First slide

Change of state

Question

A substance of mass m requires a power input of P watts to remain in the molten state at its melting point. When the power is turned off, the sample completely solidifies in time t sec. What is the latent heat of fusion of the substance?

Easy

A

$\frac{Pm}{t}$
B

$\frac{P t}{m}$
C

$\frac{m}{Pt}$
D

$\frac{t}{Pm}$

Solution

Heat lost in t sec = mL or heat lost per sec = $\frac{mL}{t} .$ This must be the heat supplied for keeping the substance in molten state per sec.
$\frac{mL}{t} = P or L = \frac{Pt}{m}$

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