Physics Assertion (A) & Reason (B) MCQ

3. A is true, but R is false.

Explanation:

Fuse wire must have high resistance because in series, current remains same. Therefore according to Joule's law $\text{H}=\frac{\text{i}^2\text{Rt}}{4.2},$ heat produced is high if R is high. The melting point must be low so that wire may melt with increase in temperature. As a current larger than the specified value flows through the circuit the temperature of the fuse wire increases. This melts the fuse wire and breaks the circuit.

2. Both A and R are true, but R is not the correct explanation of A.

Explanation:

As filament of bulb and live wire are in series, hence current through both is same. Now, because $\text{H}=\frac{\text{i}^2\text{Rt}}{4.2}$ and resistance of the filament of the bulb is much higher than that of live wires, hence heat produced in the filament is much higher than that in line wires.

2. Both A and R are true, but R is not the correct explanation of A.

Explanation:

The resistance, $\text{R}=\frac{\text{V}^2}{\text{p}},$ i.e., $\text{R}\propto\frac{1}{\text{P}}$ i.e., higher is the wattage of a bulb, lesser is the resistance and so it will glow bright.

3. A is true, but R is false.

Explanation:

Consider a conductor of length l and area of cross section A. Time taken by the free electrons to cross the conductor, $\text{t}=\frac{\text{l}}{\text{v}_\text{d}}$

Hence, current, I = Anev_d

Or $\text{I}\propto\text{v}_\text{d}$

Thus current is directly proportional to drift velocity.

1. Both A and R are true, and R is the correct explanation of A.

Explanation:

Drift velocity of free electrons is given by, $\text{v}_\text{d}=\frac{\text{eE}}{\text{m}}\tau$

Where, $\text{E}=\frac{\text{Potential difference }}{\text{ength }}=\frac{\text{V}}{\text{l}}$

$\therefore\text{v}_\text{d}=\frac{\text{eE}}{\text{ml}}\tau$ r.e., $\text{v}_\text{d}\propto\frac{\text{1}}{\text{l}}$ where, $\frac{\text{eV}\tau}{\text{m}}$ is constant.

It mean if l is doubled, the drift velocity will become half of the original value.

2. Both A and R are true, but R is not the correct explanation of A.

Explanation:

Current is a scalar quantity, it is justified by the following two observations:

1. If current carrying wire is bent at some point, then also current in the wire remains same, while a vector quantity always changes by changing its direction.
2. Current flowing in the circuit do not follow the laws of vector addition. It follows according to ordinary rule of algebra. This makes it clear that current is not a vector but a scalar quantity. Also current is defined as rate of flow of charge through the wire $\text{I}=\frac{\text{dq}}{\text{dt}}.$

1. Both A and R are true, and R is the correct explanation of A.

Explanation:

Resistance of 50 watt bulb is two times the resistance of 100 watt bulb. When bulbs are connected in series, 50 watt bulb will glow more as P = i²R (current remains same in series). ln parallel, the 100 watt bulb will glow more as $\text{P}=\frac{\text{V}^2}{\text{R}}$ (potential difference remain same in parallel).

3. A is true, but R is false.

Explanation:

As P = Vi, hence for the transmission of same power, high voltage implies less current. Therefore heat energy loses $\Big(\text{H} = \frac{\text{i}^2\text{Rt}}{4.2}\Big)$are minimized if power is transmitted at high voltage.

2. Both A and R are true, but R is not the correct explanation of A.

Explanation:

Heater wire must have high resistance and high melting point, because in series current remains same, therefore according to Joule's law $\text{H} = \frac{\text{i}^2\text{Rt}}{4.2}$ heat produced is high if R is high. Melting point must be high, so that wire may not melt with increase in temperature.