Heat

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Heat is a form of energy .
It travels from a hot region towards a cooler one.

Effects of heat

How heat travels
Heat can move three ways:

Conduction     Convection      Radiation

 

Through solids by conduction

Metals are good conductors of heat

Non metals are poor conductors of heat (insulators)

Heat being conducted along a copper rod
Tthe heat makes the copper atoms vibrate faster.
These atoms in turn make the atoms near them vibrate faster.
In this way the heat energy is gradually transferred along the rod from the hot end towards the cooler end.

 

We can demonstrate this by coating the rod in wax and timing how long it takes the wax to melt.
With a copper rod the wax will melt quicker than an iron rod because copper is a better conductor of heat.

 

Examples and uses

Good conductors of heat: Copper, gold, Aluminium

Insulators (poor conductors): Wood, plastic, glass, china, cork

 

A saucepan is made of aluminium or copper which is a good conductor of heat.

The handle is made of plastic or wood which is a poor conductor

 

Through gas or liquid (fluids) by convection

As a liquid or gas heats up its particles move faster and get further apart.

This makes it less dense (lighter) so it starts to rise.

The rising air or water is called a convection current

 

Air in a room is warmed by convection currents

Air is warmed by the heater.
Warm air is less dense than cool air so the warm air rises.
Cool air is drawn in at the bottom of the radiator to replace the air that has risen.

 

This diagram shows how the heater warms all the air in the room.
notice that air near the ceiling will be warmer than air near the floor because warm air rises.

A large city or a desert will warm air in the same way as the heater.
Cooler air will blow in from outside to replace the air that has risen.
This is one way that breezes and wind is formed

 

Examples and uses

1.    Hot water rises to the top of a hot water tank. This helps in central heating in a house.

2.    A radiator works by warming the air which rises and flows around the room.

3.    Wind (or breezes)  are often caused by hot air rising above a hot desert or city.

       Cooler  air travels in from the side to take the place of the warmer air        

 

Through space by radiation

Radiation is the only form of heat that can travel through a vacuum.

Heat radiation, like light, is made of waves and travels in straight lines.

 

 

Heat from the sun reaches us by radiation

The sun gives out a large amount of radiation.
This radiation is made of waves and travels in straight lines away from the sun.

Some of it is visible (we call it light)
Some of it is invisible and is made up of many things including heat.

The heat radiation is easily stopped by anything which gets in its way like clouds.

Examples and uses

Heat from the sun reaches us by radiation

An electric grill heats food by radiation

More about Heat Radiation

Shiny or white surfaces do not radiate heat well which is why water in a shiny jug will stay warm longer than one in a black jug.

Dark or dull surfaces absorb heat radiation much more easily than white shiny surfaces.
This is why in hot countries the houses and clothing are often white because white reflects the heat better.

Investigating how effective cups are at keeping water hot
Aim: to see which cup keeps water hot the longest

Cup 1 Cup 1
Cup 1 is made of metal painted black. It has no lid and no saucer.
 

Cup 2
Cup 2 is made from white china. It has a lid and is resting on a cork mat

Both cups are the same size and both are filled with boiling water at the same time.

1. In which cup will the water remain hot the longest?
2. Give three reasons for your answer above
3. Mention two things that are done to make this a fair test
3. Name two additional pieces of apparatus you would need to actually carry out the experiment.
answers

Cup 2

 

 

 

Answers:
1. The water will remain hot for longer in cup 2

2. Reasons:
    In cup 2:
    The lid stops convection currents. The air heated by the coffee cannot rise
    The white, shiny surface does not radiate heat well.
    Cork is a poor conductor of heat so heat cannot travel down into the table.

3. The cups are the same size
    The water is at the same temperature for each cup at the start.

4. A thermometer and stop watch

 

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