(2)

Current is constant in a series circuit as there is only one path for the flow of current. But different devices connected in a circuit have different power ratings and therefore draw different amounts of currents. Thus, both assertion and reason are correct and reason is not the correct explanation of the assertion.

(1)

According to the formula: $R=\rho \frac{l}{A}$

Where, 
R : resistance
$\rho$ : resistivity
l : length of conductor
A : area of cross section of conductor

Here, resistivity of the material never varies. If length is also kept constant and when area is halved then resistance of the material gets doubled as the resistance depends on 3 factors, i.e., length, area and nature of the material.

Thus, both assertion and reason are correct and reason is the correct explanation of assertion.

(2)

Resultant resistance is the addition of individual resistance present in the series circuit. So, according to the above statement,

$R=R_1+R_2+R_3.$

So, total resistance will be 3R. In parallel combination, resultant resistance will be 

$\frac{1}{R}=\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_3}.$

Thus, both assertion and reason are correct, but reason is not the correct explanation of assertion.

(1)

Due to high electrical conductivity of copper, it conducts the current without offering much resistance. The copper being diamagnetic material does not get magnetised due to current through it and hence does not disturb the current in the circuit. Thus, both assertion and reason are correct and reason is the correct explanation of the assertion.

(3)

Ohm’s law states that the electric current flowing through a metallic wire is directly proportional to the potential difference across its two ends. The expression is:

V=IR

Here, R (resistance of the wire) is constant value, then only the statement will be valid.

$V\propto I only if \frac{V}{I} =cons\tan t$

Thus, assertion is true, but reason is false.

(3)

Alloys are the combination of metals and they are used in heating devices due to their high resistance which produces more heat energy. Alloys are made up of weak bonds, their melting point is higher than their constituent metals. Thus, assertion is true but reason is false.

(4)

In a series circuit the current measured will be same, this is the reason why ammeter is always connected in series with the circuit for which the measurement has to be done. Thus, assertion is false, but reason is true.

(3)

Heat flow is dependent on three factors:

1. Current flowing through it

2. Time period of flow of current

3. Resistance of the conductor

So, the reason is false that heat flow only depends on square of current. Thus, assertion is true, but reason is false.

(1)

Most part of the power consumed by bulb is dissipated as heat but very less part gets converted into light. So, there is a need for thermal isolation in order to reduce heat losses. This is the reason why bulbs are filled with inactive nitrogen and argon. Thus, both assertion and reason are correct and reason is the correct explanation of assertion.