Physics MCQs for NEET — Practice Questions with Answers

$R=\frac{\rho l}{a}$ for first wire and R’ = $\frac{\rho l}{4a}=\frac{R}{4}$ for second wire. 

Because as temperature increases, the resistivity increases and hence the relaxation time decreases for conductors $\left(\tau \propto \frac{{\displaystyle 1}}{{\displaystyle \rho }}\right).$  

Since $R\propto l^2$ ⇒ If length is increased by 10%, resistance is increases by almost 20%

Hence new resistance $R\text{'}=10+20\%$ of 10

$=10+\frac{20}{100}\times 10=12\Omega .$ 

$$\begin{gathered} R_{150}=R_0\left[1+\alpha \left(150\right)\right] \\ {R}_{500}=R_0\left[1+\alpha \left(500\right)\right] \\ \frac{R_{150}}{R_{500}}=\frac{[1+\alpha (150\left)\right]}{[1+\alpha (500\left)\right]} \end{gathered}$$.

Putting $R_{150}=133\Omega$ and $\alpha =0.0045/°C,$

we get $R_{500}=258\Omega$

Temperature coefficient of a semiconductor is negative.

The reciprocal of resistance is called conductance

$\text{Resistance}=\frac{\text{Potential\hspace{0.17em}difference}}{\text{Current}}$   

Ohm’s Law is not obeyed by semiconductors.

This is because of secondary ionisation which is possible in the gas filled in it.

If E be electric field, then current density j = $\sigma E$

Also we know that current density $j=\frac{i}{A}$

Hence j is different for different area of cross-sections. When j is different, then E is also different. Thus E is not constant. The drift velocity v_d is given by $v_d=\frac{j}{ne}$ = different for different j values. Hence only current i will be constant.