Heat

Heat and TemperatureKinetic Molecular Theory –

Is the theory that matter is made up of atoms (smallest piece of matter) and that these atoms are in continuous random motion

Atoms may join together to form moleculesSolids usually maintain both their shape and their volumeLiquids maintain their volume, but not their shapeGases do not maintain their volume or their shape – they will expand to

fill a container of any sizeMolecular motion is randomMolecular motion is greatest in gases, less in liquids and least in solidsCollisions between atoms and molecules transfers energy between themMolecules possess kinetic energyMolecules in gases do not exert large forces on one another, unless they

are colliding

Thermal energy – is the average of the potential and kinetic energies possessed by atoms and molecules experiencing random motion

Energy that is stored inside matter, The total of the kinetic energy of its molecules

as it moves (colliding, spinning and vibrating) And their potential energy as they move closer

together and further apart

Heat – is the thermal energy transferred from one object to another due to differences in temperatureQ heat = E thermalHeat is measured in Joules (J)

• There is no direct method to measure heat – only indirect methods

Heat can be transferred by:Convection – it is the transfer of heat due to fluid

movement (mixing hot and cold water in a tub, air in an oven to cook food)

Conduction – it is the movement of heat through solids or between two solids that are touching (cooking on a stovetop, the pan on top of the stove. The stove heats the pan at the bottom of the pan, but it is not just the bottom of the pan that gets hot, the handle of the pan often gets hot also)

Radiation – this energy transfers as a wave, like an x-ray or gamma ray, or as thermal energy (microwave)

Temperature – is a measure of the average kinetic energy of the molecules of a substance (how hot or cold something is)It can be measures with a thermometer

Measuring the amount of thermal expansion or contraction that occurs within a given type of substance

The units are CelsiusKelvin Scale (absolute scale) sets 0K as absolute

zero (-273.15oC). Temperature increases on the scale are the same as the Celsius scale (1K = 1oC)Conversion K = oC + 273Absolute zero can never be reached – because by

definition absolute zero is the temperature at which all molecular motion ceases.

Thermal ExpansionMost substance expand when heated and

contracted when cooled. However, the amount of expansion or contraction varies, depending on the material

Experiments indicate that the change in the length L of almost all solids is (approx) directly proportional to the change in temperature T.

The change in length is also proportional to the original length of the object Lo. That is, for the same temperature change a 4m

long ion rod will increase twice as much in length as a 2m long ion rod

ThusL = Lo T

Where is the proportionality constant that is called the coefficient of linear expansion for the particular material and has the units (oC)-1 or (K)-1

It does vary slightly with temperature

This is important in the design of certain structures

Volume Expansion – the change in volume of a material which undergoes a temperature change is given by the relation

V = Vo T is the coefficient of volume expansion and has the units

(oC)-1 or (K)-1

This is key for gases – important to recognize any potential hazardous situation which could result in an increase in pressure in closed containers)

Note that for solids, is normally equal to approx 3 However this is not true for solids that are not

isotropic (isotropic – having the same properties in all directions)

Note that there is no linear expansion for liquids or gases since they do not have a fixed shape

Specific Heat Capacity and Latent HeatSpecific heat capacity (c)– is the quantity of

heat required to raise the temperature of a unit of mass of a substance by a unit change in temperature (or raise 1kg of water 1oC)

c = Q__ or Q = mcT mT

Q – change in heat content (Joules – J)m – is the mass (kg)c – is the specific heat capacity (J/kg oC)T – change in temperature (oC or K)

the specific heat capacity of a substance depends on its molecular structure and on its phase

Calorimetry – quantitative measure of heat exchange that takes place in a calorimeter. To determine the specific heat capacity of substances

Specific Latent Heat – of a substance is the quantity of heat energy required to change the state of a unit mass of substance (heat required for a change in phase)

EL = mlEL – heat transferred (Joules – J)m – mass of the substance (kg)l – latent heat of the particular process and substance

(J/kg)Specific Latent Heat of Fusion (lF)– is the quantity

of heat energy released when 1kg of a substance solidifies (fuses) without changing its temperature

Specific Latent Heat of Vaporization (lv) - is the quantity of heat energy needed to vaporize 1kg of a substance without changing its temperature

Water Has one of the largest specific latent heats of

fusion of all substancesIt exhibits a high specific heat capacity

compared to other liquidsIt exhibits anomalous behavior:

From 0oC to 4oC it contracts as heated. It also expands when it freezes. The expansion results in a decrease in density, allowing ice to float on water.

ThermodynamicsThermodynamics – is the name we give to the

study of processes in which every is transferred as heat and as workHeat – transfer of energy due to a difference in

temperatureWork – is a transfer of energy that is not due to a

temperature differenceSystem – is any object or set of objects that we

which to considerClosed system – no mass enters or leavesOpen system – mass may enter or leave

A transfer of heat energy occurs when substances having different temperatures are allowed to mix

First Law of Thermodynamics – law of conservation of energy (in any transformation of energy the total amount of energy remains constant)

Second Law of Thermodynamics – Principle of heat exchange (when 2 substances of different temperatures mix, heat travels from the hotter substance to the colder one)Heat energy lost = heat energy gained EH (lost) = EH (Gained)To look at heat exchange it must occur under

controlled conditions – calorimeter to look at:Qualitative properties – look, texture, etcQuantitative properties - measurements

Third Law of Thermodynamics – Entropy (part of the system that can increase or stay the same – the amount of randomness)

Heat EnginesIs a device which converts heat energy into

mechanical workRemember that no devise transfers energy

completely into work – for that reason it is impossible to build a perfect heat engine

Efficiency can be calculated as followsEfficiency = useful energy output x 100% Energy input

Heat Pump (or refrigerator) - Just the reverse of a heat engine – requires an application of work to transfer heat energy form a low temperature to a high temperature