resistance of a uniform a metallic conductor is directly proportional to its length
    (l) and inversely proportional to the area of cross-section (A).
    That is, R ∝ l
    and R ∝ 1/A
    By combining eqs., we get
    R ∝ l /A 
    or, R = ρ l /A where ρ (rho) is a constant of proportionality and is called the electrical resistivity of the material of the conductor.
33. The SI unit of resistivity is Ω m. It is a characteristic property of the material. The metals and alloys have very low resistivity in the range of 10^–8 Ω m to 10^–6 Ω m. They are good conductors of electricity. Insulators like rubber and glass have a resistivity of the order of 10¹² to 10¹⁷ Ω m. Both the resistance and resistivity of a material vary with temperature.
34. The resistivity of an alloy is generally higher than that of its constituent metals.
35. Alloys do not oxidize or burn readily at high temperatures. For this reason, they are commonly used in electrical heating devices, like electric iron, toasters, etc. Tungsten is used almost exclusively for filaments of electric bulbs, whereas copper and aluminium are generally used for electrical transmission lines.
36. In a series combination of resistors, the current is the same in every part of the circuit or the same current through each resistor.
37. The total resistance of the series circuit- is equal to the sum of the resistances. R_s = R₁ +R₂ + R₃.
38. The reciprocal of the equivalent resistance of a group of resistances joined in parallel is equal to the sum of the reciprocals of the individual resistances. 1/R_p = 1/R₁ + 1/R₂ + 1/R₃.
39. The disadvantage of series connection –
    (i) In a series circuit, the current is constant throughout the electric circuit. Thus it is impossible to connect an electric bulb and an electric heater in series because they need currents of widely different values to operate properly.
    (ii) When one bulb fails a circuit is broken and none of the bulbs work.
40. Advantage of a parallel circuit –
    A parallel circuit divides the current through the electrical devices. The total resistance in a parallel circuit is decreased. This will be helpful when each gadget has a different resistance and requires different current to operate properly.
    When one component or device fails the circuit will not break and other components or devices can work.
41. When current flow through a conductor, after some time conductor becomes hot due to the conversion of some electric energy passing through the conductor into heat energy. This effect of electric current is called the heating effect of current.
42. The power input to the circuit by the source is P = V Q/T
    = VI  Or H = I² Rt  ( V = IR)
43. The electrical energy dissipated in a resistor is given by W = V × I × t ( V = volt, I = current, and t = tome)
44. The unit of power is watt (W). One watt of power is consumed when 1 A of current flows at a potential difference of 1 V.
45. Joule’s law of heating –According to the Joule’s law heat produced in a resistor is
    directly proportional to the square of current for a given resistance,
    directly proportional to resistance for a given current, and
    directly proportional to the time for which the current flows through the resistor.
46. Applications of Heating Effect of Electric Current -- The electric laundry iron, electric toaster, electric oven, electric kettle, electric bulb and the electric heater is some of the devices based on Joule’s heating.
47. A strong metal with a high melting point such as tungsten (m.p  3380°C) is used for making bulb filaments. The filament is thermally isolated.
48. The bulbs are usually filled with chemically inactive nitrogen and argon gases to prolong the life of the filament. Most of the power consumed by the filament as heat radiated and a small part of it is in the form of light-emitting.
49. Fuse -- The common application of Joule’s heating is the fuse used in electric circuits. It protects circuits and appliances by stopping the flow of high electric current. The fuse is placed in series with the device. It consists of a piece of wire made of a metal or an alloy of an appropriate melting point, for example, aluminium, copper, iron, lead, etc. 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. The fuse wire is usually fitted in a cartridge of porcelain or similar material with metal ends.
50. The fuses used for domestic purposes are rated as 1 A, 2 A, 3 A, 5 A, 10 A, etc.
51. For an electric iron which consumes 1 kW electric power when operated at 220 V, a current of 1000/220 A that is, 4.54 A will flow in the circuit. In this case, a 5 A fuse must be used.
52. Electric power --The rate of consumption of energy in an electric circuit is electric power. The power P is given by P = VI Or P = I²R = V²/R
53. 1 watt - It is the power consumed by a device that carries 1 A of current when operated at a potential difference of 1 V. Thus, 1 W = 1 volt × 1 ampere = 1 V A. 
54. The commercial unit of electric energy is a kilowatt-hour (kW h), commonly known as ‘unit’. 1 kW h = 1000 watt × 3600 second = 3.6 × 10⁶ watt-second = 3.6 × 10⁶ joules (J).