chapter 17
17.1 Expansion and Contraction of matter
in liquid
in gases
in solids
e.g. when we pour boiling water too quickly into a cup made of thick glass, the glass sometimes cracks. The solid gases on the inside expands more quickly than the outside when heated. For this reason, the walls of glass containers for hot liquids are usually thin.
e.g. liquid mercury in clinical thermometers responds quickly to heat. The mercury level changes when the liquid is heated or cooled. This occurs when the volume of the mercury changes.
e.g. when baking a loaf of bread, the bread dough rises. This is partly because the air bubbles trapped in the dough expand when heated.
17.2 Effects of Expansion and Contraction
telephone wires and electricity cables
laboratory glassware
tiled and concrete pavements
water pipes
railway tracks
bridges and metal girders
railway tracks can bend and twist due to expansion in hot weather. Hence, expansion gaps in the railway tracks provide space for the tracks to expand without any damage on hot days
excessive expansion on very hot days can cause concrete to crack and pop out. Thus, expansion gaps are laid between concrete slabs and tiles.
contract on cold weather, hence may taut and snap. Thus, the wires and cables are strung loosely from pole to pole
if boiling water is poured into a thick glass, the glass is likely to crack due to the uneven expansion of the glass. Hence, most of the laboratory glassware are made of a special type of glass which expands very little when heated.
water pipes expand on hot days, causing them to burst. Hence, expansion bends are made in this pipes, allowing the pipes to expand without damage
may expand up to a metre in hot weather, hence, these bridges need to be built with allowance for expansion. Thus, they have expansion gaps or a free end resting on rollers, allowing the metal structures to expand without any damage on hot days.
17.3 Movement of heat
coduction
convection
radiation
process of heat transfer through a medium or material without any movement of the medium or material.
uses of heat conductors
radiators
keep rooms warm during the winter. Radiators have to quickly conduct heat away from the heat source before they can warm the air around them. Hence, they are made of metal.
cooking utensils
minor's safety lamp
boilers
conduct heat quickly from fire to food or water in them.
made of metal. The metal gauze conducts heat away quickly from a naked flame inside it, so that flammable gases outside never get hot enough to catch fire
made of metal and conducts heat quickly from the heat source to the water, which is converted to steam.
transfer of heat from from one place to another by the movement of fluids (liquids and gases)
uses of heat insulators
walls of buildings are made of bricks and other insulating materials. In warm climate, the insulating materials help to reduce heat gain by conduction. In very cold climates, the insulating materials also help to reduce heat loss by conduction.
handles of cooking utensils do not conduct heat easily. This allows a person to hold the utensils comfortably while cooking.
liquids
gases
when rice is boiled in water
rice grains placed into a pot of water.. As the water is heated, it expands and becomes less dense. This causes the rice grains to rise towards the surface of the water. The cooler water at the surface is denser and sinks with the rice grains.
in coal mines
in the past, coal mines were ventilated by vertical shifts sunk into the mines. These were joined by horizontal passages. A fire is lit at the bottom of one shaft. This cause hot, less dense air to rise from the shaft and cold, denser air to enter the other shafts.
applications of convection
electric kettle : the heating coil of an electric kettle is place near the base of the kettle. This makes boiling more efficient as water becomes less dense and rises when it heated. This allows convection currents to be set up easily during heating
land and sea breezes : the land absorbs the heat from the sun and warms up faster than the sea. The air above the land is heated up and rises. Cooler, denser air above the sea moves in to replace the warm air above the land.
the transfer of heat, in the form of waves, from one place from another. It does not require a medium for transmission.
factors affecting the rate of radiation
nature of the surface of the body (whether it is black or white or coloured, rough or smooth or shiny)
The darker and rougher the surface, the higher the rate of radiation or absorption.
total surface area of the body
the greater the area, the higher the rate of radiation or absorption
uses of good and bad radiators of heat
good
poor
the heating surfaces of electric stoves are black, and the heating coil has a large surface area
a house painted in light colours to keep it cool on a hot day. The bright surface of the house absorbs less heat radiation than a dark surface
the backs of refrigerators have hot pipes is a good radiator of heat. This helps the pipes to lose heat quickly to the surroundings.
the black pipes on solar water heaters are good absorbers of radiation from the sun. The water flowing in the pipes hence heat up more quickly.
light-coloured clothes are worn on hot, sunny days to keep us cool. The bright surface of the clothes is a poor absorber of heat radiation from the sun. This helps to keep us cool.
the brightly polished surfaces of cooking utensils, teapots and kettles are poor radiators of heat. This keeps their contents hot.