Electricity Class 10 Science Physics One Shot ConceptsQuestions CBSE Class 10 Board Exams

Electricity: Current, Resistance, and Ohm's Law

Electricity is one of the most important chapters in CBSE Class 10 Science, introducing the fundamental concepts of electric current, potential difference, resistance, and the relationship between them. Electric current is defined as the rate of flow of electric charge through a conductor. If a net charge Q flows through any cross-section of a conductor in time t, the current I = Q/t is measured in amperes (A), where 1 ampere equals 1 coulomb per second. Current is measured using an ammeter connected in series with the circuit. The direction of conventional current is taken as the direction of flow of positive charges, which is opposite to the direction of electron flow. Electric potential at a point is the work done per unit charge in bringing a positive test charge from infinity to that point, while potential difference (voltage) between two points is the work done per unit charge in moving charge between them: V = W/Q. It is measured in volts (V) using a voltmeter connected in parallel across the component.

Ohm's Law, discovered by Georg Simon Ohm in 1827, states that at constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends: V = IR, where R is the resistance of the conductor measured in ohms (Ω). Resistance is a property of the conductor that opposes the flow of current. It depends on four factors: the material (characterised by resistivity ρ), the length L (R increases with length), the cross-sectional area A (R decreases with increasing thickness), and the temperature. The relationship is given by R = ρL/A. Resistivity (ρ) is an intrinsic property of the material — metals like copper and silver have very low resistivity (good conductors), while materials like rubber and glass have very high resistivity (insulators). Alloys like nichrome have much higher resistivity than their constituent metals and are used in heating elements. For most metallic conductors, resistance increases with temperature because increased thermal vibration of atoms scatters electrons more frequently.

Resistors can be connected in series or in parallel, and the equivalent resistance depends on the configuration. In a series circuit, the same current flows through each resistor, the total voltage equals the sum of individual voltage drops (V = V₁ + V₂ + V₃), and the equivalent resistance is the sum of individual resistances: Rs = R₁ + R₂ + R₃. In a parallel circuit, the same voltage appears across each resistor, the total current equals the sum of individual branch currents, and the reciprocal of the equivalent resistance equals the sum of reciprocals: 1/Rp = 1/R₁ + 1/R₂ + 1/R₃. The equivalent resistance in parallel is always less than the smallest individual resistance. Electric power P = VI = I²R = V²/R measures the rate at which electrical energy is converted to other forms (heat, light, mechanical energy). The commercial unit of energy is the kilowatt-hour (kWh), which equals 3.6 × 10⁶ joules. The heating effect of current, described by Joule's law (H = I²Rt), is the principle behind electric heaters, geysers, electric irons, and the filament in incandescent bulbs. Fuses and circuit breakers protect circuits by melting or tripping when current exceeds a safe value.

• Current I = Q/t (amperes); potential difference V = W/Q (volts); Ohm's Law: V = IR.
• Resistance R = ρL/A depends on material resistivity, length, cross-sectional area, and temperature.
• Series: Rs = R₁ + R₂ + R₃ (same current, voltages add); Parallel: 1/Rp = 1/R₁ + 1/R₂ + 1/R₃ (same voltage, currents add).
• Electric power P = VI = I²R = V²/R; energy consumed is measured in kilowatt-hours (kWh).
• Heating effect: H = I²Rt — used in heaters, irons, and fuses for circuit protection.