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Thermal Physics

Chapter: Thermal Physics

Moving particles

Solids, liquids and gases

Every material is solid, liquid or a gas. Scientists have developed a model called the kinetic theory to explain how solids, liquids and gases behave. According to this theory, matter is made up of tiny particles which are constantly in motion. The particles attract each other strongly when close, but the attractions weaken if they move further apart.


A solid such as iron has a fixed shape and volume. Its particles are held closely together by strong forces of attraction called bonds.


A liquid such as water has a fixed volume but can flow to fill any shape. The particles are close together and attract each other. But they vibrate vigorously that the attractions cannot hold them in fixed positions.


A gas such as hydrogen has no fixed shape or volume and quickly fills any space available. Its particles are well spaced out and virtually free of any attractions. They move about at high speed colliding with each other and the walls of their container.

Brownian motion

Robert Brown is the scientist who first noticed the wobbling, wandering motion of pollen grains in water in 1827. The effect is called Brownian motion.

Energy of particles

The particles in solids, liquids and gases have kinetic energy because they are moving. They also have potential energy because their motion keeps them separated and opposed the bonds trying to pull them together. The total kinetic and potential energies of all the atoms or molecules in a material are called its internal energy. The hotter a material is, the faster its particles move and the more internal energy it has.

If a hot material is in contact with a cold one, the hot one cool down and loses internal energy while the cold one heats up and gains internal energy. The energy transferred is known as heat

The term thermal energy is used for both internal energy and heat.


The Celsius scale

A temperature scale is a range of number for measuring the level of hotness. Everyday temperatures are normally measured on the Celsius scale. Its unit of temperature is the degree Celsius (oC). The numbers on the scale were specially chosen so that pure ice melts at 0 oC and pure water boils at 100 oC. These are its two fixed points.


Temperature is measured using a thermometer.  Every thermometer depends on some property of a material that varies with temperature.

What is temperature?

In any object the particles are moving so they have kinetic energy. The higher the temperature the faster they move. If a hot object is placed in contact with a cold one, there is a transfer of thermal energy from one to the other.

Temperature is defined as the hotness or coldness of the body.

Absolute zero and the Kelvin scale

As the temperature decreases, the particles in a material lose kinetic energy and move more and more slowly. At -273 oC, they can go no slower. This is the lowest temperature and it called absolute zero. The temperatures are measured using the Kelvin scale. Its temperature unit the Kelvin (K) is the same size as degree Celsius but the scale uses absolute zero as its zero (0 K).

You can convert from one scale to the other like:

            Kelvin temperature/K    =   Celsius temperature/ oC   +   273

Expanding solids and liquids

If a steel bar is heated its volume will increase slightly. The effect is called thermal expansion. Most solids expand when heated. So do most liquids and by more than solids.

The expansion is more for greater lengths and higher temperatures.

Water and ice

When hot water cools, it contracts. However when water freezes it expands as it turns into ice. The force of the expansion can burst water pipes and split rocks with rainwater trapped in them. Water expands on freezing for the following reason. In liquid water, the particles are close together. But in ice, the molecules link up in a very open structure that actually takes up more space than in the liquid. This happens as Ice has a lower density than liquid water. Because of its low density, ice floats on water. When liquid water is cooled the molecules start forming into an open structure at 4 oC, just before freezing point is reached. As a result, water expands very slightly as it is cooled from 4 oC to 0 oC. It takes up least space and therefore has its maximum density.

Heating gases

A gas does not necessarily expand when heated unlike a solid or a liquid. This is because its volume depends on the container it is in. There are always three factors to consider: pressure, volume and temperature. A change in temperature can produce a change in pressure or volume or both depending on the circumstances.

Linking pressure and temperature (at constant volume)

At constant volume, as the temperature of the air rises, the pressure also rises. This is because the molecules move faster so they hit the sides of the flask with greater force.

The pressure law

For a fixed mass of gas at constant volume, the pressure is directly proportional to the Kelvin temperature.

This law applies only to an ideal gas.

Linking volume and temperature (at constant pressure)

For a fixed mass of gas at constant pressure, the volume is directly proportional to the Kelvin temperature.

Thermal conduction

All materials are made up of tiny, moving particles. The higher the temperature the faster the particles move. If one end of a metal bar is heated, the other end eventually becomes too hot to touch. Thermal energy is transferred from the hot end to the cold end as the faster particles pass on their extra motion to particles all along the bar. The process is called conduction.

Thermal conductors and insulators

Metals are the best thermal conductors. Non-metal solid tend to be poor conductors. Same is the case with most liquids. Gases are the worst of all. Many materials are insulators because they contain tiny pockets of trapped air.

Insulating materials are used to reduce heat losses from a house:

  1. Plastic foam lagging round the hot water storage tank.
  2. Glass or mineral wool insulation in the loft.
  3. Wall cavity filled with plastic foam, beads, or mineral wool.
  4. Double glazed windows: two sheets of glass with air between them.

How materials conduct

In atoms there are tiny particles called electrons. Most are strongly attached. But in metal some are loose and free to drift between the atoms. When a metal is heated these free electrons speed up. As they move randomly within the metal, they collide with atoms and make them vibrate faster. In this way thermal energy is rapidly transferred to all parts.

Metals are good electrical conductors as well as good thermal conductors.


Liquids and gases are poor thermal conductors but if they are free to circulate, they can carry thermal energy from one place to another very fast.

Convection in a liquid

When the water is heated, water particles gain energy and vibrate more rapidly. These vibrations lead to circulation of water particles and resulting in a circulating stream. This is called convection current.

Convection in air

Convection can occur in gases. Heated by the Sun, warm water from the Sea rises above and forms vapour. The result is huge convection currents in the earth’s atmosphere. Convection also causes the onshore and offshore breezes which sometimes blow at the coast during the summer.

Thermal radiation

We are warmed by the Sun. Its energy travels to us in the form of electromagnetic waves. They heat up things that absorb them and are called thermal radiation.

All objects give out some thermal radiation. The higher their surface temperature and the greater their surface area, the more energy they radiate per second.

Emitters and absorbers

Some surfaces are better at emitting thermal radiation than others. Good emitters of thermal radiation are also good absorbers.

Greenhouse effects

When the Sun’s thermal radiation reaches the Earth, the atmosphere acts as a heat trap. This happens because some gases absorb energy strongly at certain wavelengths in the infrared region of the spectrum. The heat-trapping action of the atmosphere is called the greenhouse effect.  Without it, the Earth’s surface would be around 25 oC cooler than it is.

Greenhouse acts as heat traps, which is how the greenhouse effect got its name.

The solar panel

Some houses have a solar panel on the roof. It uses Sun’s thermal radiation to warm up water for the house. The blackened layer absorbs the radiant energy and warm up the water flowing through the pipes.

The vacuum flask

A vacuum flask can keep drinks hot (or cold) for hours. It has the following features for reducing the rate at which thermal energy flows out (or in):

  1. An insulated stopper to reduce conduction and convection.
  2. A double-walled container with a gap between the walls. Air has been removed from the gap to reduce conduction and convection.
  3. Walls with silvery surfaces to reduce thermal radiation.


Even on a cool day, rain puddles can vanish and wet clothes dry out. The water becomes an invisible gas called water vapour which drifts away in the air. When a liquid below its boiling point changes into a gas, this is called evaporation.

There are several ways of making a liquid evaporate more quickly:

Increase the temperature

Wet clothes dry faster on a warm day because more of the particles have enough energy to escape.

Increase the surface area

Water in a puddle dries out more quickly than water in a cup because more of its molecules are close to the surface.

Reduce the humidity

If air is very humid, this means that it already has high water vapour content. In humid air, wet clothes dry slowly because molecules in the vapour return to the liquid almost as fast as those in the liquid escape.

Blow air across the surface

Wet clothes dry faster on a windy day because the moving air carries escaping water molecules away before many of them can return to the liquid.


It is a very rapid form of evaporation. When water boils, vapour bubbles form deep in the liquid. They expand, rise, burst and release large amounts of vapour. Boiling occurs at 100 oC.

The cooling effect of evaporation

Evaporation has a cooling effect. For example, if you wet your hands, the water on them starts to evaporate. It means it takes thermal energy from your skin. So your hands feel cold.

Refrigerators use the cooling effect of evaporation.

Sweating also uses the cooling effect of evaporation.


When a gas changes back into a liquid, this is called condensation.

Specific heat capacity

Some materials have a greater thermal capacity for absorbing thermal energy than others. Scientifically speaking, water has a specific heat capacity of 4200J/kg oC and aluminium has only 900 J/kg oC. The energy that must be transferred to an object to increase its temperature can be calculated using the following equation:

Energy transferred = mass x specific heat capacity x temperature change

Energy transferred = mcΔT

Where m is the mass in kg, c is the specific heat capacity in J/kg oC. And ΔT represents the temperature change in ΔT or K.

Thermal capacity

The quantity mass x specific heat capacity is called the thermal capacity.

Latent heat

Water can be a solid (ice), a liquid or a gas called vapour or steam. These are its phases or states.

Latent heat of fusion

The energy absorbed by any substance when it changes its phase from solid to liquid is called latent heat of fusion. Ice has a specific latent heat of fusion of 330000J/kg.

Latent heat of vaporization

The energy absorbed by any substance when it changes its phase from liquid to vapour is called latent heat of vaporization. Water has a specific latent heat of vaporization of 2500000 J/kg.