Heat and its effect

1. Heat is a form of energy that a substance possesses due to the movement or vibration of its atoms, molecules or ions.
   Solids, liquids and gases generally expand on heating causing increase in volume. On cooling, they contract and the volume decreases.
2. Thermal expansion of solids, liquids and gases.
   Solids - Solids expand the least compared to liquid and gases for the same rise in temperature. The most commonly used experiment to demonstrate the thermal expansion of solids is the ball and ring experiment. When hot, the metal ball expands and does not pass through the metal ring.
   Liquids - Liquids expand to temperature and mostly it is demonstrated using water. Various liquids expand to various levels for the same rise in temperature.
   Gases - Gases contain independent particles-in-random motion. They expend the most with temperature and the particles move faster and faster as the temperature increases. The bombardment of the gas particles against the sides of the container creates pressure. Thermal expansion of gas can be explained using the hot air balloon experiment. Warmth on the flask causes expansion of air inside resulting in production of bubbles.
3. Effects on thermal expansion and contraction
   Changes in temperature brings about expansion or contraction in railway tracks, concrete blocks, bridges, overhead cables, telephone lines, pipes etc. Allowance given in for the expansion helps to overcome the adverse effects of thermal expansion and contractions.
4. Usefulness of thermal expansion and contraction
     - Riveting --> Rivet is the nail shaped metal which helps to hold two metal plates together. Red hot rivets are put through the holes in two metal plates and the rivet is hammered flat. As the rivet cools it contracts and pulls the two metal plates together and firmly holds them together.
     - Bimetallic strip --> As the name denotes the strip is made of two metal strips having different metal expansion. When heat is applied to a bimetallic strip, made of brass and steel, brass expands more than steel and the strip bends with the brass on the outside of the curve. Till a certain temperature the bending does not take place and the contract is maintained. Once the heat increases and the strip bends due to uneven expansion of the two metals and the electrical contact is broken.
5. Thermometers
   Thermometers are instruments which are used to measure temperatures. Any physical property of a material which changes steadily with temperature can be used to measure temperature. Thermometer uses the principle of thermal expansion of liquids. The most commonly used liquid in a thermometer is mercury.
   
   Clinical thermometers are both graduated in Fahrenheit and in Celsius. In the Celsius scale the ice point is 0 degree Celsius. It is the temperature at which ice melts. The steam point is the temperature at which pure water boils. There two reference points are equally divided into 100 divisions, each of which is 1 degree Celsius.
6. The other thermometers are the constant volume thermometer, liquid crystal thermometer, the bimetallic thermometer, and the  resistance thermometer.