Unit 1 — Dynamic Electricity and Electromagnetism · Physics. These are 10 questions from the Magmo3 bank on this lesson, from easy to hard. Solve each one, then open "Show answer and explanation". Magmo3 has 74 questions on this lesson, each with an explanation.
The electric resistance of a metal wire depends on all of the following factors EXCEPT…
Correct answer: (B) the electric current passing through it
Explanation: For an ohmic conductor resistance is fixed by material, dimensions and temperature, not by the current flowing through it.
The electric conductivity (σ) of a material is related to its resistivity (ρ) by the relation…
Correct answer: (A) σ = 1/ρ
Explanation: Conductivity is defined as the reciprocal of resistivity, σ = 1/ρ.
Two wires of the same material and the same length have cross-sectional areas A and 2A. The ratio of the resistance of the thin wire to that of the thick wire (R_thin/R_thick) equals…
Correct answer: (A) 2/1
Explanation: Since R ∝ 1/A, the wire of half the area has twice the resistance, giving a ratio of 2/1.
In a metal filament lamp, as the electric current passing through the filament increases, its resistance…
Correct answer: (A) increases, because the temperature of the filament rises
Explanation: A larger current heats the filament, and the rise in temperature increases the resistance, which is why the lamp is a non-ohmic conductor.
A wire of length 10 metre and uniform cross-sectional area 1 mm² has a resistance of 0.5 Ω. The resistivity of its material equals…
Correct answer: (B) 5×10⁻⁸ Ω·m
Explanation: ρ = RA/L = 0.5 × (1×10⁻⁶) / 10 = 5×10⁻⁸ Ω·m (using 1 mm² = 1×10⁻⁶ m²).
The temperature coefficient of resistance (α) of a metal has the unit…
Correct answer: (B) per kelvin (K⁻¹)
Explanation: Since α = ΔR/(R₀·Δt) is a ratio of resistances divided by a temperature, its unit is per kelvin (K⁻¹).
A metallic conductor obeys Ohm's law only when…
Correct answer: (D) its temperature is kept constant
Explanation: Ohm's law V ∝ I holds for a metallic conductor provided its temperature and physical condition stay constant.
A metal conductor has a resistance of 50 Ω at 0 °C. If the temperature coefficient of resistance of its material is 5×10⁻³ K⁻¹, its resistance at 100 °C equals…
Correct answer: (C) 75 Ω
Explanation: R = R₀(1 + αΔt) = 50 × (1 + 5×10⁻³ × 100) = 50 × 1.5 = 75 Ω.
A wire of resistance R is replaced by another wire of the same material whose length is three times and whose cross-sectional area is twice that of the original wire. The resistance of the new wire equals…
Correct answer: (A) 3R/2
Explanation: R′ = ρ(3L)/(2A) = 1.5 × ρL/A = 1.5R = 3R/2.
Two conductors A and B are made of different materials. Conductor A has resistivity ρ, length L and cross-sectional area S, while conductor B has resistivity 2ρ, length 2L and cross-sectional area 4S. The ratio of the resistance of A to that of B (RA/RB) equals…
Correct answer: (A) 1/1
Explanation: RA = ρL/S and RB = (2ρ)(2L)/(4S) = ρL/S, so RA/RB = 1/1.
The first 10 questions in every subject are free, each with an explanation.