Electricity — Previous Year Questions (Class 10 Science)
Electricity covers Ohm's law, circuits, resistivity, and power. CBSE tests circuit analysis, calculation of current/voltage/resistance, and practical circu
TL;DR: Electricity covers Ohm's law, circuits, resistivity, and power. CBSE tests circuit analysis, calculation of current/voltage/resistance, and practical…
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
Electricity covers Ohm's law, circuits, resistivity, and power. CBSE tests circuit analysis, calculation of current/voltage/resistance, and practical circu
Electricity — Previous Year Questions with Solutions
Q (2023, 3 marks): A wire has a resistance of 12 Ω. If the wire is stretched to double its length, what will be the new resistance? (Resistivity and density remain constant.)
Answer: Resistance formula: R = ρL/A where ρ = resistivity, L = length, A = cross-section area
When wire is stretched, length becomes 2L. Volume remains constant:
Original: V = A × L
New: V = A' × 2L
A × L = A' × 2L
A' = A/2
New resistance: R' = ρ(2L)/(A/2) = ρ × 2L × 2/A = 4ρL/A = 4R = 4 × 12 = 48 Ω
Final Answer: New resistance = 48 Ω
Q (2022, 2 marks): Three resistors of 2 Ω, 3 Ω, and 6 Ω are connected in parallel. Find the equivalent resistance.
Answer: For resistors in parallel:
1/R_eq = 1/R1 + 1/R2 + 1/R3
1/R_eq = 1/2 + 1/3 + 1/6
LCM of 2, 3, 6 is 6:
1/R_eq = 3/6 + 2/6 + 1/6 = 6/6 = 1
R_eq = 1 Ω
Final Answer: Equivalent resistance = 1 Ω
Q (2021, 2 marks): A 100 W, 220 V electric bulb is connected to a 220 V supply. Calculate the current drawn and the resistance of the filament.
Answer: Given: Power P = 100 W, Voltage V = 220 V
Current: P = V × I
I = P/V = 100/220 = 5/11 ≈ 0.45 A
Resistance: R = V/I = 220 / (5/11) = 220 × 11/5 = 2420/5 = 484 Ω
Or directly: R = V^2/P = 220^2/100 = 48400/100 = 484 Ω
Final Answer: Current = 5/11 A ≈ 0.45 A; Resistance = 484 Ω
Q (2023, 3 marks): Two resistors, when connected in series, give equivalent resistance 9 Ω. When connected in parallel, the equivalent resistance is 2 Ω. Find the individual resistances.
Answer: Let the resistances be R1 and R2.
Series: R1 + R2 = 9 ... (1)
Parallel: 1/R1 + 1/R2 = 1/2
(R1 + R2)/(R1 × R2) = 1/2
R1 × R2 = 2(R1 + R2) = 2 × 9 = 18 ... (2)
From (1) and (2):
R1 + R2 = 9
R1 × R2 = 18
These are sum and product; so R1 and R2 are roots of:
x^2 - 9x + 18 = 0
(x - 3)(x - 6) = 0
x = 3 or x = 6
Final Answer: R1 = 3 Ω, R2 = 6 Ω (or vice versa)
Q (2022, 2 marks): An electric heater rated 1000 W, 250 V is immersed in water. How much heat energy is generated in 2 hours?
Answer: Power P = 1000 W, Time t = 2 hours = 2 × 3600 = 7200 s
Energy (heat) = P × t = 1000 × 7200 = 7,200,000 J = 7.2 × 10^6 J
Or in kWh: P = 1000 W = 1 kW, t = 2 h
Energy = 1 × 2 = 2 kWh
Final Answer: Heat energy = 7.2 × 10^6 J or 2 kWh
Q (2021, 2 marks): State Ohm's law and explain when it is applicable.
Answer: Ohm's Law: For a conductor at constant temperature, the current passing through it is directly proportional to the potential difference across it and inversely proportional to its resistance.
Mathematically: V = I × R or I = V/R
Applicability:
1. Valid only for ohmic materials (conductors like copper, nichrome wire).
2. Must be at constant temperature (resistance changes with temperature).
3. NOT applicable to non-ohmic materials like semiconductors, thermistors, diodes.
4. NOT applicable at very high voltages or frequencies where non-linear effects dominate.
Final Answer: V = IR; applicable to ohmic conductors at constant temperature only.
Frequently Asked Questions
Why is resistance independent of current in an ohmic conductor?
In ohmic conductors, the resistance is determined by the material's resistivity (ρ), length (L), and cross-section (A): R = ρL/A. These properties don't change with current. The ratio V/I remains constant, so resistance is independent of current.
What is the advantage of connecting devices in parallel in a house?
In parallel, each device gets full voltage (220V), devices can be switched independently, and if one fails the others still work. Series would lower voltage per device and one failure breaks the whole circuit.
More Class 10 Science PYQs
- Chemical Reactions and Equations
- Light (Reflection and Refraction)
- Acids, Bases and Salts
- Metals and Non-metals
- Life Processes
- Heredity and Evolution
🤖 Stuck on any of these? Ask Syllab's free AI Tutor to explain step by step →