Transfer of Thermal Energy (Cambridge O Level Physics)

A thermometer is used to measure the temperature inside a room in a house.

State a physical property that varies with temperature and can be used in a thermometer.

Fig. 6.1 shows how the temperature of the room changes between 6:00 pm and 11:00 pm.

A heater in the room is switched on at 6 pm. The room has a large window. A large amount of thermal energy is transferred through the window. The window in the room has thick curtains.

                                  
Closing the curtains reduces the loss of thermal energy from the room.

In cool climates, people use mineral wool to reduce heat loss from houses. Mineral wool is made of fibres and trapped air, as shown in Fig.6.2.

Use words from the box to complete the sentences. Each word may be used once, more than once, or not at all.

 

Air is a good ............................................ .

 
When air is trapped between fibres, it reduces heat loss by ..................................... and by ........................................ .

A student wants to investigate good and bad absorbers of thermal radiation. She has the apparatus shown in Fig. 5.1, a supply of cold water and a metre rule.

Explain how the student could use the apparatus she has available to carry out her investigation.

Describe the results she would expect to obtain. Draw a diagram of the set-up.

A solar panel is mounted on the roof of a house. Fig. 1.1 shows a section through part of the solar panel. 

A pump makes water flow through the copper pipes. The water is heated by passing through the solar panel. 

Select and explain three features of the solar panel that maximise the final temperature of the water.

During one day, 250 kg of water is pumped through the solar panel. The temperature of this water rises from 16 °C to 38 °C.

The water absorbs 25% of the energy incident on the solar panel. The specific heat capacity of water is 4200 J/(kg °C)

Calculate the energy incident on the solar panel during that day.

The heated water is stored in a sealed copper hot-water tank. During the day, the water cools as thermal energy (heat) passes from the water to the air surrounding the tank.

State and describe the process by which the thermal energy is transferred from the hot water to the air.

The manufacturer of the hot-water tank says that when the outside surface is polished regularly and kept bright and shiny, the hot water will cool more slowly.

Describe an experiment that shows whether a container with a bright and shiny surface is better at keeping its contents warm than one with a dull and dark surface.

The apparatus shown in Fig. 1.1 is known as Leslie’s Differential Air Thermometer.

The heater is switched off. Tap T is opened so that the air on the two sides of T has the same pressure. Tap T is then closed.

A gym has a low initial temperature. A larger radiant heater is placed on the floor in the centre of a gym to heat the room. The room has no open windows or ventilation. 

Describe and explain the subsequent motion of air in the gym, naming any processes. 

After a marathon in winter, the athlete has sweated a large amount. 

For the end of the marathon, the athlete's coach has a double-walled vacuum flask (Fig. 1.2), which is designed to maintain the temperature of its contents. It is partially filled with a hot liquid.

The lid is made of black plastic and sealed tight. The walls are silvered metal.

Eventually the liquid cools. Explain how thermal energy is transferred from the hot liquid through the lid over time.

An object in the space between Earth and the Sun has an average temperature of 3 K.

The Earth's magnetic field arises from its iron-rich molten core, shown in Fig. 1.2.

Molten iron must be in motion to generate a magnetic field. 

The inner core is very hot and solid. The outer core is also iron-rich and liquid.

Explain the origin of the Earth's magnetic field, referring to density in your answer. 

Extended

Fig. 4.1 shows a cold plastic spoon that has just been placed in hot liquid in a cup.

Describe, in terms of molecules, why the temperature of the whole of the spoon increases.

The plastic spoon is replaced by a metal spoon.

 
Describe an additional process by which the temperature of the whole of this spoon increases.

The cup contains 150 g of liquid of specific heat capacity 4.2 J / (g°C). When the cold spoon is placed into the hot liquid, the temperature of the liquid decreases from 80 °C to 56 °C.

Calculate the loss of thermal energy from the liquid.

energy loss = ......................................................... 

A student constructs a device for absorbing thermal energy from the Sun. Fig. 6.1 shows the device.

The student places the white plastic pipe in sunlight. The cold water flows slowly from Tank A to Tank B. Energy from the Sun heats the water in the pipe.

Fig. 6.2 shows the temperatures in Tank A and Tank B.

Determine the rise in temperature of the water.

temperature rise = .................................................... °C 

The student wants to increase the thermal energy absorbed by the water in the pipe. Suggest three improvements he can make to increase the thermal energy absorbed.

Describe how the thermal energy is transferred from the Sun to the water inside the pipe.

A student investigates the thermal energy lost from two metal cans, X and Y. The cans are identical apart from their outside colour.

The student pours the same volume of hot water into each can and seals each can. The student records the temperature of the water in each can at regular time intervals for a period of 35 minutes.

The equipment is shown in Fig. 6.1.

Fig. 6.2 is a graph of the results from the investigation.

For can X, use Fig. 6.2 to determine the drop in temperature of the water

Explain why the water cools at a greater rate during the first five minutes of the experiment, compared with the last five minutes.

The outside of one can is dull black and the outside of the other is shiny white.

State the colour of can Y. 

Explain your answer.

A student heats some water in a metal can, as shown in Fig. 6.2.

Complete the sentence. Choose a word from the box.

Thermal (heat) energy moves through the metal can by ............................................

Describe how thermal energy is transferred throughout the water. Include your ideas about density changes.

Fig. 2.1 shows a hollow metal cube filled with boiling water. The temperature of the four vertical surfaces are equal but each surface has a different colour or texture.

Fig. 2.1

A thermal radiation sensor is placed the same distance from each surface and the meter reading measures the thermal radiation emitted from each surface. 

The results are shown in Fig. 2.2.

Draw a line linking each type of surface with the appropriate meter reading. One line has been drawn for you.

Fig. 2.2

The hot surfaces produce a convection current in the air outside the metal cube.

Describe how the convection current is produced.