# Energy - Kinetic Theory

This GCSE Physics energy quiz takes a look at kinetic theory. The kinetic theory (sometimes referred to as the kinetic particle theory) of matter is used to explain many of the properties of solids, liquids and gases. As you might imagine from the name of this theory, it is all about movement of particles. These particles are atoms and molecules, depending on the substance. The kinetic theory links the movements of particles to heat energy - the higher the temperature, the faster the movement. It also takes into account the forces between the particles. It is not perfect, but works well at GCSE if you make the assumptions that all of the particles are identical and that any collisions between particles or the particles and the wall of a container are elastic.

For the GCSE, you need to be able to recognise simple diagrams to model the difference between solids, liquids and gases.

To be honest, this should not be too taxing as it is really just revision of concepts you first met during KS3. These usually give the impression that the kinetic theory deals with small numbers of particles as there are only ever a few seen in drawings.

This is not the case. It is really a statistical theory that describes what happens to large numbers of particles as the temperature changes. It also helps you to understand a little more about temperature which is really just a measure of the energy in a sample of particles. The lower the temperature, the lower the energy of the particles, so they move more slowly. The particles of solids, liquids and gases have different amounts of energy. In solids, the particle motions are limited to vibrations as the particles have the least energy. In liquids and gases, the particles can exchange places. The forces between the particles of a solid are greater than those between the particles of a liquid and in a gas, the forces between the particles are considered as being zero.

You will need to be able to apply the kinetic theory to describe and explain various phenomena in all three sciences. In physics, you are most likely to need to be able to explain that the transfer of energy by conduction, convection, evaporation and condensation involves particles, and how this transfer takes place. This could include the movement of the free electrons in a metal as well as the closeness of the metal atoms in the structure. Also, you should be able to use the idea that as particles move apart when fluids are heated, the fluid becomes less dense and then apply it to the formation of convection currents. Remember that the word fluid could mean either a gas or a liquid - many students use the words fluid and liquid to mean the same thing. It is possible that you may also be challenged to explain why evaporation has a cooling effect.

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1. The particles in a system are always in what kind of motion?
The theoretical exception is at absolute zero. This is defined as the point where the constant and random motion of all particles stops. Scientists have come close to achieveing absolute zero but have never yet succeeded in getting there. Some very odd effects appear at these very low temperatures such as superfluidity and superconductivity
2. How large is the number of particles within a system in kinetic theory?
The sample size needs to be large to be reliable
3. What does the kinetic energy of the particles in a system depend on?
Particles at higher temperatures have a greater kinetic energy than when they were at a lower temperature
4. Which experiment shows how kinetic theory works?
Brownian motion was first observed by the botanist Robert Brown who noticed the random movement of pollen grains in water but had no idea why this movement was taking place. We now can explain this as being caused by the constant random motion of water molecules colliding with the pollen grains. You may have seen it demonstrated at school using smoke
5. The particles in the system are considered to have what mass relative to each other?
This is one of the basic assumptions of the kinetic theory
6. It is assumed all collisions within a system are what?
Assuming all collisions are elastic allows us to make a good approximation of the kinetic energy of the system, as we are not concerned with the losses in kinetic energy that occur due to heat released during collisions in the real world
7. As a substance changes state from a liquid to a gas, the amount of energy the particles have …
During this change the temperature remains the same but a lot of heat energy needs to be added in order to make the particles move fast enough to escape from the liquid
8. What forces are assumed to exist between particles in a gas?
The energy used to change a liquid into a gas breaks any forces between particles
9. The molecules in a system are assumed to obey which laws?
Newton's Laws of Motion are used to solve kinetic theory problems as they are simple to use and offer a good approximation to the solutions needed for a system
10. What does the kinetic particle theory describe?
It relates the movement of the particles to the energy in a system