Transfer of Heat
Name |
Description |
Heat (Thermal Energy) |
Measured in Joules (J), thermal energy |
Temperature (symbol T) |
Measured in Kelvin (K), determined by the average kinetic energy of particles |
Transfer of heat |
Occurs when there is a difference in temperaturerom a region of higher temperature to a region of lower temperature |
Temperature gradient |
Affects the rate of heat transfer |
Thermal Equilibrium |
Reached when no net heat flow occurs (same temperature) |
Thermal Conductivity |
How well a substance transfers heat. In increasing order of thermal conductivity: gas, liquid, solid |
Expansion and Contraction
Phenomenon |
Definition |
Expansion |
As an object gains heat, it expands |
Contraction |
As an object loses heat, it contracts |
What do they cause? |
Results in a change of volume, While mass remains the same → density changes |
Methods of Heat Transfer
Method |
Definition |
Conduction |
Transfer of thermal energy from one place to another without any flow of the material medium, occurs in all states of matter |
Convection |
Transfer of thermal energy by means of convection currents in a fluid (gas, liquid), due to differences in density, only occurs in fluids as particles are able to move freely |
Radiation |
Transfer of heat in the form of electromagnetic waves, does not require any medium for heat transfer. Good radiators are also good absorbers of radiation. |
Applications of Conduction
Examples of Good Conductors |
Cooking utensils, Electric irons, kettles |
Examples of Poor Conductors |
Handles of cooking utensils, thermal underclothes, polystyrene cups |
Describe how the design of a vacuum flask keeps the liquid inside hot/cold. |
● Vacuum flask keeps hot liquids hot and cold liquids cold ● No heat can enter or leave the flask by conduction or convection across the vacuum ● Inner silvered surface reflects radiation from hot fluids back into the flask ● Outer silvered surface reflects external radiation away from the flask ● Plastic cap (and foam support) minimise heat transfer by conduction ● Plastic cap stops convection and evaporation |
Why does a stone floor feel cold to bare feet? |
● Feet has higher temperature than stone floor ● Stone floor will conduct heat away from feet |
Why does the cloth with the coin inside not burn when exposed to fire? |
● The coin is made of metal and has delocalised electrons ● Thus the coin conducts heat better ● The coin conducts heat energy from the coin ● Prevents the cloth from burning |
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Applications of Convection
Examples of Convection |
Hot water system, Cooling system in car radiator, Refrigerators and air conditioners |
Why does placing a lid over a pot of hot soup keep the soup warm longer? |
● Lid does not allow hot air to rise and escape ● Lid prevents convection current from existing |
Factors influencing radiation
Factor |
Description |
Temperature of surface |
The higher the surface temperature, the higher the rate of heat transfer by radiation |
Colour of surface |
Black, dull surfaces absorb and radiate heat much faster compared to bright, shiny surfaces; Shiny surfaces are better reflectors of radiation |
Surface area |
The larger the surface area, the higher the rate of heat transfer by radiation |
Emitters of Radiation
Good Emitter |
Poor Emitter |
Black colour fins of refrigerators to cool the coolant in the condenser coil |
Shiny teapot to keep tea hot |
Absorbers of Radiation
Good Absorber |
Poor Absorber |
Solar panels are painted dull black to absorb maximum radiation from the sun |
Houses are painted white to keep them cool |
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Roofs are coated with aluminum paint to reduce heat absorption during the day and minimize emission of heat at night |
Absorbers of Radiation
Good Absorber |
Poor Absorber |
Solar panels are painted dull black to absorb maximum radiation from the sun |
Houses are painted white to keep them cool |
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Roofs are coated with aluminum paint to reduce heat absorption during the day and minimize emission of heat at night |
Applications of Radiation
I am at home and need a warm drink quickly. There is a cup of cold black coffee and a cup of cold milk. Which drink should I take to warm up and get my drink? |
Black coffee (that is black) is a better absorber of heat radiation than milk which is white, hence it will gain heat faster |
Applications of Radiation
I am at home and need a warm drink quickly. There is a cup of cold black coffee and a cup of cold milk. Which drink should I take to warm up and get my drink? |
Black coffee (that is black) is a better absorber of heat radiation than milk which is white, hence it will gain heat faster |
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