thermal expansion and contraction. have you ever wondered why this sign is posted at gas stations? ...
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Thermal Expansion and Contraction
Have you ever wondered why this sign is posted at gas stations?
What would happen if you filled your gas tank completely?
Thermal Expansion and Contraction
Thermal Expansion and Contraction
Or why highways in the winter seem more bumpy?
Thermal Expansion and Contraction
Also why do the sidewalks have gaps?
Thermal Expansion and Contraction
Or why a full pan of water will overflow when you heat it?
Thermal Expansion and Contraction
And what are those teeth like things on bridges?
Arranging MoleculesThermal Expansion and Contraction
Thermal Expansion and Contraction
Demonstration
Arranging Molecules
Thermal Expansion and Contraction
Molecules are similarly arranged
The less they move the more of them can fit in a space
Adding heat or thermal energy (remember therm means heat) causes molecules to move more
Arranging Molecules
Thermal Expansion and Contraction
This energy that makes molecules move is described as KINETIC ENERGY
Kinetic Energy = energy of movement
Arranging Molecules
Thermal Expansion and Contraction
More energy = more movement
More movement = more space
Molecules will push out with a lot of force (well for them) to get that space
Arranging Molecules
Thermal Expansion and Contraction
This “pushing” out is called THERMAL EXPANSION
Therm=heat
Arranging Molecules
Thermal Expansion and Contraction
This works in the opposite as well
Take away heat (or thermal energy) you take away Kinetic Energy
Molecules move slower so they need less space
The result is shrink
Arranging Molecules
Thermal Expansion and Contraction
So if adding Heat causes things to expand and taking it away makes them shrink
Let’s look back at our questions from earlier
Arranging Molecules
When gas is stored underground it is colder than you expect
On a hot summer day that gas will expand and if you filled you car up too much it would leak out!
Thermal Expansion and Contraction
Thermal Expansion and Contraction Highways have gaps built into them so
that when it gets hot they expand and seal the gap, in the winter though the road has shrunk and now the gap is noticable when you drive
Thermal Expansion and Contraction
Same thing with sidewalks, gaps are left so the cement has somewhere to go
Thermal Expansion and Contraction
Otherwise….
Thermal Expansion and Contraction
That water that fit before, well now it is taking more room which means it needs to get out of that pan
Thermal Expansion and Contraction
Bridges don’t have gaps, not good for crossing a space.
Instead they have expansion joints, that look like teeth
Thermal Expansion and Contraction
They give room for the bridge to grow but it is still connected the whole time
Also only one side of the bridge is anchored, the other side is only resting on a pillar
Hold on a second!Water when it freezes (gets colder) expands...
Why is that?
Water breaking the rules
Water molecules when liquid flow very close to each other, in fact they are always constantly touching
But when it freezes they form crystals and leave gaps between themselves
Kind of like K’nex
Thermal Expansion and Contraction
Different materials expand and contract at different rates
That is because their molecules are connected differently from each other
Some have very strong connections so the molecules stay close, while others have loose connections
These are measured and given values called coefficients
Volume and length each have one, we will focus on length or linear for now
Thermal Expansion and Contraction
The larger the coefficient the more the material changes with heat
Make a chart in your notes to compare 7 common materials
Let’s start with Glass
Glass has a coefficient of 8.5
Thermal Expansion and Contraction
But there are different types of glass
Pyrex is a type of glass
Pyrex glass has a coefficient of only 3.2
Many scientist spend their time trying to develop new types of materials, thermal expansion is one thing they are working on
Thermal Expansion and Contraction
A common substance in homes is copper
It is used in wiring and pipes to hold water
It has a coefficient of 17
Thermal Expansion and Contraction
But in the winter if you are not careful water in pipes can freeze and burst the pipe
That is because water has a very high coefficient
69
Thermal Expansion and Contraction
Steel is used in bridges and skyscrapers
Engineers have to plan on how much the building will expand or contract
It has a coefficient of 11~13 depends on the type of steel
Thermal Expansion and Contraction
In homes windows need to allow light in but not the outside air
The glass has a low coefficient but it still changes enough to let in some
We put silicon caulk around windows to keep this from becoming a problem
Silicon has a coefficient of 2.56
Thermal Expansion and Contraction Many times people end up
getting rings stuck on their fingers
Our fingers react to the cold contracting as well but so will the gold in the ring
Gold has a coefficient of 14
Thermal Expansion and Contraction
Get familiar with these materials and their coefficients
Look for patterns that will help you determine how likely a material is to change
Hint: metals have higher coefficients than non-metals