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TP Be 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. 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 Expansion 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 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. Bimetallic Strip 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 (brass on other side) Compensations for thermal expansion Expansion Joint Pipelines carry very hot gases (such as steam), therefore need expansion joint (in ring form) to avoid damages given by expansion. 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 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 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. 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. 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) (b) (c) Metal Tyre Fixing Application of thermal expansion Hot riveting is a common method to fasten two metal plates firmly. How ? (c) (a) (b) Hot Riveting Expansion of various Solids Different materials expands by different amount when heated through the same increase in temperature. Materials Aluminium brass iron steel glass invar Increase in length (mm) per m 0.25 0.19 0.12 0.11 0.09 0.01 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 During Winter…… Due to the unusual behaviour of water, at 4°C during Winter time, at, water at the surface of a pond will si (Why ?). Later, even the surface freezes, the fish ins the pond still alive. ( Why ? )