Chapter 10 - Worked Solutions
to selected extension questions from the OUP text Senior Physics -
Concepts in Context by Walding,
Rapkins and Rossiter
Q 39.
A rating of 5 kW means it gives heat at the rate
of 5 kJ per second
If we consider 1 minute:
5 kW = 5000 J s-1 = 5000 x
60 J min-1 = 300000 J min-1
In 1 minute 5 Kg of water pass through the
shower unit.
Q = M c DT.
30000 = 5 x
4180 x DT
DT = 14.4 oC
The final temperature = 15
oC + 14.4 oC = 29.4 oC
Q 40
Assume that the heat lost from the iron goes
into just melting the ice and does not raise its temperature
Q (lost) = Q
(gained)
mcDT (iron) = mLf (ice)
0.1 x 460 x (150
- 0) = m x 3.34 x 105
6900 = m x 3.34 x 105
m = 0.0206 kg (= 20.6 g)
Density (D) =
m/V
V = m/D = 20.6/7.9 = 2.6 cm3
Length of cylinder =
Volume/Area = 2.6/(p r2)
L =
2.6/(p x 22) = 0.2 cm (sorry for the mistake
in the back of the book!!).
Q 41
Assume that the steam cools to 100°C,
condenses to water at 100°C and then cools to 0°C. At the same time the ice cube melts and turns to water at
0°C
Q (lost) = Q (gained)
mcDT + mLv + mcDT =
mLf
m(2.02 x 10 + 2250 + 4.2 x 100) = 20 x 334
m = 2.48
g
\ m (total) = 55 g + 2.48 g = 57.48 g
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