TPBe able to identify problems caused

by thermal expansion.

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.

A solid expands when heated, contracts when cooled.

Ball and Ring

Before heating the ballAfter heating the ball

Expansion of Solids When cold, the bar fits into the gaps. When bar is heated it will not. Why?

ExpansionWhen 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

Problems posed by Thermal expansion

Rails expand and may buckle on a hot summer day.Engineers leave expansion gaps between rails during construction.

Rollers and gap at one end allows for bridge to slide during expansion and contraction

Telephone and power cables sag (bend) during hot weather.

Bimetallic Strip

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

steel (brass on other side)

Click for Internet DemoUsed to make thermostat switches.

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.

Expansion Joint

Compensations for thermal expansion

Concrete blocks for pavements and road surfaces arelaid with gaps or joints between them. This allow it expands or contracts during hot or cold weather

Concrete Pavement

Liquids Expand more than solids

(a) (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 ?)

EXPANSION OF GASESGases 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.

Fire Alarm If the fire breaksout, the heat from the fire will causethe bimetallic stripsto bend upwardsand complete thecircuit. The alarmbell then ring.

Bimetallic strip & its application

Application of Thermal Expansion

(a)

(b) (c) Metal Tyre Fixing

Fig. (a) shows the tyreis too small when cold.Fig. (b) shows tyre hasbigger diameter thanwheel when heated.Fig. (c ) shows that tyreis firmly secured on thewheel. (Why ?)

(a)(b)

(c)

Hot Riveting

Application of thermal expansion

Hot riveting is a common method to fasten twometal plates firmly. How ?

Different materials expands by different amount when heated through the same increase intemperature.

Materials Increase in length (mm) per m

Aluminium 0.25brass 0.19iron 0.12steel 0.11glass 0.09invar 0.01

Expansion of various Solids

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

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 insidethe pond still alive. ( Why ? )

During Winter……