NEET Current Electricity MCQs & PYQs

Assertion (A) is true. Increased temperature in semiconductors generates more charge carriers, decreasing resistivity. Reason (R) is also true, as thermal vibrations increase electron-ion collisions in conductors, increasing resistivity. However, (R) explains conductors, not semiconductors, so it's not the correct explanation for (A).

Assertion (A) is false. The terminal potential difference \( V \) is equal to emf \( E \) when no current is drawn (open circuit) and can be greater than \( E \) during charging. Reason (R) is false. The potential drop across internal resistance (\( Ir \)) actually decreases the terminal potential difference (\( V = E - Ir \) for discharging).

The electromotive force (EMF) of a cell depends only on the nature of the electrodes and electrolyte, not its size. Size affects internal resistance and current capacity. Both assertion and reason are incorrect.

Assertion (A) is true: \(8 times 1.5\text{ V} = 12\text{ V}\). Dry cells have high internal resistance and cannot provide the high current needed to start a car. Reason (R) is also true, as engine oil is less viscous on a warm day. However, (R) does not explain (A).

When identical cells are connected in parallel, the effective EMF remains the same as that of a single cell, while the current capacity increases. Thus, Assertion (A) is false. For identical cells, current distribution should be equal. Thus, Reason (R) is also false.

In a balanced Wheatstone bridge, the current through the galvanometer is zero. Hence, the galvanometer's resistance does not affect the equivalent resistance of the bridge and thus the current drawn from the cell. Both Assertion (A) and Reason (R) are false.

If an observer moves with the electron drift speed (in the direction of electron flow), electrons are at rest relative to them. However, the positive lattice ions, which were stationary in the conductor's frame, are now moving. This motion of positive ions constitutes a current and produces a magnetic field.

Thus, (A) is false. Also, in this frame, only electrons are at rest, while positive ions are moving, so (R) is false.

Silver is a metal with a very high electrical conductivity, which means it has the lowest resistivity among the given options (glass is an insulator, while silicon and germanium are semiconductors).

Using \(R = \rho \frac{l}{A} = \rho \frac{l}{\pi (d/2)^2} \propto \frac{1}{d^2}\). Since diameter \(d_B = 3d_A\), the resistance \(R_B = \frac{R_A}{3^2} = \frac{81}{9} = 9 \Omega\).

Using \(I = n e A v_d ⇒ v_d = \frac{I}{n e \pi r^2}\). Substituting values, we get \(v_d = \frac{10}{10^{22} \times 1.6 \times 10^{-19} \times \pi \times (10^{-3})^2} = \frac{6.25}{\pi} \times 10^3 \text{ms}^{-1}\).