Electricity

Revision Summary: Electricity (NCERT Class 10, Ch 11)

Chapter at a glance

• Electric current is the rate of flow of electric charge; in metallic conductors it is due to electrons, but conventionally taken as flow of positive charge in the opposite direction.
• A continuous and closed path for current is an electric circuit; a switch makes or breaks this path.
• Potential difference between two points is the work done per unit charge to move charge between them; it is maintained by a cell/battery.
• Ohm’s law states that potential difference across a conductor is directly proportional to current through it at constant temperature.
• Resistance of a conductor depends on its length, area of cross-section and the material (resistivity).
• Resistors can be combined in series (same current, resistances add) or parallel (same potential difference, reciprocals of resistances add).
• In a purely resistive circuit, electrical energy is converted entirely into heat (Joule’s heating).
• Electric power is the rate of consumption of electrical energy; commercial unit of energy is kilowatt-hour (kWh).

Definitions and laws (exact NCERT framing) - Electric current: If a net charge Q flows across any cross-section of a conductor in time t, then I = Q/t. SI unit: ampere (A); 1 A = 1 C s⁻¹. - Electric potential difference: V = W/Q. SI unit: volt (V); 1 V = 1 J C⁻¹. - Ohm’s law: The potential difference, V, across the ends of a given metallic wire in an electric circuit is directly proportional to the current flowing through it, provided its temperature remains the same. V = IR or R = V/I. SI unit of resistance: ohm (Ω); 1 Ω = 1 V A⁻¹. - Resistivity: R = ρl/A. SI unit: Ω m. - Joule’s law of heating: H = I²Rt (also H = VIt). - Electric power: P = VI = I²R = V²/R. SI unit: watt (W); 1 W = 1 V A. Commercial unit of energy: kilowatt-hour (kWh); 1 kWh = 3.6 × 10⁶ J.

Important diagrams and activities

• Fig. 11.1: Schematic circuit (cell, bulb, ammeter in series, plug key) – shows conventional current direction and ammeter connection.
• Fig. 11.2 & 11.3: Circuit with nichrome wire + cells, ammeter, voltmeter; V–I graph – verifies Ohm’s law (straight line through origin).
• Fig. 11.4 & Activity 11.2: Circuit with different components (nichrome, torch bulb, 10 W bulb) – shows different currents for different resistances.
• Fig. 11.5 & Activity 11.3: Circuit to study effect of length, thickness and material on resistance.
• Table 11.1: Standard circuit symbols (cell, battery, switch open/closed, resistor, rheostat, ammeter, voltmeter).
• Fig. 11.6 & 11.8: Series combination – same current through each resistor; V = V₁ + V₂ + V₃.
• Fig. 11.7, 11.10 & 11.11: Parallel combination – same potential difference across each resistor; I = I₁ + I₂ + I₃.
• Fig. 11.13: Purely resistive circuit – illustrates conversion of electrical energy into heat.

Common misconceptions and exam pitfalls

• Confusing conventional current direction with actual electron flow (electrons move opposite to conventional current).
• Assuming electrons are “used up” in the circuit (energy is supplied by the source; charge is conserved).
• Connecting ammeter in parallel or voltmeter in series.
• Forgetting temperature dependence when applying Ohm’s law.
• Writing series/parallel resistance formulae incorrectly (resistances add in series; reciprocals add in parallel).
• Using power or energy units wrongly (W vs kWh) or omitting time in energy calculations.
• Assuming same current or same resistance in mixed series-parallel circuits without redrawing equivalent circuits.

Formula sheet