1. Expansion & Contraction
D. Crowley, 2007

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

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

4. 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?!

5. 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?

6. 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

7. 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!

8. 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…

9. 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

10. 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

11. 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

12. Revision Try this online test: -