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Electricity – Class 10 Science notes, important questions and MCQs

NCERT Class 10 Science, chapter 11. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Electric current is the flow of charge. Ohm's law links voltage, current and resistance, and resistors can be joined in series or parallel. Current also produces heat.

Basics

Current I = Q/t (ampere); potential difference V = W/Q (volt).
Ohm's law: V = IR at constant temperature.
Resistance R = ρl/A depends on length, area, material and temperature.

Combinations

Series: R = R₁ + R₂ + …; same current through each.
Parallel: 1/R = 1/R₁ + 1/R₂ + …; same voltage across each.
Household wiring uses parallel connection so each appliance gets full voltage and can be switched independently.

Heating and power

Heat H = I²Rt (Joule's law); used in heaters, irons, bulbs, fuses.
Power P = VI = I²R = V²/R; unit watt.
Commercial unit 1 kWh = 3.6 × 10⁶ J (one "unit" on the bill).

Key terms

Resistivity: resistance of a 1 m long, 1 m² cross-section piece of a material
Ammeter: measures current, connected in series
Voltmeter: measures potential difference, connected in parallel

Formulas

V = IR
R = ρl/A
H = I²Rt
P = VI, 1 kWh = 3.6 × 10⁶ J

Important questions with answers

Why are coils of electric toasters made of an alloy rather than a pure metal?2 marks

Alloys have higher resistivity than pure metals and do not oxidise (burn) easily at high temperatures, so they produce more heat and last longer.

Find the equivalent resistance of 4 Ω, 6 Ω and 12 Ω connected in parallel.3 marks

1/R = 1/4 + 1/6 + 1/12 = (3 + 2 + 1)/12 = 1/2, so R = 2 Ω.

An electric iron of 1000 W is used for 2 hours a day for 30 days. Find the energy used in kWh and its cost at ₹6 per unit. Also find the current it draws at 220 V and the resistance of its element.5 marks

Energy = 1 kW × 2 h × 30 = 60 kWh = 60 units. Cost = 60 × 6 = ₹360. Current I = P/V = 1000/220 ≈ 4.55 A. Resistance R = V²/P = 220²/1000 = 48.4 Ω.

Quick revision

Series adds resistances; parallel reduces total resistance.
Fuse wire has a low melting point.
Tungsten is used in bulbs (high melting point).

Exam tips

Doubling the length doubles resistance; doubling the area halves it.
For energy bills: units = kW × hours.

Practice MCQs

SI unit of resistance is
1. ohm
2. ampere
3. volt
4. watt
Show answer
Answer: (a) ohm. Ohm (Ω).
An ammeter is connected in
1. parallel
2. series
3. either
4. none
Show answer
Answer: (b) series. It must carry the full current.
1 kWh equals
1. 1000 J
2. 3.6 × 10³ J
3. 3.6 × 10⁶ J
4. 36 J
Show answer
Answer: (c) 3.6 × 10⁶ J. 1000 W × 3600 s.
A current of 0.5 A flows through a 10 Ω resistor. The voltage is
1. 5 V
2. 2 V
3. 20 V
4. 0.05 V
Show answer
Answer: (a) 5 V. V = IR = 5 V.
Equivalent resistance of 4 Ω and 4 Ω in parallel is
1. 2 Ω
2. 8 Ω
3. 4 Ω
4. 1 Ω
Show answer
Answer: (a) 2 Ω. 4 × 4/(4 + 4) = 2 Ω.
Power of a bulb drawing 0.5 A at 220 V is
1. 220 W
2. 440 W
3. 110 W
4. 55 W
Show answer
Answer: (c) 110 W. P = VI = 110 W.
A wire of resistance R is stretched to double its length. The new resistance is
1. 4R
2. 2R
3. R
4. R/2
Show answer
Answer: (a) 4R. Length doubles and area halves, so R × 2 × 2 = 4R.
Three 6 Ω resistors can give which total resistance by suitable connection?
1. 5 Ω
2. 9 Ω
3. 7 Ω
4. 15 Ω
Show answer
Answer: (b) 9 Ω. Two in parallel (3 Ω) plus one in series = 9 Ω.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.