Chapter 12
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The document discusses how solids, liquids, and gases expand when heated and contract when cooled. It specifically mentions how expansion can cause problems for roads, bridges, and other structures if not properly accounted for through the use of expansion gaps. It then discusses how the principle of unequal expansion in bimetallic strips is used in applications like thermostats, thermometers, and fire alarms to regulate temperature and detect fires.
Chapter 12
- 1. - When heated , solids , liquids and gas expand ( expansion) - When cooled , solids , liquids and gas contract ( contraction) - Some solids expand so little that we may not notice the expansion Eg. 1 metre brass rod increase 1mm when heated and temperature increase by 100°c - When cooking soup / boiling water , do not fill pot/kettle to brim because water expand when heated and it will spill out - Pg173 ( bottom ) - Holes in slice of bread because when a loaf of bread is baked , thousands of tiny air bubbles are trapped in the dough . When dough heated , air bubbles gain heat and expand , creating holes that causes the bread to rise . - When body expands / contracts , large forces may be set up and it may help/hinder us . - Pg175 ( top – experiment 5 ) - Large forces given rise and exerted from expansion . Problems caused by expansion Category Problem Solution Examples Roads and Excessive Tiles on 1.Tiles pop out Pathways expansion on very pavements and due to in sufficient hot days cause walls are laid with expansion gaps roads / pavements expansion gaps 2. Expansion gaps to crack , tiles to help prevent tiles pop out from getting damaged MRT and railway Railway tracks Gaps made in Distorted tracks on tracks warp due to tracks to allow abandoned expansion. expansion. railway due to expansion caused by heat . Bridges and metal Long bridges Expansion gaps or Eg. Golden gate girders expand up to a a free end resting bridge , Benjamin metre in hot on rollers to allow Sheares Bridge in weather and expansion / Singapore without contraction allowance , without damage structures are at risk of damage Water pipes Water pipes/oil Expansion bends See pg177 ( top – pipelines expand made to allow graphic ) and burst expansion Telephone wires Raised on poles Strung loosely Transmission wires and cables contract in cold from one pole to strung loosely . weather and another pole for become taut and contraction may snap
- 2. Laboratory Boiling water Glassware at -NIL- glassware poured into thick home and lab glass , glass crack made of special due to uneven glass unlikely to expansion crack when hot liquids are poured in due to little expansion when heated . Warp : to turn /twist/bent out of shape. Uses of expansion ( pg. 178-179) - Metal tyres : Metal tyre heated to fit over wooden wheel , tyre cools , clings on very tightly to rim of wheel - Rivets: Join steel plates and girders When red-hot , the rivet is put in position then hammered into a head and cooled, contraction on cooling pulls steel plates together - Bimetallic strip Rivet a strip of brass and strip of steel together , it is bimetallic strip Brass on top , steel below , rivets fasten both Brass expands more than steel when heated and strip bends with brass on outside curve , when cooled brass contracts more than steel so strip bends with steel on outside curve Uses of bimetallic strips - Thermostat Bimetallic strips used in thermostat, used to keep temp. constant Seen in ovens, refrigerators to keep temp. either hot / cold Pg. 180 ( top ) - Bimetallic thermometer Can be made into a long , nearly flat coil One end fixed and other end has a pointer Coil tightens or unwinds as the temperature goes up and down and pointer move over scale marked in °C temp. degrees Uses uneven expansion to measure temp. Other thermometers pg.181 (top) - Fire alarm Brass over iron and when fire occurs , heat cause brass to expand and bend in towards iron and close circuit enables current to flow and activate fire alarm Pg. 181 ( bottom )
- 3. Questions 1. What substances expand the most when heated? Ans: 2. What is bimetallic strip made of? Ans: Brass and iron. 3. Pg. 183 (qn 7) Ans: