Expansion & Contraction D. Crowley, 2007

Expansion & Contraction Monday, August 26, 2019 Expansion & Contraction To understand expansion and contraction, and to be able to explain this using particles

Javelin Why do you think javelin measuring tapes are not made from metal? It is all due to expansion…

Heating Look at the ball and ring - the ball can pass through the ring easily But… when the ball is heated it cannot pass through the ring! However, if we heat them both, the ball passes through the ring Why is this so?!

Cooling When the ball is allowed to cool, it suddenly passes through the hoop again Also, a heated glass rod will shatter if placed into iced water What is going on here?

Expansion & Contraction What happened when we heated the ball? Substances expand (get bigger) when they are heated up What happened when we rapidly cooled the glass? Substances contract (get smaller) when they are cooled

Expansion & Contraction Substances expand (get bigger) when they are heated up The particles stay the same (the number of particles + their size is the same) But they take up more room! When heated: - Solids - particles vibrate more and take up more room Liquids - move around each other more quickly and take up more room Gases - move more quickly in all directions, and take up more room Substances contract (get smaller) when they are cooled down When we cool objects, the number of particles and their size remains the same, they just take up less room!

Expansion & Contraction Look at the example of the cylinder and the hole Substances expand when they are heated Substances contract when they are cooled Role play expansion & contraction…

Particle Theory Copy out the particle box of a substance Then draw out what happens to the particles when they are a) heated; and b) cooled Cooling Heating

Particle Theory Copy out the particle box of a substance Then draw out what happens to the particles when they are a) heated; and b) cooled Cooling Heating Remember, the number of particles and their size does not change The only thing that changes is the amount of space between the particles - heat a substance up and this spaces become bigger, cool a substance down and the spaces become smaller

Expansion joint in a bridge Problems & Uses Expansion joint in a bridge Expansion of substances can be both useful, and cause problems for designers and engineers - can you think of any? Problem - bridges expand in the summer, and need special joints to stop them bending out of shape Concorde (the supersonic plane) expanded when it was going at very high speed by ~30cm! Thermometers use liquids which expand when they get hotter, and contract when the get colder - very useful for us to measure heat! At very high speed friction causes a great deal of heat - this expands the plane in a variety of areas

Revision Try this online test: - http://www.bbc.co.uk/schools/ks3bitesize/science/chemistry/particle_model_test.shtml