Expansion & Contraction
D. Crowley, 2007
Monday, December 12, 2011
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
Problems & Uses
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/ch
emistry/particle_model_test.shtml