THERMAL EXPANSION OF SOLIDS AND LIQUIDS
DEMOSTRATIONS OF SOLID EXPANSION
The size of most solids is altered when their temperatures change.Solids EXPAND when heated,and CONTRACT when cooled.This implies that temperature depends on the kinetic energy and movement of molecules.As a substance gets hotter,its molecules vibrate faster and take up more room inside the substance.Thus it expand.
The rate of expansion is different for different substances.
For example an iron bar 1m long will increase in length by about 1mm if heated through a temperature change of 100°C,but brass bar will increase by about 1.5mm over the same range of temperature.
The Following Experiments Demostrate Expansion In Solids.
1.BALL AND RING EXPERIMENT : The apparatus consist of a metal ring through which a metal ball just fits.When the ball is strongly heated for several minutes it is found that the ball does not pass through the ring,the ring becomes heated and expands while the ball cools and contracts.When ball and ring reach the same temperature the ball will pass through the ring again.
2.STEEL ROD EXPERIMENT : A steel rod is clamped at one end and allowed to roll on a needle near the other end.Attached to the rod is a pointer.When the rod is heated,the pointer is observed to move to the right,showing that the rod is expanding.When the flame is removed and the rod allowed to cool,it is noticed that the pointer returns to its original position,showing that the rod has contracted to its original size.
Effects Of Expansion And Their Applications
1.In hot weather you can hear creaking noises if you are under a roof made of galvanised iron sheets.This due to the iron expanding as it gets hot.As the temperature come down in the evening,the creaking noise is reapted as the sheet contract.
2.A metal structure such as a bridge expands when heated.Such expansion has to be allowed for during the design of a bridge so that the structure does not fracture under the action of large force resulting from expansion.In order to allowe for the expansion of the structure of a bridge,one end of the bridge is supported on rollers while the other end is fixed under this arrangement the metal structure is free to expand or contract at various temperature without damaging the bridge.
3.To allow for expansion,gaps are left between sections of rail on a railway track.The ends are held in line with fishplates,which are strips of metal bolted to the ends of the rails by slotted holes.
4.If hot water is poured into a thick glass bottle or tumbler,it is liable to crack.This is due to the uneven expansion of glass.The glass is a poor conductor of heat.As a result,the part of the glass nearer the hot water,the inside,expands more than that part on the outside.The stress causes the glass to crack.
LINEAR EXPANSIVITY
The linear expansity of a metal is defined as the increase in length per unit length per degree rise in temperature.
a =α l2-l1 / l1Φ per K
where α = linear expansivity
L2 = length of metal at temperature Φ2
L1 = length of metal at temperature Φ1
Φ = temperature rise,Φ2 - Φ1
CUBIC EXPANSIVITY OF A LIQUID : the real (or absolute) cubic expansivity of a liquid is defined as the increase in volume per unit volume per unit rise in temperature.
It is denoted by γ.
THE APPARENT CUBIC EXPANSIVITY OF A LIQUID : is defined as the increase in volume per unit volume per unit rise in temperature when the liquid is heated in an expansible vessel.
It is denote by γa
γP = γa + γ
The formula used to calculate the amount of volume expansion a solid experiences when there is a change in temperature is ΔV = βVΔT β is the coefficient of volume expansion (m-3) For solids, β is approximately equal to 3α.
temperature is ΔL = αLΔT α is the coefficient of linear expansion (m-1) L is the original length (m) ΔL is the change in length (m) ΔT is the change in temperature When ΔT is positive, ΔL is positive. The new length L' = L + ΔL.
