# Thermo-fluids Thermal Expansion and Contraction (*)

Bi-metallic strips

The principle behind a bimetallic strip is that different metals expand to different extents with temperature changes. By combining two different metals one on top of another into a strip, a bimetallic strip is formed. As the two metals expand or contract differently under the same temperature change, the strip bends. It can then be used to switch on or off a circuit at certain temperatures. Bimetallic strips are often found in ovens. The typical structure of this type of control is shown in Fig. 5.

 Fig. 4   A typical bimetallic strip Fig. 5   The structure of a bimetallic strip

The device shown in Fig. 5 is typical of those used in ovens. The upper metal (blue) expands more when heated and contracts more when cooled than the lower metal. Thus, when the temperature inside the oven drops below a certain point, the bimetallic strip bends upwards enough to complete the circuit, switching on the heating element. In a refrigerator, the reverse set-up is used. When the temperature inside the refrigerator rises, the bimetallic strip bends to switch on the compressor which starts the cooling cycle.

# Internal Energy

Matter in general expands when heated. This can be explained using the molecular model of matter. When a solid is heated, its molecules vibrate faster about their fixed positions. As a result of this, the molecules move slightly further apart than when they are cold. The cumulative effect of all the molecules result in the volume expansion of the solid.

### SOLID EXPANSION

The expansion of a solid when heated is small. A metre rule may expands by 1 – 2 mm when heated. Through small, this expansion can create a very large force if it is restrained.  Railway tracks expand during a hot day. If the tracks are not designed for the expansion, the entire track may bend out of shape during expansion. However, the expansion of solid may also be put into good use. Two pieces of different metals with different expansion coefficients may be bound together. When temperature changes, the two metals expands differently. This causes the strip to bend according to the temperature. This bimetallic strip may be used to open and close an electric circuit to control temperature.

### LIQUID EXPANSION

Liquid also expands for the same reason. However, since liquid particles are usually less tightly bound to each other molecules, they generally move further than solid particles when heated. Hence, liquid expands more than solid if the temperature rise is the same. This expansion of liquid may be used in a liquid-in-glass thermometer. The volume increase of alcohol or mercury may be calibrated to provide a temperature reading since the expansion is almost directly proportional to the temperature rise.

### GAS EXPANSION

Gases behave differently from solids and liquids. Gas molecules are far apart and weakly attracted to each other. Heat causes the molecules to move faster and the volume increases much more than solids and liquids. However, gas do not have to expands when heated. If the gas is confined to a fixed volume, the increase in temperature may cause the pressure to increase if the volume of kept constant. If the gas is allowed to expand, the pressure may be kept constant. We may use the kinetic model to explain this. When a gas is heated, the molecules move faster. The higher speed of molecules result in a higher frequency of collision with the container walls. These collisions are also harder as the molecules are faster. Together, these cause the pressure to increase. As the pressure is higher than original value, the gas will push the piston out. This result in a volume increase. As the volume is now larger, the frequency of collisions with the container walls is less and the pressure decrease. The volume stops expanding when the internal pressure equals to the original external pressure. This is expansion of gas under constant pressure.