# 15.2 Convection - Woodlands Meed

```15.2 Convection
It is the material itself that moves
Learning Objectives
• What is convection?
• How can we change the rate of convection?
• Where do we make use of convection in the
home?
Particles
• In fixed positions in solids
• Can change places in liquids and gases
Liquids and Gases
• Both fluids.
• Heat is transferred
by convection in
fluids.
• The particles in the
fluid move, carrying
energy with them.
When a fluid is warm
•
•
•
•
•
•
Less dense than when it is cool
Particles gain energy and vibrate more
Particles move further apart
Fluid takes up more space
Fluid expands and rises
Cooler denser water sinks
• Same for gases such as air
• A wind is felt when cool air is sucked into a
bonfire to replace the hot smoke
• Hot air in the balloon is less dense than the
surrounding air, so it floats.
Watch this
• http://www.bbc.co.uk/learningzone/clips/hotair-balloons-powered-by-a-toaster/8452.html
Heat loss
• Heat is lost by convection through gaps in
badly fitting window frames and doors
• Hot smoke is less dense than surrounding air
so it rises up a chimney. Heat is lost up the
chimney because of convection.
Ovens
• Fan ovens use fans to move the hot air inside
an oven so the food cooks evenly.
• No fan – food at the top of the oven cooks
quicker because convection means the hot air
rises
• Burners in gas oven are at the bottom, so
convection helps the heat to spread more
evenly.
Heat
• Heat spreads quickly if a liquid or gas is heated
from the base or cooled from the top
• This sets up convection currents
Kettle
The heating element is
fitted at the base of a
kettle. When the kettle is
on it heats up the water
next to it. The warm
water rises and cool
water sinks and takes its
place.
Fridge
• Some fridges have
an ice box at the
top of the fridge.
Cooler air near the
ice box sinks and
warmer air rises to
take its place,
transferring heat
away from the
fridge.
convection
• The hottest air in a room is near the ceiling.
• Gliders can rise because of “thermals” in the atmosphere.
• Breezes occur on the coast because of air rising above the
land or sea.
• The outlet for the hot water is at the top of the hot water
tank.
• The ice box is always at the top of a fridge.
• Motor cycle engines are air cooled and are fitted with fins.
• The element in a kettle is near the bottom of the kettle.
• Some hot water cylinders have two immersion heaters, one
small one near the top of the tank and one large one
nearer the bottom.
Problems
1. Why is it not vital to keep the lid of a chest
freezer closed?
2. Why is the ice box always at the top of the
fridge?
3. Why do you think that convection is important in
a house hot water system?
4. Why does a coal, fire help to ventilate a room as
well as heat it?
Experiments
1. Convection in water
•
• Fill a beaker with cold water and then
carefully drop a few crystals of potassium
permanganate into it so that they fall close to
one side of the beaker. Now heat the base of
the beaker just under where the crystals have
fallen.
• You should see the colour rise up this side, go
across the top and then fall down the other
side of the beaker — this is a convection
current.
• You can also use the special piece of
apparatus shown in the diagram. It is a
"square " glass tube filled with water. Drop a
crystal of potassium permanganate into the
top and then heat one of the bottom corners
gently. You will see the colour begin to move
round the tube going down the limb opposite
the heating and then rising up the other side
above the Bunsen due to convection currents
in the water.
Unwanted convection
•
•
•
•
•
•
Block up gaps
Create small pockets of air
Block up old fireplaces
Install draft excluders
Keep doors and windows closed.
Insulate the loft – stop warm air escaping
through the roof
Did you know
• Molten rock inside the Earth is constantly
moving due to convection currents.
• These currents move tectonics plates, causing
earthquakes and volcanoes.
Watch this
• http://www.bbc.co.uk/learningzone/clips/198
9-santa-cruz-california-earthquake/5722.html
Key points
• Convection occurs when particles move,
carrying heat with them
• Convection takes place in liquids and gases
• Convection currents spread heat in fluids
where they are heated from the base or
cooled from the top
Watch this
• http://www.bbc.co.uk/learningzone/clips/elni-o-and-la-ni-a-ocean-currents-andwinds/1504.html
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