(^234) Aptitude Test Problems in Physics
We write the energy conservation law:
4 X Am (cice cw) m A T.
Let us estimate the work A done by the external
force. The change in the volume of the mixture
as a result of melting ice of mass Am is
AV
Am
—
Am
Pice (^) Pw
Am Pw—Pice < 0.1 --, 10-4 m 3.
PwPice Pice
We have taken into account the fact that the den-
sity of water decreases as a result of freezing by
about 10%, i.e. (Pw — Pice)/Pw 0.1. Therefore,
we obtain an estimate
A < pi AV = 0.25 kJ.
The amount of heat AQ required fot heating the
mass m of ice and the same mass of water by AT is
AQ = (rice cw) m AT , 1.1 kJ.
Since A < AQ,. we can assume that X Am = AQ,
whence
AQ
Am = = 3.2 g.
The change in volume as a result of melting ice
of this mass is
Am Pw —Pice 3.5 x 10-7 ins.
p,‘ Pice
Considering that for a slow increase in pressure the
change in the volume A V, is proportional to that
in the pressure Ap, we can find the work clone by
the external force:
p c Vi
—
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