1. Heat energy is required to increase the temperature of a substance and increase its internal energy. The amount of heat needed depends on the substance, its mass, and the temperature change. 2. The heat capacity of an object is the heat needed to raise its temperature by 1K. The specific heat capacity is the heat needed to raise the temperature of 1kg of a substance by 1K. Substances with higher specific heat capacities require more energy to heat up or cool down. 3. Mechanical energy can be converted to heat energy. For example, the gravitational potential energy of falling water is converted to a small increase in temperature as friction produces heat.

1. Shahjahan notes:Heat energyCONCEPT
The jam ina sponge puddingstayshotterforlongerthanthe puddingaroundit
Water isa verygoodcoolantfor use inengines
Saucepansare made of materialswithalow massif possible
It takesa longtime to heatup enoughwatertohave a hotbath
Housesbuiltwiththickstone wallskeepwarminwinterandstaycool in summer
People usedtowarmupheavychinateapotsbefore puttingthe hotwaterandtea inthem
Storage radiatorsare usedto retainheatenergyforuse laterinthe day
You can put out a candle flame (temperature 800o
C) withmoistfingerswithoutpainbutputtingyour
handinto a bowl of boilingwater(100 o
C) wouldhurta lot!
Electroniccircuitsare builtwith"heatsinks"
Whenyourun "hot" waterintoa basinit coolsas soonas it touchesthe material of the basin
Liquidsodiumisusedasa coolantin some nuclearreactors
All these factsare relatedtoheat energyandto a quantity
knownas specificheatcapacityandwe can explainthemby
lookingmore closelyatheatenergy.
Overthe past fewcenturiesscientistshave putforwardsome
verystrange theoriesaboutthe nature of heat.One of these
was thatheat wassome sort of a fluidthatyouaddedto a
bodyto make it hotand took awayfroma bodyto cool it
down.
However,duringthe lastcenturytwomen,RumfordandJoule,bothproposedthe ideathatheatwas
relatedtoenergy.Whenheatenergypassesintoabodyitincreasesthe internal energyof the body.
Rumforddemonstratedthisinsome well-knownexperimentsincannon boringandJoule showedthat
the frictiongeneratedbetweenapaddle wheel andsome waterwouldheatupthe water.
We can summarise these resultsas:
To heatup a substance requiresenergy.Thisenergyincreasesthe internal energyof the substance by
increasingthe kineticenergyof itsmoleculesandsothe temperature of the substance rises
Heat capacity and specificheat capacity
The amount of heatenergyneededtochange the temperature of asubstance dependson:
(a) what the substance is;
(b) howmuch of it isbeingheated;
(c) what rise intemperature occurs.

2. Shahjahan notes:Heat energyCONCEPT
The heat energyneededtoraise the temperature of anobjectby1 K iscalledthe HEAT CAPACITY of the
object.
However,arather more useful quantityisthe heatenergyneededfor1 kg only.
The SPECIFIC HEAT CAPACITY of a substance isthe heatneededtoraise the temperature of 1kg of the
substance by1K (or by 1o
C).
Specificheatcapacityisgiventhe symbol c.The unitsfor c are J/(kgK) or J/(kgo
C).
The valuesforthe specificheatcapacitiesof some commonsubstancesare giveninthe followingtable:
Substance Specificheatcapacity(J/(kgK) Substance Specificheatcapacity(J/(kgK)
Water 4200 Aluminium 913
Cast iron 500 Brick 840
Copper 385 Concrete 880
Lead 126 Marble 880
Rememberthatsubstanceswithhighspecificheatcapacities
take a lotof heat energyandtherefore alongtime toheatup
and alsoa longtime to cool down.
One interestingeffectisthe wayinwhichthe landheatsup
quickerthanthe sea - the specificheatcapacityof seawateris
greaterthan that of the landand so more heat energyis
neededtoheatitup by the same amountas the land andso it
takeslonger.Italsotakeslongerto cool down.
The heat energyneededtoraise the temperature of mkg of a substance of specificheatcapacityc by a
certaintemperature difference isgivenbythe equation:
Heat energy= massx specificheatcapacityx temperature change

3. Shahjahan notes:Heat energyCONCEPT
If the objectcoolsthenitgivesoutheatenergyandif itheatsup ittakesin heatenergy.
Example problems
1. How much heatenergyisneededtoraise the temperature of 3kg of copperby 6 K?
(Specificheatcapacityof copper= 385 J/(kgK)
Heat energy= massx specificheatcapacityx temperature change =3 x 385x 6 = 6930J
2. What is the rise intemperature of 5 kg of waterif itis given84 000 J of heat energy?
Specificheatcapacityof water= 4200 J/(kgK).
Heat energyinput=84000 = 5x4200x temperature rise
Temperature rise =84000/[5x4200] = 4 K
3. How much heatis lost by3 kg of leadwhenitcoolsfrom 1000 o
C to 200 o
C?
Specifcheatcapacityof lead= 126 J/(kgK)
Heat energygivenout= 3x126x80 = 30240J
4. A heaterof 800W is use to heata 600 g cast iron cookerplate.
How longwill ittake toraise the temperature of the plate by200 o
C?
Specificheatcapacityof iron= 500 J/(kgK)
Heat energyneeded=0.6x500x200 = 60 000J
Time needed=60 000/800 = 75 s = 1 minute 15 s
Conversionofmechanical energyto heat
Whenan objectfallstothe ground,the potential energythat
it hadat the top isconvertedtokineticenergythatfinally
becomesheatenergy.
Assumingnolossof energytoany otherformswe can work
out the rise intemperature of waterfallingoverahigh
waterfall.
Example problem
Heightof waterfall 84 m.
Consideramass of waterm kg
Specificheatcapacityof water= 4200 J/(kgo
C).
Gravitational fieldof the Earth= 10 N/kg
Potential energylost=Heat energygained

4. Shahjahan notes:Heat energyCONCEPT
m x10 x 84 = m x 4200 x temperature rise
Temperature rise =[10 x 84]/4200 = 0.2o
C.
There mustbe noresidual kineticenergyof sprayandno soundmustbe made!(Clearlynottrue butit is
the bestwe can dowithoutmakingthe problemverydifficult).
Thisconversionof gravitational potential energyintoheatenergycanbe usedin the laboratoryto
measure the specificheatcapacityof leadshot.
Example problem
Lengthof cardboardtube = 1 m
Mass of leadshot= m kg
The tube is upendedtentimesgivingatotal heightfallenof 10m
Temperature rise 0.80 o
C
Heat energygainedbythe leadshot= potential energylostbythe leadshot
Expressedasa formula: m x specificheatcapacityx 0.8 =m x10x10
Therefore specificheatcapacityof lead= 10x10/0.8 = 125 J/(kgo
C)