Chapter 11

Heat

Chapter 11 Objectives

• Define heat

• Identify specific heat

• Differentiate between types of latent heat

• Identify types of heat transfer

• Account for insulation

Heat• Recall that heat is defined as the amount of energy that is

transferred between a system and its environment.• A common unit of measurement for heat is the calorie (cal).

– The heat required to raise one gram of water 1 oC

• A related unit is the kilocalorie, or 1000 calories.– The Calorie (with a capital C) is equivalent to 1 kilocalorie.

• This is the Calorie that you hear of when talking of food.

• Another common unit of measurement for heat is the British Thermal Unit (BTU).– The heat required to raise 1 lb of water 1 oF

• Common unit of heat rating for furnaces and other appliances

• The SI base unit for heat is the same as energy– Joule (J)

Specific Heat

• The specific heat of a substance is defined as the amount of heat requires to raise 1 kg of it by 1 oC.

• This varies for different materials.• Can be measured in J/kg•oC or cal/g•oC

Q = mcTHeat

Specific Heat

Tempertaure (oC)

Phase Change

• While an object is transferring heat without changing phase, then you must use the equation involving specific heat.

• An object will also transfer heat and change phase.• This is called latent heat.

– Also known as hidden heat because this occurs where there is no temperature change.

• Units are J/kg

• There are two types of latent heat– Heat of Fusion– Heat of Vaporization

Fusion vs Vaporization

• Heat of Fusion is the heat transferred during a phase change between solid and liquid.

• Denoted by Lf

• Heat of Vaporization is the heat transferred during a phase change between liquid and gas.

• Denoted by Lv

Q = mLLatent Heat

Heating Curve for H2O

solid

liquid

gas

heatof

fusion

heatof

vaporization

use specific heat whilesubstance remains in

same phaseQ = mcT

use latent heat whilesubstance changes

phaseQ = mL

Types of Heat Transfer

• There are three types of heat transfer– Remember heat is not transferred between a system and its

surroundings when they are at the same temperature.

• Conduction– Heat transferred as a result of direct contact.

• Convection– Heat transferred as a result of the movement of a heated substance

past another substance.

• Radiation– Heat transferred as a result of energy given off from a substance

without contact or movement of any molecules.

Conduction

• An example of conduction would be ice cubes in a glass of water.

• We can classify the rate at which that heat is conducted called the heat transfer rate, H.– Often called a heat current.

QtH =

Convection

• Convection can occur in two ways– Natural convection

• Movement of molecules as a result in different densities of the substances.

– Ex: Hot air rises

– Force convection• When the heated substance is forced to move by a

fan or pump.– Ex: Furnace systems

Radiation

• Radiation is the heat felt by placing your hands next to a fire.– On top of the fire would be convection!

• Example of this would be how the sun heats the Earth.– Also what you experience by cooking in a

microwave.