Senior Physics - Solutions

## Chapter 10. Heat and Temperature - Worked Solutions

### ...to selected Complex Reasoning questions from the OUP text "New Century" Senior Physics - Knowledge, Processes and Reasoning 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 = mLf
0.1 x 4200 x (150 - 0) = m x 3.34 x 105
m = 0.1886 kg (= 188.6 g)
Density (D) = m/V
V = m/D = 188.6/7.9 = 23.87 cm3
Length of cylinder = Volume/Area = 23.87/(p r2)
L = 23.87/(p x 22) = 1.9 cm

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