•Heat flows from warmer objects to cooler objects. When a warmer object is in contact with a cooler object, it will transfer heat to the cooler object.

•This will go on until both objects have the same temperature. At this point, they are in the state of thermal equilibrium.

•For example, a bottle of soda is taken out from refrigerator and placed on a table. The table which is at room temperature, will transfer heat to the bottle. Eventually, their temperature will be equal and thermal equilibrium will be achieved.

•Most matter expands when heated and contracts when cooled.

•Two common examples of the effects of heat are a boiled egg and a thick glass cracking when hot water is poured onto them.

•The increase in size of objects when they are hot is called expansion. The decrease in their size when they are cooled is called contraction.

•The atoms or molecules in a solid vibrate at all temperatures.

•As the temperature increases, they vibrate more vigorously and this pushes the atoms further apart. The volume of the solid increases and the expansion is said to occur.

•When the solid is cooled, the atoms vibrate more slowly and they become closer together. The volume of the solid decreases and contraction occurs.

cold hot

Arrangement of particles in a solid

•When a liquid is heated, the molecules of the liquid have more energy and move more vigorously. The movement of the molecules gradually overcomes the forces of attraction between molecules, allowing them to have greater freedom to move over greater volumes. Thus, the liquid expands.

Arrangement of particles in a liquid

cold hot

•At a lower temperature, the molecules of the liquid have less energy and move closer to each other. This causes the volume of the liquid to decreases and the liquid contrasts.

•The molecules of a gas are far part compared with the molecules in a solid and a liquid. The gas molecules move at a high speeds in all direction.

•If a gas is confined in a container whose volume is variable, the volume of the gas will increase with increasing temperature. The volume will decrease as the temperature drops.

•When the gas is heated, the molecules become more energetic, move faster and are further apart. This causes the volume of the gas to increase and expansion is said to occur. At a lower temperature, the molecules move very much slower due to less energy. They are closer together, causing the volume to decrease and contraction occurs.

cold hot

Arrangement of particles in gas

•The effect of expansion and contraction of matter can be very troublesome. Precautions have to be taken against these effects. We can also apply the principle of expansion and contraction of matter in making instruments that are useful in our daily life.

•The following are some examples to show the use of expansion and contraction of matter.

a. Mercury in a thermometer

- Mercury is a liquid metal that can expand and contract when there is a change in temperature.

- This make it suitable for temperature measurement and it is used in a thermometer.

Mercury in a laboratory thermometer

Clinical thermometer

b. Bimetallic strip in a fire alarm

-An automatic fire alarm uses a bimetallic strip to switch on the electric bell when there is a fire.

-The heat from the fire causes the bimetallic strip to bent towards the contact point.

-When the bending strip touches the contact point to complete the circuit, the fire alarm rings.

-An automatic fire alarm

c. Bimetallic strip as a thermostat

-The bimetallic strip is also used as thermostat in an electric iron for controlling and maintaining temperature.

-As temperature rises, the bimetallic strip bends away from the contact point and cuts off the current.

-When the bimetallic strip cools down, contact is made again and current flows once more to heat up the iron.

Thermostat in an electric iron

d. Bimetallic thermometer

-A bimetallic strip wound in a spiral can be used to make a thermometer.

-In the figure below, the metals used are brass and invar, with brass on the outside. As the temperature decreases, the spiral is wound tighter and the pointer will move to the left.

A bimetallic thermometer

•When laying railways tracks, gaps have to be left between successive lengths of rail to allow for expansion on the hot days. Without the gaps, the tracks buckles and this affects the safety of the trains.

A gap in railway tracks

•When concrete roads are laid down, gaps (normally filled with bitumen) are left between sections in order to allow for expansion on hot days.

Gaps between concrete sections

•Structures like steel bridges and overhead bridges and built with gaps to allow for expansion . Sometimes, one end is supported by rollers which allow the bridges to expand easily when heated.

•Rollers of steel bridge

•Electric transmission cable and cable cars’ cable sag on hot day and tighten during a cold night. Therefore, allowances have to be made for the expansion and contraction of the cables.

•Overhead cable sag in hot weather and tighten during a cold night

Rivetsa.Rivets are steel pins used to join pieces of metal slightly together.b.They are heated before being inserted through two pieces of metal.c.While the rivet is still hot, the end is hammered flat. d.When the rivet cools, it contracts, pulling the two pieces of metal tightly together.

Stages of riveting

A very tight bottle cap can removed easily by immersing it in hot water. The cap expand much faster than the bottle and so, the cap can be removed easily when it expands.

The hub of a wheel us usually slightly smaller than the size of its axle. To fix the axle into the hub of the wheel, the axle has to be put in liquid nitrogen (at -190 C) to cool it so that it contracts until it can be fitted into the hub.

•Metallic tires

The metallic wheels of a train are fitted with metal tires. To ensure a tight fit, the tire is slightly smaller in diameter than the wheel. Before fitting, the tire is heated uniformly and the resulting expansion enables the tire to be slipped over the wheel. Upon cooling, the steel tire contracts and make a tight fit.

Fitting a metal tire onto a metal wheel

•All objects can absorb and give out (radiate) heat. But some objects absorb or give out heat better than others.

•The ability of an object to absorb and give out heat depends on the type and color of its surface (dull or shiny and light colored or dark- colored)

•The rate of which heat is absorbed and given out also depends on the surrounding temperature of an object.

•Most of the buildings and houses are painted with white or brightly colored paint so that less heat is absorbed and the interior can be kept cool. White and bright surfaces are good reflectors of heat and poor absorbers of heat.

•The oil tank of an oil tanker is painted silver so that it becomes good reflectors of heat. This is safer as less heat will be absorbed by the petrol inside the tank. Oil storage tanks are also painted with shiny aluminium paint to reflect the radiated heat away so as to keep the contents (oil cool).

•The base of the heating utensil or a pot is usually dull and black. This makes it good absorber of heat.

Kitchen utensils