Physics is where JEE ranks are usually won or lost, since a single conceptual slip changes the whole answer. Questions here span mechanics, rotational motion, thermodynamics, waves and oscillations, electrostatics, current electricity, magnetism, optics, and modern physics. Numerical problems carry full derivations so you can see which step you skipped, not just which option was right.
Conductivity σ = 1/ρ where ρ is resistivity. Since resistivity is in Ω·m, conductivity is in (Ω·m)⁻¹ = S/m or mho/meter.
Q.2Easy
A copper wire and an aluminum wire of the same length and cross-sectional area carry the same current. Which wire has greater power dissipation?
Answer: B
Resistivity of aluminum (2.65 × 10⁻⁸ Ω·m) is greater than copper (1.68 × 10⁻⁸ Ω·m). Since P = I²R and R = ρL/A, aluminum has higher resistance and thus higher power dissipation for same current.
Q.3Medium
In a balanced Wheatstone bridge, if the resistance of one arm is changed by 10%, by what percentage should another arm be changed to maintain balance?
Answer: A
For Wheatstone bridge: P/Q = R/S. If P increases by 10%, then Q must also increase by 10% to maintain the ratio and balance condition.
Q.4Easy
The drift velocity of electrons in a copper wire is approximately:
Answer: B
Drift velocity vd = I/(nAe) is typically 10⁻⁴ to 10⁻³ m/s for normal currents, much slower than thermal velocity (~10⁶ m/s) but much slower than light speed.
Q.5Easy
Two resistances R₁ and R₂ are connected in parallel. If R₁ = 4Ω and R₂ = 6Ω, what is the equivalent resistance?
A metallic conductor's resistance increases with temperature. This is primarily because:
Answer: B
As temperature increases, atoms vibrate more vigorously, increasing collision frequency with drifting electrons. This increases mean free path reduction and thus resistance increases.
Q.11Medium
A heating element rated 1000W, 200V is connected to a 100V supply. The heat generated will be:
Answer: A
Resistance R = V²/P = (200)²/1000 = 40Ω (constant). Heat at 100V: P = V²/R = (100)²/40 = 4010000 = 250W.
Q.12Medium
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.13Hard
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.14Hard
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.15Easy
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.16Medium
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
Q.17Easy
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.19Easy
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.20Medium
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?
