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Heat transfer


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heat-transfer


Summary

Transfer of Thermal Energy
  • When 2 objects are placed in contact with one another, their temperature eventually becomes the same, known as thermal equilibrium.
  • Heat travels from a region of high temperature to low temperature.
Describe in molecular terms, how energy transfer occur in solids
  • The molecules at the hotter part of the solid vibrate vigorously. They collide with the neighbouring slower moving molecules in the colder part of the solid.
  • As the molecules collide, the faster molecules transfer some of their kinetic energy to the slower molecules.
  • The slower molecules gain more thermal energy and collide with other neighbouring molecules in the colder part of the solid.
  • This process continues until heat from the hotter part spreads to the colder part.

Conduction

Heat is transmitted layer by layer through a medium from one particle to another.

Mechanism

- collision between neighbouring particles

a. Particles nearer to heat source gain energy and vibrate faster.
b. Particles collide into less energetic neighbouring particles which gains kinetic energy.
c. The less energetic particles vibrate faster, collides into other particles.
d. Process continues layer by layer to spread the heat to cooler parts.

- flow of free electrons in conductors only

a. Electrons near heat source gain energy, move faster.
b. Free electrons can move between the particles and collide with other electrons, allowing the less energetic electrons to gain energy and move faster.
c. Process continues to spread the heat to cooler parts.

Convection

  • Process whereby heat is transmitted from one place to another by the movement of heated particles of a gas/liquid.

Mechanism

- change in density

a. fluid nearer to heat source gains heat and expands.
b. Expansion causes decrease in density for the fluid nearer to heat source, causing it to rise.
c. The hotter fluid rises over the cooler fluid while the cooler fluid rushes in to take the space.
d. The movement of the fluid due to difference in density sets up a convection current.
e. Convection is faster than conduction as there is bulk movement (all the molecules get hot and move up, thus it is faster than conduction.

Radiation

  • A method of heat transfer where the source of heat transmit energy through electromagnetic waves. A medium is not required.

Factors affecting radiation

- Temperature of object
- surface of object
- surface area of object
- Good emitters are also good absorbers of radiation.

Reducing Energy Wastage

  • Reducing our demand for energy is as important in reducing greenhouse gas emissions as finding renewable energy resources. 

Bonus: Radiation

  • Everything we know about space outside the solar system comes from analyzing the electromagnetic radiation collected from space by telescopes.
  • Different objects in space emit different wavelengths.

Bonus: Exploring the Universe



MCQ Questions

1. How may heat be transferred though a vacuum?
a. by convection only
b. by radiation only
c. by conduction only
d. by convection and radiation

2. Which of the following is the poorest conductor of heat?
a. air
b. brass
c. vacuum
d. water
e. wool

3. How is heat transferred through the walls of a steel radiator?
a. conduction only
b. convection only
c. radiation only
d. conduction and convection
e. convection and radiation

4. Which of the following will be the best absorber of infra-red radiation?
a. dark animal fur
b. shiny metal tray
c. white plastic bag
d. window glass
e. writing paper

5. A vacuum will prevent heat transfer by
a. conduction only
b. convection only
c. radiation only
d. conduction and convection
e. conduction, convection, and radiation

6. How is heat transferred through the base of a metal saucepan?
a. conduction only
b. radiation only
c. conduction and convection
d. convection and radiation

7. The diagram shows a vacuum flask and an enlarged view of a section through the flask wall.
The main reason for the silvering is to reduce heat transfer by
a. conduction only
b. conduction and convection
c. radiation only
d. radiation and convection

MCQ Answers

1. b
2. c
3. a
4. a
5. d
6. a
7. c

Structured Question Worked Solutions

1. State briefly how energy is transferred in the following processes
a. conduction
b. convection
c. radiation

Solution

1a. Conduction occurs only in all solids whereby the heat energy is transferred via increased vibrations from one layer of molecules to its neighbouring layer and so on. For solids which are good conductors of electricity, free electrons can carry heat energy quickly by diffusing from the hotter end of the solid to the colder sections

1b. Convection occurs only in fluids whereby the fluid at the heat source warms up and becomes less dense than the surrounding fluid. This causes the fluid to rise carrying the heat quickly away from the source. The process is repeated as colder fluid move in to replace the moving warmer fluid

1c. In radiation, the heat energy travels in the form of waves from one place to another without any medium.


Solution

2. A bunsen burner is used to heat a beaker full of water
a. explain how energy is transferred through the bottom of the beaker
b. explain how energy is transferred through the water
c. A student's hand, several cm to one side of the bunsen burner, starts to feel hot. Name the process by which energy is transferred from the burner to the student's hand

2a. the glass molecules gain energy which is passed on to neighbouring molecules

2b. the water near the bottom of the beaker heats up first, expands, and rises up to the top. The colder water, being denser sinks to the bottom to be heated up. Thus, a convection current is set up which transfers heat energy through the water

2c. radiation


3. A saucepan with a thick copper base contains water and is placed on a flat electric hot plate.
a. state the process by which energy is
i. transferred from the hot plate to the water
ii. spread through the water

b. state one reason why the water would reach boiling point more rapidly with a lid on the pan

c. the sides of saucepans are often polished. How does this reduce heat loss?


Solution

3ai. conduction
aii. convection
b. heat lost by convection is reduced by the lid
c. polished surfaces are poor emitters of heat. Hence heat lost by radiation is minimized.


4a. A curved metal mirror focuses the heat energy from the Sun on to a small tank containing water. The area of the mirror is 1.2 m2, Assuming that the average amount of energy received from the Sun at the mirror per square metre per second is 550 J, calculate the minimum time taken to raise the temperature of 5 kg of water from 20oC to 100oC. The energy needed to raise the temperature of 1 kg of water by 1oC is 4200 J.

4b. Only some of the energy incident on the mirror actually reaches the water. Give two reasons why this is so.

4c. The graph illustrates how the temperature of the water changes with time when the mirror is used.



Suggest explanations for the forms of sections AB and BC of the graph


4d. Explain why the temperature of the water rises to 100oC whereas the surface of the mirror remains quite cool.


Solution

4a. energy supplied per second by the mirror = 550 x 1.2 J

quantity of heat required = 5 x 4200 x (100 - 20) = 1680000

minimum time taken = energy/power = 1680000/(550 x 1.2) = 2545.45 s or 42.42 min

4bi. some energy is absorbed by the water container

4bii. some energy is absorbed by the metal mirror

4c. AB: The temperature of water rises slowly with increase in temperature due to greater loss of heat at higher temperature

BC: The temperature of water is at 100oC. Temperature remains constant when water is transformed into steam.

4d. Only a small amount of the incident energy is absorbed by the mirror which has a larger area. The heat can be conducted away quite easily. The small tank holds a small amount of water which absorbs most of the incident energy. This leads to a rapid rise in temperature.


5. Small amount of ice at -10ºC is poured into a big glass of coloured water at 0ºC.
a. Why does thermal energy transfer from water to ice?

b. Assuming that there is no loss of energy to the surrounding, what will happen to the ice and the coloured water?

c. Assuming that there is no energy loss to the surrounding, when will the transfer of thermal energy between the ice and coloured water stop?

Solution

5a. Thermal energy always transfers from a region of higher temperature to a region of lower temperature.

5b. The temperature of the ice increases. Some coloured water freezes to ice at 0ºC.

5c. The transfer of thermal energy between the ice and coloured water stops when the ice gains enough energy to be at the same temperature as the coloured water.

6. The delivery bag for pizza is made of soft plastic with many air sacs in it. The bag has a silvery surface. Why is pizza sealed in such a delivery bag for delivery service?

Solution
  • Air is a poor conductor of heat. The air sacs minimise the transfer of thermal energy from the hot pizza to the surrounding by conduction.
  • The pizza is sealed in the bag, such that hot air does not flow out of the bag, to prevent thermal energy transfer from the pizza to the surrounding by convection.
  • A silvery surface is a bad emitter of radiant heat. The silvery surface thus minimise thermal energy transfer from the pizza to the surrounding by radiation.
7. State a situation where the colour black is used to maximise heat loss and a situation where it is used to maximise heat gain.

Solution
  • Cooling fins at the back of some refrigerators and in motor cars are painted black to maximise thermal energy transfer to the surrounding as black colour is a good emitter of radiant heat. 
  • Solar panels are painted black to maximise the thermal energy absorption as black colour is a good absorber of radiant heat.
8. State a situation where the colour white is used to minimise heat loss and a situation where it is used to minimise heat gain.

Solution
  • During winter when the surrounding is colder than the human body, people wear white to minimise thermal energy loss to the surrounding as white colour is a poor emitter of radiant heat.
  • During a sunny day when the surrounding is hotter than the human body, people wear white to minimise thermal energy gain from the sun as white colour is a poor absorber of radiant heat.
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