Class 10 Science · Chapter 11 NotesElectricity

When electric charge flows through a conductor such as a metallic wire, we say there is an electric current in it. A continuous and closed path of an electric current is called an electric circuit. If the circuit is broken anywhere, the current stops flowing. Electric current is the amount of charge flowing through a particular area in unit time, that is, the rate of flow of electric charges. In metallic wires, electrons constitute the flow of charge, but conventionally the direction of current is taken as opposite to the direction of flow of electrons. An ammeter measures current and is always connected in series in the circuit.

• I = Q/t, where Q is the net charge flowing in time t
• SI unit of chargecoulomb (C); 1 C is nearly the charge of 6 × 10¹⁸ electrons
• SI unit of currentampere (A); 1 A = 1 C/1 s
• 1 mA = 10⁻³ A and 1 µA = 10⁻⁶ A

Charges do not flow in a conductor by themselves. They move only when there is a difference of electric pressure, called the potential difference, along the conductor. A cell or battery produces this potential difference through chemical action. The potential difference between two points is the work done to move a unit charge from one point to the other. It is measured with a voltmeter, which is connected in parallel across the points. Georg Simon Ohm found that the potential difference across the ends of a metallic wire is directly proportional to the current through it, provided its temperature remains the same. This is Ohm's law.

• V = W/Q; SI unit of potential difference is volt (V), 1 V = 1 J C⁻¹
• Ohm's lawV ∝ I, so V = IR
• R = V/I; SI unit of resistance is ohm (Ω), 1 Ω = 1 V/1 A
• I = V/R, so current is inversely proportional to resistance

Resistance is the property of a conductor that resists the flow of charges through it. Experiments show that the resistance of a uniform metallic conductor is directly proportional to its length and inversely proportional to its area of cross-section. It also depends on the nature of the material of the conductor. The constant of proportionality is called the electrical resistivity of the material, and its SI unit is ohm metre (Ω m). Metals and alloys have very low resistivity and are good conductors, while insulators such as rubber and glass have very high resistivity. Both resistance and resistivity vary with temperature.

• R = ρl/A, where l is length, A is area of cross-section and ρ is resistivity
• Alloys generally have higher resistivity than their constituent metals and do not oxidise readily at high temperatures
• Tungsten is used for bulb filaments; copper and aluminium are used for transmission lines
• A component used to regulate current without changing the voltage source is called variable resistance; a rheostat is used for this purpose

Resistors can be joined in two ways. In a series combination, resistors are joined end to end, the current through each resistor is the same, and the total potential difference is the sum of the potential differences across the individual resistors. The equivalent resistance is greater than any individual resistance. In a parallel combination, resistors are connected between the same two points, the potential difference across each resistor is the same, and the total current is the sum of the separate currents through each branch. The equivalent resistance is less than the smallest individual resistance.

• SeriesRs = R₁ + R₂ + R₃ + ...
• Parallel1/Rp = 1/R₁ + 1/R₂ + 1/R₃ + ...
• In series, current is the same in every part of the circuit
• In parallel, potential difference across each resistor is the same

A cell or battery is a source of electrical energy. In a purely resistive circuit, the energy supplied by the source is dissipated entirely in the form of heat. This is known as the heating effect of electric current. If a current I flows through a resistor of resistance R for time t, with potential difference V across it, the heat produced is H = VIt. Applying Ohm's law gives H = I²Rt, which is Joule's law of heating. The law implies that heat produced is directly proportional to the square of the current, directly proportional to the resistance, and directly proportional to the time for which the current flows.

• H = VIt
• H = I²Rt (Joule's law of heating)
• Heat ∝ I², heat ∝ R, heat ∝ t
• In a purely resistive circuit, the source energy is continually dissipated as heat

The rate at which electric energy is dissipated or consumed in an electric circuit is called electric power. It is given by P = VI, and by applying Ohm's law this can also be written as P = I²R or P = V²/R. The SI unit of power is the watt (W), which is the power consumed by a device carrying 1 A at a potential difference of 1 V. Electrical energy is the product of power and time. The commercial unit of electric energy is the kilowatt hour (kW h), commonly called a unit.

• P = VI = I²R = V²/R
• 1 W = 1 V × 1 A
• 1 kW h = 1000 W × 3600 s = 3.6 × 10⁶ J
• Energy consumed = power × time

The heating effect of current is used in many everyday devices. Electric laundry irons, toasters, ovens, kettles and heaters work on Joule's heating. In an electric bulb, the filament must retain heat so that it becomes very hot and emits light; tungsten, with its high melting point, is used for filaments, and bulbs are filled with chemically inactive nitrogen and argon gases. Another important application is the fuse, which protects circuits and appliances by melting and breaking the circuit when an unduly high current flows. The fuse is placed in series with the device and is made of a metal or alloy of appropriate melting point.

• Electric heating deviceselectric iron, toaster, oven, kettle, heater
• Bulb filamenttungsten, melting point 3380°C
• Fuseplaced in series, melts and breaks the circuit on excessive current
• Domestic fuses are rated as 1 A, 2 A, 3 A, 5 A, 10 A, etc.