Correct Answer: c) Fuse

Explanation:
– A fuse is a safety device connected in series with the circuit.
– If an overload or short circuit occurs, the current rises above a safe level.
– The fuse element heats up and melts (“blows”), opening the circuit and cutting off power.
– This prevents damage to equipment and reduces the risk of electric shock and fire.

Motors and generators are energy-conversion devices, not protective devices. A “current controller” regulates current but does not automatically interrupt dangerous fault currents like a fuse does.

Correct Answer: a) Increases very high

Explanation:
– A short circuit provides a path of (almost) zero resistance.
– By Ohm’s law, I = V/R. As R → 0, the current I becomes extremely large (theoretically infinite).
– In practice, it is limited by the source and wiring impedance, but it is still very high compared to normal operation.

Correct Answer: b) IR^2

Explanation:
– Electric power is given by P = VI.
– Using Ohm’s law (V = IR) for a resistive circuit:
– P = VI = (IR)·I = I^2R
– Also, P = VI = V·(V/R) = V^2/R
– Thus, VI, I^2R, and V^2/R are valid power expressions.
– IR^2 = I·R^2 is not a power expression; its units are A·Ω^2 = V^2/A, which are not watts (V·A). Hence, it does not denote electric power.

Correct Answer: d) Infinity

Explanation:
– For mirrors, focal length f is related to the radius of curvature R by f = R/2.
– A plane mirror can be thought of as a spherical mirror with an infinite radius of curvature (R = ∞).
– Therefore, its focal length is f = ∞/2 = ∞.
– Physically, a plane mirror does not converge or diverge parallel rays; they remain parallel, so the focus is at infinity.

Correct Answer: a) 2100 Joule

Explanation:
– Work done (W) = Force (F) × Displacement (s) × cos(θ)
– Here, the force is along the direction of motion, so θ = 0° and cos(0°) = 1.
– W = 140 N × 15 m × 1 = 2100 N·m = 2100 J
Hence, the work done is 2100 Joule.

Correct Answer: b) and d)

Explanation:
– In free fall (ignoring air resistance), as height decreases, gravitational potential energy U = mgh decreases. The lost potential energy is converted into kinetic energy K = 1/2 mv^2.
– Since ΔK = −ΔU, the total mechanical energy E = K + U remains constant. So the principle of conservation of energy is not violated (option b).
– This behavior is exactly what a conservative gravitational force predicts; there is no “violation” of gravity (option d). Options a and c are false.

Correct Answer: c) Steel

Explanation:
The speed of sound depends on how stiff (elastic) a medium is and how dense it is. In general:
– Solids > Liquids > Gases for speed of sound.

This is because solids have a much higher elastic modulus compared to their density, allowing sound to travel faster.

Approximate speeds:
– Air: ~343 m/s
– Hydrogen (fastest gas): ~1,300 m/s
– Water: ~1,480 m/s
– Steel: ~5,000–6,000 m/s

Thus, among the options, sound travels fastest in steel.

Correct Answer: a) 9 × 10^8

Explanation:
– 1 “unit” of electrical energy = 1 kWh = 3.6 × 10^6 J
– For 250 units: Energy = 250 × 3.6 × 10^6 J = 900 × 10^6 J = 9 × 10^8 J

Correct Answer: d) 14000 Newtons

Explanation:
– The truck starts from rest and covers s = 400 m in t = 20 s with constant acceleration a.
– Use s = (1/2) a t^2 ⇒ 400 = (1/2) a (20)^2 ⇒ 400 = 200a ⇒ a = 2 m/s^2.
– Mass of the truck: 7 tons = 7000 kg (taking 1 ton = 1000 kg).
– Net force causing the acceleration: F = m a = 7000 × 2 = 14,000 N.

Correct Answer: b) 200 Newtons

Explanation:
– The box is moving at constant velocity, which means its acceleration is zero.
– By Newton’s First Law (or using ΣF = m·a), if acceleration is zero, the net force must be zero.
– Along the direction of motion: Applied force − Friction force = 0 ⇒ Friction force = Applied force.
– Therefore, the kinetic (sliding) friction balances the push: Friction = 200 N, opposite to the motion.