Transcript Understanding Heat Transfer, Conduction, Convection and
If a cup of hot coffee and an ice cube were left on the table in this room what would happen to them? Why?
The cup of coffee will cool until it reaches room temperature. The ice cube will melt and then the liquid will cool to room temperature.
Heat always moves from a warmer place to a cooler place.
Hot objects in a cooler room will cool to room temperature.
Cold objects in a warmer room will heat up to room temperature.
These are: Conduction Convection Radiation
When you heat a metal strip at one end, the heat travels to the other end.
As you heat the metal, the particles vibrate, these vibrations make the adjacent particles vibrate, and so on and so on, the vibrations are passed along the metal and so is the heat. We call this?
Conduction
Conduction is the process
of heat transfer through a medium or material
without any movement
the medium or material of
6
Metal is a conductor, wood is an insulator. The metal conducts the heat away from your hands, the wood does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal.
Some materials conduct heat more rapidly than others Such materials are called thermal
conductors
Most metals are good thermal conductors 8
Times CD-rom 9
Poor heat conductors are called insulators They conduct heat much more slowly Examples : wood, rubber, asbestos, plastic, glass, water and air 10
Times CD-rom 11
Material
Brass Iron Water Wood Air
Thermal conductivity (W/mk)
106 84 0.56
0.13
0.024
Which is the best conductor?
--> Brass 12
Thermal conductors: We use metals to make cooking utensils so that the food inside cooks or heats up quickly.
13
Boilers in a factory or power station have to rapidly convert heat to steam.
14
Radiators have to conduct heat from a hot source before they can warm the air around them.
15
Thermal insulators: Thermal insulators are good at keeping heat out AND keeping heat in.
Your winter scarf is a thermal insulator.
It prevents the heat from your body from escaping into the cold air.
16
Handles of cooking utensils Walls of buildings 17
Second method of heat transfer
What happens to the particles in a liquid or a gas when you heat them?
The particles spread out and become less dense.
Cooler, more d____, fluids sink through w_____, less dense fluids. In effect warmer liquids and gases r___ up.
Cooler water sinks Cools at the surface Convection current Hot water rises
Definition!
Convection
is the transfer of heat from one place to another by the movement of currents in liquids or gases
23
As air is heated up on the road, it rises and cool air replaces the warm air. This sets up a convection current.
Where is the freezer compartment put in a fridge?
It is put at the top, because cool air sinks, so it cools the food on the way down.
Freezer compartment It is warmer a at the bottom, so this warmer air rises and convection current is set up.
C A Explanation: B
A B Explanation:
Third method of heat transfer
How does heat energy get from the Sun to the Earth?
There are no particles between the Sun and the Earth so it CANNOT travel by conduction or by convection.
RADIATION
?
Radiation travels in straight lines True/False Radiation can travel through a vacuum True/False Radiation requires particles to travel True/False Radiation travels at the speed of light True/False
Four containers were filled with warm water. Which container would have the warmest water after ten minutes?
Dull metal Shiny black Shiny metal Dull black radiation.
Four containers were placed equidistant from a heater. Which container would have the warmest water after ten minutes?
Dull metal Shiny black Shiny metal Dull black
Radiation is the transfer
of heat, in the form of waves, from one place to another without a medium.
34
1.
2.
3.
Nature of surface of body Total surface area of body Temperature difference between the body and its surrounding 35
1. Black, white or coloured 2. Rough, smooth or shiny
Which types of surfaces make better radiators?
36
Times CD-rom 37
Increase in surface area, increases amount of heat radiated per second 38
The greater the temperature difference, the more heat is radiated per second
39
How does heat from the Sun reach the Earth?
Sun Earth
40
Sunlight is radiated through space to our planet in the absence of matter.
The sun transfers heat through millions of kilometres of space. 41
White clothes Houses painted in light colours Black pipes on solar water heaters 42
Times CD-rom 43
Styrofoam for food is white Box covered with lid Brightly polished kettle keeps the liquid warm Times CD-rom 44
How does the design of the spacesuit help astronaut survive in outer space?
45
Which features of these buildings help keep the heat out?
46
Why does hot air rise and cold air sink?
Cool air is more dense than warm air, so the cool air ‘falls through’ the warm air.
Why are boilers placed beneath hot water tanks in people’s homes?
Hot water rises.
So when the boiler heats the water, and the hot water rises, the water tank is filled with hot water.
Why are houses painted white in hot countries?
White reflects heat radiation and keeps the house cooler.
Why are shiny foil blankets wrapped around marathon runners at the end of a race?
The shiny metal reflects the heat radiation from the runner back in, this stops the runner getting cold.
1.
Which of the following is not a method of heat transfer? A. Radiation B. Insulation C. Conduction D. Convection
2.
In which of the following are the particles closest together?
A. Solid B. Liquid C. Gas D. Fluid
3.
How does heat energy reach the Earth from the Sun?
A. Radiation B. Conduction C. Convection D. Insulation
4.
Which is the best surface for reflecting heat radiation?
A. Shiny white B. Dull white C. Shiny black D. Dull black
5.
Which is the best surface for absorbing heat radiation?
A. Shiny white B. Dull white C. Shiny black D. Dull black