Answer the questions.[Phy-2M]

Question 12 Marks

a. Write the mathematical expression for Joule's law of heating.
b. Compute the heat generated while transferring 96000 coulomb of charge in two hours through a potential difference of 40 V .

Answer

a. The mathematical expression of the Joules Law of heating is: $\mathrm{H}=\mathrm{I}^{2} \mathrm{Rt}$
Here, H is a heating effect, I is the current flowing through the device and t is the time taken.
b. Given:
Amount of charge transferred $=96000 \mathrm{C}$
Time taken $=2 \mathrm{hrs}=2 \times 60 \times 60 \mathrm{sec}=7200 \mathrm{sec}$
Potential difference $=40 \mathrm{~V}$
Heat generated $=\mathrm{V} \times \mathrm{I} \times \mathrm{t}$
and we know that; $\mathrm{I}=\frac{Q}{t}$
So, $\mathrm{H}=$ VQ
$=40 \times 96000$
$=3.84 \times 10^{6} \mathrm{~J}$

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Question 22 Marks

An electric heater rated 1100 W operates at 220 V . Calculate (i) its resistance, and (ii) the current drawn by it.

Answer

i. $\mathrm{P}=\frac{V \times V}{R}$
$\mathrm{R}=\frac{V \times V}{P}$
$\mathrm{R}=\frac{220 \times 220}{1100}$
$\mathrm{R}=\frac{48400}{1100}$
$\mathrm{R}=44 \mathrm{ohm}$
ii. $\frac{V}{I}=\mathrm{R}$
$\mathrm{I}=\frac{V}{R}$
$\mathrm{I}=\frac{220}{44}$
$\mathrm{I}=5 \mathrm{~A}$

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Question 32 Marks

Draw ray diagram showing the image formation by a convex lens when an object is placed at twice the focal length of the lens.

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