Short Summary

Chapter 12, Electricity, covers the fundamental principles governing electric current, potential difference, circuit behaviour, resistance, heating effects, and electrical power consumption. Electric current (I) is the rate of flow of electric charge through a conductor and is given by I = Q/t, where Q is the charge and t is the time. It is measured in amperes (A) using an ammeter connected in series. Electric potential difference (V) is the work done to move a unit charge between two points and is given by V = W/Q, where W is the work done and Q is the charge. It is measured in volts (V) using a voltmeter connected in parallel. Ohm's law states that, at a constant temperature, the potential difference across a conductor is directly proportional to the current flowing through it, giving the relation V = IR, where R is the resistance. Resistance is measured in ohms (Ω) and depends directly on the length of the conductor, inversely on its cross-sectional area, and on the resistivity (ρ) of the material, which is measured in ohm-metres (Ω·m). In a series combination of resistors, the current remains the same through all resistors while the total resistance increases according to Rₛ = R₁ + R₂ + R₃. In a parallel combination, each branch receives the full supply voltage while the equivalent resistance decreases according to 1/Rₚ = 1/R₁ + 1/R₂ + 1/R₃. When electric current passes through a resistive conductor, electrical energy is converted into heat. Joule's law of heating states that the heat produced is given by H = I²Rt, meaning that the heat produced is directly proportional to the square of the current, the resistance, and the time for which the current flows. This principle is used in devices such as electric irons, tungsten-filament bulbs, and safety fuses. Electric power (P) is the rate at which electrical energy is consumed or dissipated and is given by P = VI = I²R = V²/R, with power measured in watts (W). The commercial unit of electrical energy is the kilowatt-hour (kWh), where 1 kWh = 3.6 × 10⁶ J.