1. TP
Be able to identify problems caused by thermal expansion.

2. A rectangular sheet of metal has a circular hole
A rectangular sheet of metal has a circular hole. What happens to the size of the hole when the metal is heated?
A) It gets larger.
B) It gets smaller.
C) It stays the same.

3. Expansion of Solids
When cold, the bar fits into the gaps. When bar is heated it will not. Why?
A solid expands when heated, contracts when cooled.
Ball and Ring
After heating the ball
Before heating the ball

4. Expansion When cooled, they contract.
When heated, the particles (atoms or molecules) in a substance vibrate more and move apart to occupy more space
So the size and volume increases but the size of the particles remains the same.
When cooled, they contract.
Heat

5. Problems posed by Thermal expansion
Rollers and gap at one end allows for bridge to slide during expansion and contraction
Rails expand and may buckle on a hot summer day.
Engineers leave expansion gaps between rails during construction.
Telephone and power cables sag (bend) during hot weather.

6. Bimetallic Strip Click for Internet Demo
(brass on other side)
steel
A bimetallic strip is a strip of two different metals.
Brass’s expands more than steel and becomes longer and the strip to bend toward the steel side.
The bending is reversed when the strip cools
Used to make thermostat switches.
Click for Internet Demo

7. Expansion Joint Compensations for thermal expansion
Pipelines carry very hot gases (such as steam), therefore need expansion joint (in ring form) to avoid damages given by expansion.

8. Compensations for thermal expansion
Concrete Pavement
Concrete blocks for pavements and road surfaces are
laid with gaps or joints between them. This allow it expands or contracts during hot or cold weather

9. Liquids Expand more than solids
(b)
(c)
Fig. (a) shows a glass flask filled with coloured water.
Fig. (b) the water level is seen to fall for first few
seconds when heated. (Why ?)
Fig. (c) later, water level rises quickly. (Why ?)

10. EXPANSION OF GASES
Gases expand much more than solids and liquids.
When heated, gases move faster and exert a greater force on the piston pushing it outwards.
If the piston is not moved, the particles hit the piston more frequently causing more pressure.

11. Bimetallic strip & its application
Fire Alarm
If the fire breaks
out, the heat from
the fire will cause
the bimetallic strips
to bend upwards
and complete the
circuit. The alarm
bell then ring.

12. Application of Thermal Expansion
Fig. (a) shows the tyre
is too small when cold.
Fig. (b) shows tyre has
bigger diameter than
wheel when heated.
Fig. (c ) shows that tyre
is firmly secured on the
wheel. (Why ?)
(a)
Metal Tyre Fixing
(c)
(b)

13. Application of thermal expansion
Hot riveting is a common method to fasten two
metal plates firmly. How ?
(c)
(a)
(b)
Hot Riveting

14. Expansion of various Solids
Different materials expands by different amount
when heated through the same increase in
temperature.
Materials Increase in length (mm) per m
Aluminium
brass
iron
steel
glass
invar

15. The unusual behaviour of Water
Most liquids expand when heated and contract when cooled. Water, however is an exception.
Between 00 C to 40 C, water contracts when temperature increases while water expands when temperature decreases.
Water has highest density and smallest volume at 40C

16. During Winter…… Due to the unusual behaviour of water, at 4°C during
Winter time, at, water at the surface of a pond will sinks
(Why ?). Later, even the surface freezes, the fish inside
the pond still alive. ( Why ? )