Download Transfer of thermal energy (heat)

CONDUCTION
EXPERIMENT TO DEMONSTRATE THE HEAT CONDUCTIVITY OF A SOLID
Take a few rods each made of a different substance (for example, three different metal rods). Attach
some wax to the end of each one, and then stick a coin onto the wax. Heat up all three rods equally. If
the metal is a good conductor of heat, the wax will melt fairly quickly (because the heat will have
passed all the way through the rod), dropping the coin. If the metal is not a good conductor, the wax
will take longer to melt. You can put different substances in order of conductivity by measuring how
much time it takes for the coins on each one to fall off.
Conduction is a mode of transfer of energy within and between bodies of matter (such as metal rods)
due to a temperature gradient. Heat travels from areas of high temperature to areas of low
temperature. Therefore, in the experiment above, the heat travels from the left of the rod to the right,
in order to equalise the temperature throughout the body.
Conduction occurs mainly in solids. In solids, the particles are in a fixed arrangement, and can only
vibrate around a fixed point, but cannot move. However, when the object is heated, its particles gain
thermal and kinetic energy. Since they cannot move, they vibrate instead. Particles with the most
kinetic energy vibrate the most, and pass on the vibration to other molecules. Hence, the thermal
energy is also passed on.
CONVECTION
Convection is the main method of heat transfer in liquids and gases.
The following diagram shows how water can be heated through the process of convection.
 The bottom of the container is slowly heated.
 This causes the water at the bottom of the container to become
warmer.
 Since liquids expand when heated, the water will expand and
become less dense.
 The less dense water will rise, and the denser water will sink,
taking the place of the previously heated water.
 This dense, cooler water is now at the bottom of the container, and can now be heated.
This is called a convection current. Note that if the container is heated from the top, the same will not
occur. The water at the top of the container will be heated, and will remain there, as the denser,
cooler water will stay at the bottom, and the already heated water will remain at the top.
The following are two experiments which can illustrate convection currents in:
A GAS
1. Hold an ice cube with a pair of tongs or a pair of pliers. Hold your hand just underneath the ice
cube. You will feel cool air. Slowly move your hand downwards to about a metre underneath it.
You will also probably feel cool air while moving your hand.
2. Now, place your hand just above the ice cube. You will probably also feel cool air. Repeat the
process, but this time raising your hand to about a metre above the ice cube. This time,
however, you will probably only feel cool air near the ice cube, not far above it.
Warm air rises, and cool air sinks. The cool air coming off the bottom off the ice cube sinks below the
warmer air, which is why you are able to feel it quite a long way down. However, it is also why you
can’t feel it a long way above the ice cube. The cool air doesn’t rise, and so doesn’t reach your hand. If
any mist is coming out of the ice cube, it probably falls downwards, because the mist is cool.
A LIQUID
1. Take two empty bottles and fill one of them with hot water, the other with cold water. Put a
few drops of food colouring or dye into the cold bottle.
2. Put a piece of cardboard over the cold bottle. Carefully turn it round, with the cardboard, and
place it on top of the hot bottle. Then, remove the cardboard so that the water in the two
bottles can mix.
3. You will see that the hot water (the clear water), which is on the bottom, will begin to rise
above the cold water (with the food colouring) into the upper bottle, and the cold water will
begin to sink into the lower bottle.
This is another example of a convection current; two bodies of water with different temperatures will
move about so that the warmer water rises above the cold water, and displaces it. If you repeated the
exact same experiment but with the hot and cold water reversed (so that the hot water was on the top
and had the food colouring in it), nothing would change, as the hot water would be at the top and the
cold water at the bottom.
RADIATION
Radiation is a method of heat transfer that does not need a medium to travel through. This
means that it does not need a solid, a liquid or a gas to pass through; it can pass even through a
vacuum. Perhaps the best example of heat transfer through radiation is the heat we receive from
the sun. In space, there is no air for heat to pass through, it is just a vacuum.
Certain substances absorb (or reflect) radiation better than others. Dark surfaces absorb
radiation (and therefore heat), while light surfaces reflect radiation. Reflective surfaces also
reflect radiation very well.
EXPERIMENT:
(apparatus needed: three test-tubes, water, white paint, black paint, aluminium foil, a lamp, a
clamp, a clamp stand, a thermometer, a stop-watch)
Take three test-tubes. Paint the bottom half of one of them black. Paint the bottom half of
another one white. Cover the bottom half of the last one with aluminium foil. Fill each of the
test-tubes with an equal amount of water.
Clamp the first test-tube in place just below a lamp. Insert a thermometer into the test-tube, and
start the stopwatch. After each minute has passed, read the temperature of the water and make
a note of it. Do this for up to ten minutes. Repeat the experiment with the two other test-tubes.
Dark colours absorb heat much better than light colours. Therefore, you should notice that the
water in the test-tube painted black becomes hotter more quickly than the water in the test-tube
painted white. Reflective surfaces reflect heat very well, better than white ones. Therefore, you
should notice that the water in the test-tube covered with foil heats up less quickly than both the
painted test-tubes.
Infrared (also spelled ‘infra-red’) radiation is the part of the electromagnetic spectrum often
involved in heat transfer by radiation.
EXAMPLES OF EVERYDAY THERMAL ENERGY TRANSFER
METHOD OF HEAT
TRANSFER
PROCESS
Pans can be used to heat
up water and all sorts of
other liquids and foods.
CONDUCTION
CONVECTION
RADIATION
EXPLANATION
Pans are usually made of metal, which is a
good conductor of heat. This means the heat
of the stove or the cooker transfers well to
the whole pan, and to what’s heating up
inside it.
Pan handles are usually
Wood and plastic are not good conductors of
made of wood or plastic. heat. If the handles were made of metal,
we’d burn ourselves while holding the pan.
Air conditioning cools
Air conditioners are usually placed near the
rooms and buildings
ceiling (at the top). This is so it can cool the
down using convection
air near the ceiling, which will sink and be
currents.
replaced by warmer air. This means that all
the air in the room is cooled down. If the air
conditioner was placed at the bottom of the
room, it would only cool the air near the
floor.
Transparent surfaces
Greenhouses keep very warm because they
absorb heat very well
absorb sunlight through their glass panes,
through the process of
and more heat gets trapped inside than
radiation.
manages to escape. This is also why
greenhouse gases are not good for the
planet, because they produce the same
effect and make the planet gradually
warmer.
The same process applies Solar panels are reflective or light, and so
to solar panels.
absorb heat well through the process of
radiation.