Question: Q. 14. Two cells of E.M.F. $10 \mathrm{~V}$ and $2 \mathrm{~V}$ and internal resistances $10 \Omega$ and $5 \Omega$ respectively, are connected in parallel as shown. Find the effective voltage across $R$
A [CBSE SQP 2015-2016]
- Calculate the current drawn from the battery by the network of resistors shown in the figure.
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Solution:
Ans. The given network has the form given below :
It is a balanced Wheatstone Bridge.
Its equivalent resistance, $R$ is given by
$$ \begin{aligned} \frac{1}{R} & =\frac{1}{1+2}+\frac{1}{2+4} \ & =\frac{1}{2} \end{aligned} $$
$$ \mathrm{R}=2 \Omega $$
$\therefore \quad$ Current drawn $=\frac{4 \mathrm{~V}}{2}=2 \mathrm{~A}$
AI Q. 16. Estimate the average drift speed of conduction electrons in a copper wire of cross-sectional area $2.5 \times 10^{-7} \mathrm{~m}^{2}$ carrying a current of $1.8 \mathrm{~A}$. Assume the density of conduction electrons to be $9 \times 10^{28} \mathrm{~m}^{-3}$.
A [O.D. I, II, III 2014]
$$ \text { Ans. } \because \quad \begin{align*} I & =n e A v_{d} \ \Rightarrow \quad v_{d} & =\frac{I}{n e A} \ v_{d} & =\frac{1.8}{9 \times 10^{28} \times 1.6 \times 10^{-19} \times 2.5 \times 10^{-7}} \mathrm{~m} / \mathrm{s} \ & =0.05 \times 10^{-2} \mathrm{~m} / \mathrm{s} \tag{1} \end{align*} $$
[CBSE Marking Scheme 2014]
