Chapter 11Chapter 11

HeatHeat

HeatHeat

Heat is transferred thermal (internal) Heat is transferred thermal (internal) energy resulting in a temperature energy resulting in a temperature change.change.

Heat is similar to Work, which is a Heat is similar to Work, which is a transfer of mechanical energy.transfer of mechanical energy.

Heat is measured in Joules or calories Heat is measured in Joules or calories or kilocalories.or kilocalories.

calories and kilocaloriescalories and kilocalories 1 calorie is the amount of energy needed to 1 calorie is the amount of energy needed to

raise the temperature of 1g of H2O by 1 raise the temperature of 1g of H2O by 1 degree C.degree C.

1 kcal is the amount of energy needed to 1 kcal is the amount of energy needed to raise the temperature of 1 kg of H2O by 1 ºC.raise the temperature of 1 kg of H2O by 1 ºC.

Food energy is measured in Food energy is measured in CCalories (kilo alories (kilo calories)calories)

1 cal = 4.18 J1 cal = 4.18 J 1 kcal = 4180J1 kcal = 4180J

ExampleExample

If you eat a slice of cake with an If you eat a slice of cake with an energy value of 400 Calories, how energy value of 400 Calories, how long will you have to ride your bike long will you have to ride your bike in order to work this off? Assume in order to work this off? Assume the body’s power output while riding the body’s power output while riding is 200 Watts.is 200 Watts.

Specific HeatSpecific Heat Heat, Q, is transferred energy and is measured Heat, Q, is transferred energy and is measured

in [Joules] or [kcal].in [Joules] or [kcal].

Different materials require different amounts Different materials require different amounts of heat to change the temperature by one of heat to change the temperature by one degree. degree.

c = Q/mc = Q/mДДT = specific heatT = specific heat

Specific heat give the heat required to change Specific heat give the heat required to change the temperature of one kg by one degree the temperature of one kg by one degree CelsiusCelsius

Common Specific HeatsCommon Specific Heats

AluminumAluminum 920 J/kgºC920 J/kgºC 0.220 0.220 cal/gºCcal/gºC

CopperCopper 390 J/kgºC390 J/kgºC 0.0932 0.0932 cal/gºCcal/gºC

WaterWater 4186 J/kgºC 1.00 cal/gºC 4186 J/kgºC 1.00 cal/gºC

See Table 11.1 page 370 for other specific See Table 11.1 page 370 for other specific heatsheats

Calculating Heat Calculating Heat TransferTransfer

Q = mcQ = mcДДT T

gives the heat transferred as a mass, gives the heat transferred as a mass, m, undergoes a temperature change m, undergoes a temperature change ДДT.T.

ExampleExample

At the birthday party, another At the birthday party, another student ate the same slice of cake student ate the same slice of cake (400 Cal) but reasoned that if she (400 Cal) but reasoned that if she drank enough ice water at 0ºC, the drank enough ice water at 0ºC, the Calories would be used in warming Calories would be used in warming up the ice water. How much ice up the ice water. How much ice water would she need to drink?water would she need to drink?

CalorimetryCalorimetry Calorimetry is a technique for Calorimetry is a technique for

measuring heat exchanges by measuring heat exchanges by minimizing heat transfer with the minimizing heat transfer with the environment. environment.

In an isolated container in which no heat In an isolated container in which no heat is exchanged with the environment, the is exchanged with the environment, the total heat lost by an object is equal to total heat lost by an object is equal to the total heat gained by another.the total heat gained by another.

ΣΣ Q = 0 where Q = mc Q = 0 where Q = mcДДTT

Phase ChangesPhase Changes

Melting - solid to liquidMelting - solid to liquid Freezing – liquid to solidFreezing – liquid to solid Boiling – liquid to gasBoiling – liquid to gas Condensing – gas to liquidCondensing – gas to liquid

Sublimation – solid to gasSublimation – solid to gas

Latent HeatLatent Heat Latent Heat is the heat required by a mass to change Latent Heat is the heat required by a mass to change

phase. During a phase change, temperature remains phase. During a phase change, temperature remains constant in spite of heat being added (or removed). constant in spite of heat being added (or removed). The added heat goes into breaking bonds instead of The added heat goes into breaking bonds instead of changing temperature.changing temperature.

LLff –Heat of Fusion – solid to liquid phase change –Heat of Fusion – solid to liquid phase change [J/kg][J/kg]

Lv – Heat of Vaporization – liquid to gas change Lv – Heat of Vaporization – liquid to gas change [J/kg][J/kg]

Q = mL heat required to change phaseQ = mL heat required to change phase

Latent HeatLatent Heat

For Water:For Water:

LLff = 3.33 X 10 = 3.33 X 10 55 J/kg J/kg

LLvv = 22.6 X 10 = 22.6 X 1055 J/kg J/kg

ExampleExample

Heat is added to 2kg of ice at -10ºC. Heat is added to 2kg of ice at -10ºC. How much heat is needed to How much heat is needed to generate steam at 110º C??generate steam at 110º C??

Phase Change Diagram for Phase Change Diagram for 1kg H2O1kg H2O

Phase Change DiagramsPhase Change Diagrams

Phase Change DiagramsPhase Change Diagrams

ExampleExample

A liver for transplant is packed in ice A liver for transplant is packed in ice in an insulated container. Liver has in an insulated container. Liver has mass 0.50kg and is 29º C when mass 0.50kg and is 29º C when packed. It is surrounded by 2.0kg of packed. It is surrounded by 2.0kg of ice at -10º C. If the specific heat of ice at -10º C. If the specific heat of liver is cliver is cLL = 3500 J/kgºC, find the = 3500 J/kgºC, find the equilibrium temperature of the ice equilibrium temperature of the ice and liver.and liver.

Heat TransferHeat Transfer

ConductionConduction – heat transfer by direct – heat transfer by direct contact; energy is transferred by contact; energy is transferred by molecular collisions.molecular collisions.

ConvectionConvection – heat is transferred – heat is transferred indirectly via currents set up in a fluid; indirectly via currents set up in a fluid; energy is transferred via molecular energy is transferred via molecular collisions.collisions.

RadiationRadiation – transfer of heat involving NO – transfer of heat involving NO mass but instead electromagnetic energymass but instead electromagnetic energy