Junior Certificate Physics
Heat
Examples of convection:
Electric kettle
The element is placed at the bottom of a kettle.
As the water is heated, the heated particles rise
by convection and cooler ones take their place.
In this way, all of the water will be heated.
Heat is a form of energy. The unit of heat
energy is the joule (J).
There are three methods of moving heat from
place to place.
1. Conduction – this is the transfer of heat
from one place to another, through a
solid, without the particles, of the solid,
moving out of position.
2. Convection – this is the movement of
heat, through a liquid or gas, by the
upward movement of heated particles.
3. Radiation – this is the movement of
heat, by invisible rays, from a hot object
without the need for a medium to pass
through.
Electric immersion heater
This works in a similar way to a kettle.
Convection heaters
These are usually called radiators, but this is
not a good name for them, as they heat a room,
mostly, by convection.
Land and sea breezes
During the day, the land is hotter than the sea.
The air above the ground gets heated and rises
upwards. Cold air blows in from the sea to
replace it. This is called a sea breeze.
An insulator – is a substance which will not
allow heat to pass through it easily.
Examples of conduction:
Metal pots
Cooking pots are made of metal because they
are good conductors and will allow food, placed
in them, to heat-up.
A poker
A poker can get extremely hot if the end is left
in the fire. The heat will travel out by
conduction to the handle.
Gerard Nugent
Junior Certificate Physics
The effects of heating
When solids, liquids and gases are heated
they expand. The only exception to this rule
is water, between 40 and 00C. W
Mandatory experiment: To compare the
conductivity of various metals
During the night, the sea is hotter than the land.
The air above the sea gets heated and rises.
Colder air from the land blows out to replace it.
This is called a land breeze.
Wooden ring
x
Examples of radiation:
Metals
Solar energy
Energy from the sun travels through space to
the earth by radiation. There is no medium just
a vacuum.
All hot objects
All hot objects radiate heat, in all directions.
If you put your hand over a lighted candle you
will feel great. This is heat by convection.
If you put your hands around a candle you will
also feel heat. This is radiated heat and it is not
as much as the convected heat.
Examples of insulators:
Fibre glass wool – used for attic insulation.
Polystyrene – used for burger boxes, pizza
boxes (keeping food hot).
Polystyrene board – used for wall insulation to
prevent heat being lost from the walls of a
house.
Wool clothing – wool is a good insulator and
prevents us from losing body heat.
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Set up the apparatus as shown.
A thumbtack is attached with wax to
each of four metal strips with wax.
A Bunsen flame is placed at ‘x’ and
the four strips are heated evenly.
The thumbtack which falls first
indicates the best conductor.
Mandatory experiment: To show that water
is a poor conductor.
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Fill a test tube with water.
Hold the test tube at the bottom and
heat the mouth with a Bunsen burner.
The water at the mouth of the test tube
will be boiling but you will still be
able to hold the bottom of the tube.
This is because the water is a poor
conductor.
Gerard Nugent
Junior Certificate Physics
Mandatory experiment: To show convection
in water
Coloured
water rises
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This creates a convection current.
The smoke allows this current to be
seen by an observer.
Mandatory experiment: To show heat
transfer by radiation
Dye
Thermometer 2
Bunsen
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Heat the water as shown.
The hot water rises as a convection
current and the dye goes with it.
The dye makes the current visible.
Mandatory experiment: To show convection
in a gas
Candle
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Smoke
Candle
Box with glass front
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The candle creates an updraft
(convection current) of hot air.
The hot air rises and leaves through the
chimney on the left.
Cold air is drawn in from outside,
through the chimney on the right, to
replace it.
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Thermometer 1
Place two thermometers equal distances
from a candle, as shown above.
Thermometer 1 shows a small increase
in temperature. This is due to radiated
heat only.
Thermometer 2 shows a large increase
in temperature. This due to radiated
heat and convection.
Solids, liquids and gases expand when they
are heated. The following experiments are
used to demonstrate this fact.
Mandatory experiment: To show that solids
expand when heated.
 Put the ball through the hoop, to check
that it fits through the hoop.
 Heat the ball for 30 seconds with a
Bunsen burner.
 Try to fit the ball through now.
 It cannot be done.
Gerard Nugent
Junior Certificate Physics
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The solid metal ball has expanded with
the heat.
Heat the flask
Metal hoop
Metal ball
Mandatory experiment: To show that liquids
expand when heated
Experiment: To show the action of a
bimetallic strip
Cell
Bell
Flask filled
with water
Metal contact
Heat
Bimetallic strip
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Heat the flask as shown.
Since the flask is already full, any
expansion of the water will be seen as
the water rises up the tube.
The water level in the tube will fall if
the flask is cooled.
Bunsen
Mandatory experiment: To show that gases
expand when heated
 Heat the flask as shown.
 The air in the flask will expand.
 The expanded air has only one escape
route, out through the top of the tube.
 If the tube is held under the water, the
expanded air can be seen bubbling out.
 If the flask is allowed to cool, the air in
the flask will contract and water will be
sucked into the flask.
Gerard Nugent