O lvl Phy: Transfer of Heat Energy

The figure below shows a household hot water system.

(a) State the process by which the hot water rises from the boiler to the hot water tank.

(b) Through which pipe, P or Q, does the heated water rise from the boiler to the hot water tank? Explain your choice.

(c) State the purpose of pipe R.

(d) What is the name and purpose of pipe S.

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Answer:

(a) Convection

(b) Pipe P.
Hot water is less dense than cold water. Thus, hot water in the boiler will rise, and exit the boiler through pipe P to reach the hot water tank. In the hot water tank, the colder water, being denser, will sink, and exit through pipe Q to reach the boiler.

(c) Pump cold water to the lower half of the water tank to be transferred to the boiler via pipe Q for boiling/heating.

(d) Pipe S is the overflow pipe attached to the tap of the hot water tank just in case the temperature of the water becomes too high and cause a large expansion of the hot water. The excess water will flow back into the cistern.

O lvl Phy: Transfer of Heat Energy

Explain the following observations

(a) Birds usually fluff up their feathers during cold weather.

(b) Cooking utensils, kettles and boilers are usually made of aluminium or stainless steel.

(c) Sawdust is used to cover ice blocks.

(d) A black car becomes hotter than a white car when both are parked under direct sunlight for several hours.

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Answer:

(a) By fluffing up their feathers, birds trap tiny air pockets within their feathers. Since air is a poor conductor of heat, the bird loses less heat during cold weather.

(b) Aluminium and stainless steel (or metals) conducts heat well. This helps to conduct heat quickly to the food to enable it to be cooked faster.

(c) Sawdust is a bad conductor heat. By using sawdust to cover ice blocks, heat transfer from surroundings to ice is reduced, hence, melting of ice blocks is delayed.

(d) Black surfaces are better than white surfaces in absorbing radiation. Thus, the black car absorbs more heat from radiation as compared to the white car, resulting in a hotter temperature for the black car.

O lvl Phy: Transfer of Heat Energy

(a) State the three processes of thermal energy transfer by which a hot object cools down.

(b) Of the three processes in (a), which one of them
(i) involves thermal energy travelling from the hot object to its surroundings without heating the air around it? Why?
(ii) occurs only in fluids (ie liquids or gases)? Why?

(c) Metals are good conductors of heat whereas wood is a poor conductor of heat (i.e. an insulator).
Discuss this statement with reference to the mechanism of thermal energy transfer in solids.

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Answer:

(a) Conduction, convection, radiation.

(b)
(i) Radiation. Radiation is the only process that does not need a medium to transfer the energy. The energy can be transferred through vacuum. Hence, it can travel without heating the air around it.

(ii) Convection. It occurs by means of convection currents set up in fluids due to differences in density.

(c) For conduction of heat within solids, the two mechanisms are molecular vibrations and free electron diffusion. Since metals contain many free electrons, they transfer heat faster.

O lvl Phy: Transfer of Heat Energy

Anderson Sec Sch 2007 Sec 4 CA1 Q13

The figure below shows two identical glass cylinders containing equal volumes of water, initially at room temperature. Identical electric heaters were placed in the cylinders, near the top in cylinder A and near the bottom in cylinder B.

The heaters were switched on at the same time. The readings of the thermometers then changed as shown in the following figure.

(a) Explain why the thermometer reading in cylinder A initially shows a slower rate of increase than that in B.
(b) Two metal cans are identical except that one has a blackened outer surface and the other has a polished outer surface. They are both filled with a hot liquid at the same initial temperature and then placed in a shaded room. The freezing point of the liquid is 70°C. The following figure is a temperature-time graph for the blackened can.

Draw on the figure, a temperature-time graph for the polished can.

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Answer:

(a) In A, the heater is placed nearer to the top. This heats up the water around the heater (nearer the top). Hot water, being less dense, remains above and the cold denser water remains near the bottom. This makes it difficult to heat up water by convection. The thermometer, being at the bottom, thus takes a longer time to register a temperature increase.

In B, the heater is placed near to the base. Warm water, being less dense, rises to the top, and the cold denser water sinks. This cycle continues, resulting in a convection current current in the water. Heat transfer is faster here than in A. The thermometer, being at the top, measures the temperature increase faster.


(b)

O lvl Phy: Transfer of Heat Energy

Anderson Sec Sch 2007 Sec 4 CA1 Q12

When a house is heated, energy is lost to the outside.
The figure below shows where the energy is lost from the house.

(a)
(i) Describe how the house loses energy to the surroundings by the three processes of heat transfer.
(ii) Fitting carpets on the floor reduces energy loss. Explain how a carpet reduces energy loss.

(b) The table gives information about three methods of reducing energy loss.

	Method of reducing energy loss	Installation cost	Saving on energy costs in one year	Number of years of saving needed to cover installation costs
A	Fitting carpets on the floor	$600	$10	60
B	Insulating the roof	$300	Y	3
C	Fitting modern windows	X	$20	40

(i) Calculate the values of X and Y.
(ii) Which one of these three methods should the house owner choose? Explain your answer.
(iii) State two other ways, not already mentioned, of reducing energy loss from the house.

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Answer:

(a)
(i) Heat is lost by conduction through the roof, windows, grounds and walls as solid is a good conductor of heat and energy is transferred from one particle to another. Heat is lost by convection through the air near the ground; hot air which is less dense rises and cold air sinks. Heat is lost be radiation as electromagnetic waves travel through the walls of the house.

(ii) Carpets trap tiny pockets of air among its fibre and air is a poor conductor of heat.


(b)
(i) Y = $300 / 3 = $100
X = 40 * $20 = $800

(ii) Method B, insulating the roof. This is because in 3 years, the cost of insulation can be recovered.

(iii) (State three ways here instead)
Paint the house with lighter colours to reduce heat loss by radiation.
Install double glazed windows to reduce heat loss through the windows by conduction.
Reflective insulation within the roof/walls of the house to reduce heat loss by radiation.

O lvl Phy: Transfer of Heat Energy

RI Prelims 2004 P2 Q9a, b

(a) Water has a specific heat capacity of 4200 J/(kg °C).
What is meant by specific heat capacity of 4200 J/(kg °C)?

(b) A student conducted an experiment to determine the specific heat capacity of a liquid X. He immersed a heater in a beaker of liquid X at room temperature as shown in the figure below. The heater was switched on and a graph was obtained as shown.

(i) Describe briefly how thermal energy is transferred from the heating element to all the liquid in the beaker.
(ii) Another student suggested that liquid X should first be cooled to about 10 °C below room temperature, then heated to about 10 °C above room temperature.

Explain, in terms of heat transfer with the surroundings, why this suggested procedure will give a more accurate result.

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Answer:

(a) The amount of energy needed to raise the temperature of 1 kg of water by 1 °C is 4200 J.

(b)
(i) The heating element heats up the layer of liquid molecules nearest to it by conduction. This heater layer expands upon heating, causing its density to decrease. The heated less dense liquid rises, and is replaced by the colder denser liquid above. The bulk movement of liquid of different densities set up a convection current, causing the liquid to be heated up.

(ii) When the temperature of the liquid is lowered by 10 °C below room temerature, it gains heat from the surroundings. This compensates for the heat loss to the surroundings when it is heated to a temperature 10 °C higher than its surroundings, and hence, there is no net heat transfer to its surroundings.

O lvl Phy: Transfer of Heat Energy, Thermal Properties of Matter

RGS Prelims 1997 P2 Q10

(a) The figure below shows an experiment on heat transfer.

(i) State the means of heat transfer from the heater to thermometers X and Y.
(ii) Which thermometer will have a higher temperature change after a certain time?
(iii) If the heater is placed between thermometers X and Y, determine which one will register a higher change. Give reasons.

(b) A 3 kW immersion heater is used to keep the water in a domestic hot water tank at a steady temperature. The electrical supply to the immersion heater is switched off when the average water temperature is 60 °C. Assuming that the rate of heat loss is constant at 1.8 kW and no water is run off, and that the total heat capacity of the tank and the water it contains is 5.2 * 10⁵ J K^-1, calculate the average temperature 40 minutes later.

(c) Liquid air boils at a very low temperature at normal atmospheric pressure. Explain why liquid air contained in an open vacuum flask in a laboratory boils steadily and continuously. Why does liquid air boil much more rapidly when contained in an ordinary glass beaker?

(d) A 20 g lump of iron is placed in liquid air for several minutes. It is then removed and quickly placed in water at 0 °C. A 5.2 g layer of ice forms over the iron. Determine the temperature of the liquid air. (The specific heat capacity of iron is 440 J kg^-1 °C^-1 and the latent heat of fusion of ice is 3.34 * 10⁵ J kg^-1.)

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Answer:

(a)
(i) Conduction (Not convection because heater is on top, and hot water rises)
(ii) Thermometer X (nearer to heater)
(iii) Thermometer X. This is because hot water rises and cold water sinks. The water above the heater (where thermometer X is) will get heated up faster.

(b) Using Q = CΔӨ,
1.8 * 1000 * (40 * 60) = 5.2 * 10⁵ * ΔӨ
ΔӨ = (4.32 * 10⁶) / (5.2 * 10⁵)
ΔӨ = 8.3 °C

Hence, average temperature = 60 °C - 8.3 °C = 51.7 °C

(c) Liquid air contained in an open vacuum flask in a laboratory boils steadily and continuously because the room temperature is always higher than the boiling point of liquid air; heat energy is being constantly supplied from the surroundings to the liquid air to boil it continuously.

Liquid air boils much more rapidly when contained in an ordinary glass beaker because in an ordinary glass beaker, heat energy is also supplied through conduction via the sides of the beaker. Heat transfer to liquid air in a vacuum flask is limited to conduction via the hole/entrance of the vacuum flask. Thus, we can see that heat energy is being supplied at a higher rate for the glass beaker as compared to the vacuum flask (more channels/surface area for heat transfer). This explains why liquid air boils much more rapidly when contained in an ordinary glass beaker.


(d) Let the temperature of liquid air be T
Heat gained by Iron = Latent heat loss to change from water at 0 °C to ice at 0 °C.
mc_iron ΔӨ = ml_fusion
(0.020)(440)(0 - T) = (0.0052)(3.34 * 10⁵)
-8.8T = 1736.8
T = -197 °C

Note: A quick way to check... Boiling point of liquid nitrogen is around -196 °C, so since air is mainly nitrogen, -197 °C sounds very logical.