Heat and conduction

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```What is heat?
Heat is a type of energy.
Heat is the name for the type of kinetic energy possessed
by particles.
Heat energy is measured in joules (J).
How many joules are there in a kilojoule (kJ)?
If something gains a lot of heat energy, it becomes hot
– so what is temperature?
What is temperature?
Temperature is a measure of how hot or cold an object is.
(It is not the total amount of energy contained in the object.)
Temperature is measured in degrees Celsius (&ordm;C).
The freezing point of water is defined as 0&ordm;C (at 1 atm.).
The boiling point of water is defined as 100&ordm;C (at 1 atm.).
Temperature can be measured by a variety of different
thermometers. These include liquid in glass, digital,
thermocouple and bimetal strip thermometers.
Heat transfer
The correct phrase for heat transfer is ‘thermal transfer’.
Heat energy can be transferred (moved) by four processes:
1. conduction
2. convection
3. evaporation
4. radiation
In each process, thermal energy is also transferred in the way:
HOT
COLD
Heat energy only flows when there is a temperature difference.
The particle model
The differences between solids, liquids and gases can be
explained by the particle model:
1. All substances are made up of particles (atoms, ions or
molecules).
2. These particles are attracted to each other, some strongly
and others weakly.
3. These particles move around (i.e. have kinetic energy).
4. The kinetic energy of particles increases with temperature.
Temperature and energy
What happens to the gas as the temperature increases?
Would this be the same for solids and liquids?
Conduction experiment
Conduction of heat in different materials can be investigated
in an experiment.
Apparatus:
8 cm strip of copper
8 cm piece of wood
Bunsen burner
stopwatch
blob of wax
Take care whilst doing this experiment:
 Wear safety glasses.
 Do not touch mot metal objects!
 Wash any burn with cold water for ten minutes.
Conduction experiment
 Add a blob of wax to one end of the
copper and hold the other end in the
yellow Bunsen flame.
 How long does it take for the wax to
melt and drop off the metal strip?
 Now do the same for wood.
 Why are the times very different?
Conduction – compare a metal and a non-metal
Which material feels warmer if you touch a piece of wood
and a piece copper metal, both at room temperature
(i.e. both at 25&ordm;C)?
The wood feels warmer because it is a poor conductor.
It cannot conduct heat away from your hand as quickly as
the copper which is a good conductor.
Conduction in metals
Metals are good conductors
of heat because:
l metals contain many free
electrons
l the free electrons can
move between atoms at
high speed
l the free electrons carry
energy from high-energy
atoms to low-energy
atoms further away from
the heat source.
Conduction in metals
How do non-metals conduct heat?
Conduction in liquids
Metals are good conductors of heat and non-metals are
poor conductors of heat (insulators).
Are liquids good at conducting heat?
 Use some gauze to hold an
ice cube at the bottom of a
tube of water.
 Carefully heat the water at
the top of the tube only,
until this water is boiling.
100&ordm;C
0&ordm;C
 If the liquid is good at
conducting, the ice should
quickly melt – it doesn’t.
Liquids are poor conductors of heat (i.e. good insulators).
Conduction in gases
Liquids are poor conductors of heat (good insulators).
Are gases good at conducting heat?
 Carefully hold a safety match
1 cm away from a Bunsen
burner flame. (Do not put the
match in the flame!)
 If a gas is a good conductor, the
air between the flame and the
match should conduct heat and
light the match – it doesn’t.
 Gases are poor conductors of heat (i.e. good insulators).
Conductor or insulator?
Conduction – summary table
Material
Conductor or Insulator?
metals
very good conductors
non-metals
insulators
liquids
insulators
gases
good insulators
vacuum
excellent insulator
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