Question 9
Part II. Candidates are expected to answer any three questions from this part.
- Name the energy transformations which occur in a functioning:
- car engine;
- steam turbine.
- List the three main features of a clinical thermometer.
- Explain why water becomes cooler when kept in a clay pot for sometime in a room.
- Water of mass 0.4 kg loses 210kJ of energy when taken from the Earth’s surface to Mars of surface temperature – 60o. The water temperature decreases until the water freezes. The temperature of ice formed decreases until it becomes equal to that on Mars. Calculate the temperature of the water while on Earth.
- (Specific latent heat of fusion of ice = 3.3 x 105 Jkg-1; specific heat capacity of water = 4200 Jkg-1K-1 ; specific heat capacity of ice = 2100 Jkg-1K-1. )
Observation
Part (a). This question was poorly attempted by most candidates. Energy transformations were not given in the correct order by most candidates. Only very few percentage of the respondent managed to give the correct full energy transformations in the two cases.
Part (b). Surprisingly, many candidates misinterpreted the question to score zero. However, some succeeded to state one or two features of the clinical thermometer. Only very few candidates were able to state the three features demanded for in the question.
Part (c). Candidates performed poorly in this part to demonstrate their inability to handle questions on application of the principle of Physics to real life situations. Some candidates stated that the water molecules withdrew latent heat from the surrounding air.
They could not recall that the pot had tiny pore through which energetic water molecules escapes to the atmosphere.
Part (c): The basic principle of conservation of energy (Total heat lost = Total heat gained) was tested. It was poorly managed by most candidates. Majority of the candidates failed to identify the three stages that constituted the total heat lost by the water.
The expected answers are:
) Energy transformations
(i) Chemical -----> Heat -----> Mechanical
(ii) Heat -----> Mechanical -------> Electrical
(b) Features of a clinical thermometer
- Short temperature range (35o C to 42o C)
- (Very) narrow capillary / high sensitivity
- Constriction / kink
(c) Why water becomes cooler in clay pot
A clay pot is porous. Energetic water molecules escape through the pores.
The escaping molecules carry away latent heat, causing cooling
(d) Q total = mwCw w + mwf + mwCi Δ θi
210000 = 0.4 x 4200 x ( -0) + (0.4 x 330000) + 0.4 x 2100 (0 – - 60)
210000 = 1680 + 132000 + 50400
1680 = 27600
= 16.4oC