Electricity Class 10 Notes (2026-27) — CBSE
Class 10 Science Chapter 11 notes: electric current, Ohm's law, resistance and resistivity, series and parallel circuits, heating effect and electric power.
Electricity — Class 10 Science Notes
Chapter Snapshot
This chapter builds the physics of electric circuits: what current and potential difference are, how Ohm's law links them through resistance, what resistance depends on, how resistors combine in series and parallel, and how current produces heat and consumes power. It is one of the most numerical-heavy and highest-scoring chapters of Class 10 Science.
Board relevance: expect a 3-mark numerical (Ohm's law, series/parallel, or power), a circuit-diagram question, and short reasoning questions. Getting the formulas and units exactly right is where the marks are.
Key Concepts & Definitions
Electric current (I) — the rate of flow of electric charge: I = Q/t. SI unit ampere (A); 1 A = 1 coulomb/second. Measured with an ammeter connected in series. Conventional current flows from + to − terminal (opposite to electron flow).
Electric charge is quantised; charge of one electron = 1.6 × 10⁻¹⁹ C, so 1 coulomb ≈ 6.25 × 10¹⁸ electrons.
Potential difference (V) — the work done to move a unit charge between two points: V = W/Q. SI unit volt (V); 1 V = 1 joule/coulomb. Measured with a voltmeter connected in parallel across the component.
Resistance (R) — the opposition a conductor offers to the flow of current. SI unit ohm (Ω). A component's resistance is 1 Ω if 1 V drives 1 A through it.
Rheostat — a variable resistance used to change the current in a circuit without changing the source.
Resistivity (ρ) — a property of the material, independent of its dimensions. SI unit ohm-metre (Ω·m). Metals and alloys have low resistivity (good conductors); rubber and glass have very high resistivity (insulators). Alloys generally have higher resistivity than pure metals and resist oxidation at high temperature.
Formulas
Ohm's law and resistance
Quantity Formula Unit
Electric current I = Q/t ampere (A)
Potential difference V = W/Q volt (V)
Ohm's law V = IR —
Resistance from dimensions R = ρL/A ohm (Ω)
Ohm's law: at constant temperature, V ∝ I, so V = IR. A graph of V (y-axis) against I (x-axis) is a straight line through the origin; its slope is R.
Resistance depends on: length L (R ∝ L), area of cross-section A (R ∝ 1/A), the material (ρ), and temperature (R usually rises with temperature). A thick wire has less resistance than a thin one of the same material and length.
Combination of resistors
Combination Equivalent resistance Shared quantity
Series R = R₁ + R₂ + R₃ + … Same current through each
Parallel 1/R = 1/R₁ + 1/R₂ + 1/R₃ + … Same voltage across each
- In series, R is larger than the largest resistor; the current is the same everywhere; voltages add: V = V₁ + V₂ + V₃.
- In parallel, R is smaller than the smallest resistor; the voltage is the same across each; currents add: I = I₁ + I₂ + I₃.
- Two equal resistors R in parallel give R/2; in series give 2R.
Household circuits use parallel connection so each device gets the full 220 V, works independently, and draws its own current.
Heating effect and electric power
Joule's law of heating: the heat produced in a resistor is
H = I²Rt
where I is current, R resistance, t time. (Using V = IR, also H = VIt = V²t/R.)
Electric power (P) — rate of consuming electrical energy:
Formula Use
P = VI when V and I are known
P = I²R when I and R are known
P = V²/R when V and R are known
SI unit of power: watt (W); 1 W = 1 J/s. Common larger unit: kilowatt (1 kW = 1000 W).
Electrical energy = power × time. The commercial unit is the kilowatt-hour (kWh) = "1 unit":
1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J
Applications of the heating effect: electric heaters, geysers, irons, toasters (nichrome elements), and the filament of an electric bulb (tungsten, very high melting point). The electric fuse is a safety device — a thin wire of low melting point that melts and breaks the circuit if the current exceeds a safe value.
Important Question Patterns
1. Ohm's law numerical (2–3 marks): find I, V, or R given the other two; interpret a V–I graph (slope = resistance; steeper line = more resistance).
2. Series/parallel network (3 marks): compute equivalent resistance, then total current, then current/voltage in each branch. State which quantity is shared.
3. Resistance from dimensions (2 marks): effect of doubling length or halving area; R = ρL/A; what happens to R when a wire is stretched to double its length (R becomes 4× — length doubles, area halves).
4. Power and energy (3 marks): compute power from any of the three formulas; energy in kWh; cost of running an appliance; compare two bulbs' resistances from their ratings (higher wattage → lower resistance at same voltage).
5. Reasoning (1–2 marks): why nichrome for heating elements; why the bulb filament is tungsten; why ammeter in series and voltmeter in parallel; why household wiring is parallel; function of a fuse.
⚡ Quick Revision
- I = Q/t (ampere), V = W/Q (volt). Ammeter in series, voltmeter in parallel.
- Ohm's law: V = IR. V–I graph is a straight line through the origin; slope = R.
- R = ρL/A — R ∝ length, R ∝ 1/area; ρ is a material property (unit Ω·m). Stretching a wire to 2× length makes R = 4× original.
- Series: R = R₁+R₂+R₃, same current, R is largest. Parallel: 1/R = 1/R₁+1/R₂+1/R₃, same voltage, R is smallest.
- Household appliances are in parallel (full voltage, independent, own current).
- Joule's law: H = I²Rt. Power: P = VI = I²R = V²/R (watt).
- Energy: 1 kWh = 3.6 × 10⁶ J = 1 unit.
- Nichrome heating elements (high resistivity, high melting point, doesn't oxidise); tungsten bulb filament; fuse = safety wire with low melting point.
- Same voltage: a higher-wattage bulb has lower resistance (P = V²/R).
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