In a meter bridge, the null point is found at 60cm from left. If a standard resistance of 15Ω is on the right, the unknown resistance is:
Answer: A
In meter bridge: R/S = l₁/l₂ where l₁ = 60cm, l₂ = 40cm. R/15 = 4060 = 23 → R = 15 × 23 = 22.5Ω.
Q.302Hard
The sensitivity of a galvanometer can be increased by:
Answer: D
Sensitivity θ ∝ NAB/k. It increases with more turns (N) and weaker torsional constant (k). Both B and C increase sensitivity.
Q.303Hard
A wire of length L and cross-sectional area A has resistance R. If the wire is stretched to length 2L without change in volume, the new resistance will be:
Answer: B
When stretched, volume constant: A × L = A' × 2L → A' = A/2. New resistance R' = ρ(2L)/(A/2) = 4ρL/A = 4R.
Q.304Easy
The EMF of a cell is 1.5V. When a current of 2A is drawn, the terminal voltage is 1.2V. The internal resistance is:
Answer: A
E = V + Ir → 1.5 = 1.2 + 2r → 0.3 = 2r → r = 0.15Ω.
Q.305Medium
A copper wire of diameter 1 mm carries a current of 5 A. If the drift velocity of electrons is 0.5 mm/s, find the number density of free electrons in copper. (Given: e = 1.6 × 10⁻¹⁹ C)
Answer: A
Using I = nAve, where n is number density. A = π(0.5×10⁻³)². Solving gives n = 8.05 × 10²⁸ m⁻³
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Q.306Easy
Two wires of the same material have lengths in ratio 2:3 and diameters in ratio 1:2. Their resistances are in the ratio?
In a Wheatstone bridge, if the four resistances are P, Q, R, and S connected in order, and the galvanometer shows zero deflection, which condition is satisfied?
Answer: D
At null deflection: P/Q = R/S, which is equivalent to P·S = Q·R. Both are valid balanced bridge conditions
Q.308Easy
An ideal ammeter connected in series with a circuit has resistance of:
Answer: C
An ideal ammeter must have zero resistance to not affect the circuit current. In practice, ammeters have very low resistance (mΩ range)
Q.309Medium
A potential divider uses a 100 cm wire of total resistance 10Ω. A cell of EMF 2V and negligible internal resistance is connected across it. What is the potential gradient along the wire?
Answer: A
Potential gradient = V/L = 2V / 100cm = 0.02 V/cm
Q.310Medium
A battery of EMF E and internal resistance r is connected to an external resistance R. The terminal voltage is:
Answer: D
Terminal voltage V = E - Ir. Since I = E/(R+r), we get V = E - Er/(R+r) = ER/(R+r). Both expressions are equivalent
Q.311Easy
In carbon resistors, color bands are used for identification. What does the third band represent?
Answer: C
Standard color code: 1st band = first digit, 2nd band = second digit, 3rd band = multiplier, 4th band = tolerance
Q.312Easy
The internal resistance of a Leclanche cell is approximately:
Answer: B
Standard Leclanche cells have internal resistance in the range of 1-2Ω depending on size and condition
Q.313Medium
In a potentiometer, if the balancing length for a cell is 60 cm and for standard cell is 50 cm, what is the EMF of the unknown cell? (Standard cell = 1.018V)
Answer: B
E/E_std = L/L_std, so E = 1.018 × (5060) = 1.018 × 1.2 = 1.2216 ≈ 1.221 V
Q.314Easy
An electric heater rated 500W, 250V is used for 4 hours daily. The energy consumed in one day is:
Answer: A
Energy = Power × Time = 500W × 4h = 2000Wh = 2 kWh
Q.315Hard
The equivalent resistance between two opposite corners of a cube made of 12 wires of 1Ω each is:
Answer: B
Due to symmetry, current divides into three paths of 1Ω each in parallel at the first vertex, then similar distribution at other vertices. Equivalent = 31 + 61 + 31 = 65Ω
Q.316Easy
If the cross-sectional area of a wire is doubled while keeping length and material constant, how does resistivity change?
Answer: C
Resistivity is a material property independent of shape and size. It depends only on the nature of material and temperature
Q.317Easy
The temperature coefficient of resistance for a conductor is positive. This means:
Answer: A
Positive temperature coefficient means R = R₀(1 + αΔT) where α > 0, so resistance increases with increasing temperature
Q.318Medium
A galvanometer with coil resistance 100Ω shows full-scale deflection at 10 mA. What shunt resistance is needed to convert it into an ammeter reading up to 1 A?
In a circuit with pure resistance, the phase difference between voltage and current is:
Answer: A
In a purely resistive circuit, voltage and current are in phase, so phase difference is 0°. V = IR is always satisfied
Q.320Medium
A rectangular conductor with dimensions 2cm × 3cm × 10cm is placed such that current flows through the 10cm length. If resistivity is 1.7 × 10⁻⁸ Ω·m, find the resistance:
