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.
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.2Hard
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.3Hard
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.4Hard
A heating element of resistance R is connected to a battery of EMF E and internal resistance r. Maximum power is dissipated in R when:
Answer: B
By maximum power transfer theorem, maximum power is transferred to external load when load resistance equals internal resistance
Q.5Hard
Which of the following best explains why semiconductor resistance decreases with increase in temperature?
Answer: C
In semiconductors, increased temperature promotes more electrons from valence to conduction band, increasing carrier concentration and decreasing resistance
Q.6Hard
A battery of EMF 12V and internal resistance 2Ω is connected to a load resistance R. For maximum power transfer, R should be:
Answer: B
Maximum power transfer theorem: Load resistance equals internal resistance. R = r = 2Ω for maximum power
Q.7Hard
The electric field inside a copper conductor carrying current is approximately:
Answer: B
By Ohm's law in microscopic form: E = ρJ. Inside the conductor, electric field maintains the drift of electrons
Q.8Hard
When a superconductor is cooled below its critical temperature, its resistance becomes zero. What is the effect on current flowing through it when connected to a constant voltage source?
Answer: A
With R = 0, by I = V/R, current becomes infinite. In practice, voltage source cannot be maintained across a superconductor
Q.9Hard
The length of a conductor increases by 10% when stretched. Assuming volume remains constant, the resistance increases by approximately:
Answer: C
R = ρL/A. If L increases by 10% and volume constant, A decreases by ~9.1%. New R = 1.1R₀/0.91 ≈ 1.21R₀, so 21% increase.
Q.10Hard
The equivalent resistance of an infinite ladder network of 1Ω resistors (each rung) is:
A superconductor exhibits zero resistance below its critical temperature because:
Answer: B
BCS theory explains superconductivity: below critical temperature, electrons form Cooper pairs with no scattering, resulting in zero resistance.
Q.12Hard
In a Wheatstone bridge, arms P, Q, R, and S have resistances 10Ω, 15Ω, 20Ω, and 30Ω respectively. A galvanometer is connected between junctions of P-Q and R-S. The galvanometer reading will be:
Answer: A
For balanced bridge: P/Q = R/S. Check: 1510 = 3020 → 32 = 32. The bridge is balanced, so no current flows through galvanometer (zero reading).
