•Heat flows from warmer objects to cooler objects. Whena warmer object is in contact with a cooler object, it willtransfer heat to the cooler object.•This will go on until both objects have the sametemperature. At this point, they are in the state ofthermal equilibrium.•For example, a bottle of soda is taken out fromrefrigerator and placed on a table. The table which is atroom temperature, will transfer heat to the bottle.Eventually, their temperature will be equal and thermalequilibrium will be achieved.
•Most matter expands when heated and contractswhen cooled.•Two common examples of the effects of heat area boiled egg and a thick glass cracking when hotwater is poured onto them.•The increase in size of objects when they are hotis called expansion. The decrease in their sizewhen they are cooled is called contraction.
•The atoms or molecules in a solid vibrate at alltemperatures.•As the temperature increases, they vibrate morevigorously and this pushes the atoms furtherapart. The volume of the solid increases and theexpansion is said to occur.
cold hot Arrangement of particles in a solid•When the solid is cooled, the atoms vibrate moreslowly and they become closer together. The volumeof the solid decreases and contraction occurs.
•When a liquid is heated, the molecules of the liquidhave more energy and move more vigorously. Themovement of the molecules gradually overcomes theforces of attraction between molecules, allowing themto have greater freedom to move over greatervolumes. Thus, the liquid expands. cold hot Arrangement of particles in a liquid
•At a lower temperature, the molecules of theliquid have less energy and move closer to eachother. This causes the volume of the liquid todecreases and the liquid contrasts.
•The molecules of a gas are far part compared with themolecules in a solid and a liquid. The gas moleculesmove at a high speeds in all direction.•If a gas is confined in a container whose volume isvariable, the volume of the gas will increase withincreasing temperature. The volume will decrease asthe temperature drops.
cold hot Arrangement of particles in gas•When the gas is heated, the molecules become moreenergetic, move faster and are further apart. Thiscauses the volume of the gas to increase and expansionis said to occur. At a lower temperature, the moleculesmove very much slower due to less energy. They arecloser together, causing the volume to decrease andcontraction occurs.
•The effect of expansion and contraction of matter canbe very troublesome. Precautions have to be takenagainst these effects. We can also apply the principleof expansion and contraction of matter in makinginstruments that are useful in our daily life.•The following are some examples to show the use ofexpansion 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
b. Bimetallic strip in a fire alarm-An automatic fire alarm uses a bimetallic strip toswitch on the electric bell when there is a fire.-The heat from the fire causes the bimetallic strip tobent towards the contact point.-When the bending strip touches the contact point tocomplete the circuit, the fire alarm rings.
c. Bimetallic strip as a thermostat-The bimetallic strip is also used as thermostat in anelectric iron for controlling and maintainingtemperature.-As temperature rises, the bimetallic strip bends awayfrom the contact point and cuts off the current.-When the bimetallic strip cools down, contact is madeagain and current flows once more to heat up the iron.
d. Bimetallic thermometer-A bimetallic strip wound in a spiral can be used tomake a thermometer.-In the figure below, the metals used are brass andinvar, with brass on the outside. As the temperaturedecreases, the spiral is wound tighter and the pointerwill move to the left.
•When laying railways tracks, gaps have to be leftbetween successive lengths of rail to allow for expansionon the hot days. Without the gaps, the tracks bucklesand this affects the safety of the trains. A gap in railway tracks
•When concrete roads are laid down, gaps (normallyfilled with bitumen) are left between sections in order toallow for expansion on hot days. Gaps between concrete sections
•Structures like steel bridges and overhead bridges andbuilt with gaps to allow for expansion . Sometimes,one end is supported by rollers which allow the bridgesto expand easily when heated. •Rollers of steel bridge
•Electric transmission cable and cable cars’ cable sagon hot day and tighten during a cold night. Therefore,allowances have to be made for the expansion andcontraction 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 metalslightly together.b.They are heated before being inserted through twopieces of metal.c.While the rivet is still hot, the end is hammered flat.d.When the rivet cools, it contracts, pulling the twopieces of metal tightly together.
A very tight bottle cap can removed easily by immersingit in hot water. The cap expand much faster than thebottle and so, the cap can be removed easily when itexpands.The hub of a wheel us usually slightly smaller than thesize 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) tocool it so that it contracts until it can be fitted into thehub.
•Metallic tiresThe metallic wheels of a train are fitted with metaltires. To ensure a tight fit, the tire is slightly smaller indiameter than the wheel. Before fitting, the tire isheated uniformly and the resulting expansion enablesthe tire to be slipped over the wheel. Upon cooling,the steel tire contracts and make a tight fit.
•All objects can absorb and give out (radiate) heat. Butsome objects absorb or give out heat better thanothers.•The ability of an object to absorb and give out heatdepends on the type and color of its surface (dull orshiny and light colored or dark- colored)•The rate of which heat is absorbed and given out alsodepends on the surrounding temperature of anobject.
•Most of the buildings and houses are painted with whiteor brightly colored paint so that less heat is absorbed andthe interior can be kept cool. White and bright surfacesare good reflectors of heat and poor absorbers of heat.•The oil tank of an oil tanker is painted silver so that itbecomes good reflectors of heat. This is safer as less heatwill be absorbed by the petrol inside the tank. Oil storagetanks are also painted with shiny aluminium paint toreflect the radiated heat away so as to keep the contents(oil cool).
•The base of the heating utensil or a pot is usually dulland black. This makes it good absorber of heat. Kitchen utensils
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