1. Electricity is one of the most convenient and widely used forms of energy in the world.
2. Conductors are those substances in which electric charge can flow freely. Metals are electric conductors. Copper and aluminium are the two most commonly used metals in electricity.
3. Electrical insulators are those materials in which electric charges do not move freely. Rubber, glass and plastic are examples of insulators. Electrical Insulators
4. Current electricity is the electricity which deals with moving charges.
5. A continuous and closed path of an electric current is called an electric circuit. Electric Circuit
6. Electric current is defined as the rate of flow of electric charge through a cross-section of a conductor.
The flow of electric current
7. S.I. unit of electric current is ‘ampere' (A). One-ampere current is equivalent to the flow of 1 coulomb of charge per second through a section of a conductor. Thus, 1 A = 1 C s-1.
8. Direction of flow of electric current is taken as opposite to the direction of the flow of negatively charged electrons. In other terms, the direction of conventional current is taken as the direction along which positive charges will move, if allowed to do so.
9. In conductors flow of electric current is due to the flow of electrons. The flow of 6.25 x 1018 electrons per second across a cross-section of a conductor constitutes a current of 1 ampere.
10. The flow of electric charge in a circuit is similar to water flow in a pipe.
11. Potential difference between two points in an electric circuit carrying current is defined as the work done to move a unit positive charge from one point to another. If be the amount of work done in taking a charge Q from point B to point A, then
12. The SI unit of electric potential difference is volt (V), where
13. George Simon Ohm established a relationship between the electric current flowing through a conductor and the potential difference across its ends, due to which current flows.
V ∝ I or V = IR
14. Electrical resistance of a conductor may be considered as a measure of the opposition offered by it for the flow of electric charge through it.
15. At a given temperature resistance of a conductor depends on its:
(i) Length L
(ii) Cross-section area A
(iii) Nature of the material of the conductor.
Mathematically, R = ρL/A
16. Metals and alloys have low resistivity in the range of 10-8 Ω m to 10-6 Ω m and are good conductors of electricity.
17. Resistance, as well as the resistivity of a material, vary with temperature. For metallic conductors resistance as well as resistivity increases with increase in temperature.
18. Resistivity of pure metals is very-very low. On account of this reason, metals like copper and aluminium are used for electrical transmission lines.
19. Resistivity of alloys is greater than that of pure metals. However, alloys do not oxidise easily, and their change in resistivity with rise in temperature is small. Due to these properties, they are used as electrical elements in most of the electric appliances. Generally, nichrome elements are used in electric iron, heater, radiator, geyser etc.
20. Tungsten filaments are used in electric lamps because the melting point of tungsten is extremely high.
21. A rheostat is a variable resistance device used in electrical circuits. It is used to regulate the current in the circuit without changing the voltage source.
22. An ammeter is an instrument which can measure the electric current flowing in an electric circuit directly in amperes and its submultiples.
23. A voltmeter is an instrument used to measure the potential difference across two given points in an electrical circuit directly in volts.
24. In an electric circuit resistances may be connected in:
(ii) Parallel Arrangement
25. In the series grouping of resistances:
(i) The current flowing through all the resistances is the same.
(ii) The total voltage across the combination is equal to the sum of the voltage drops across the individual resistors.
(iii) The total resistance of the combination is equal to the sum of the individual resistances.
26. In the parallel grouping of resistances:
(i) The voltage across each resistor is the same and equal to the voltage across the whole combination.
(ii) The currents in various resistors are inversely proportional to the resistances, and the total current is the sum of the currents flowing through different resistances, and (Hi) the sum of the reciprocals of the separate resistances is equal to the reciprocal of equivalent resistance. If R1, R2, R3,... be the individual resistors joined in parallel then the equivalent resistance Rp is given as :
27. In household electric circuits, a series circuit is not followed. It is because if one component of the series circuit fails, the entire circuit is broken and none of the components works. Moreover, the same current flows through all the components irrespective of their operating needs.
28. A parallel circuit divides the current in various components (appliances), and each component can draw current as per its appropriate operation. Separate on/off switch can be put with each component in the parallel circuit. Moreover, the total resistance in parallel circuit arrangement decreases and hence a stronger current can be drawn from a voltage source. Due to these reasons, all appliances are connected in parallel in our household electric circuit.
29. If on applying, a potential difference V across the ends of a conductor of resistance R, the current I flows for a time t, then as per Joule’s law of heating the electric energy consumed is given by:
30. Time rate of doing electric work is called electric power. Thus,
31. A watt-hour and kilowatt-hour are practical units of electric energy. The commercial unit of electric energy is a kilowatt-hour (kW h), where
1 kW h = 1000 Wh = 3.6 x 106 J.
32. Heating effect of electric current is used in a large number of devices like electric iron, electric toaster, electric oven, electric kettle, electric heater, electric radiator, geyser etc.
33. Electric heating is also used to produce light in electric bulbs. As bulb filament operates at very high temperature, the filament should be thermally insulated. The bulb is filled with chemically inactive nitrogen and argon gases to prolong the life of bulb filament.
34. Electric fuse protects an electric circuit or appliance by stopping the flow of any unduly high electric current.