thermal concepts

8/11/2019 Thermal Concepts

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Topic 3: Thermal Physics

3.1 Thermal concepts

This chapter is an introduction to thermal physics. Itintroduces the concepts of temperature, heat, internal

energy and thermal equilibrium.

You should:

understand the concept of thermal equilibrium;relate the Kelvin and Celsius scales of temperature;know that internal energy is the total kinetic energy of the molecules ofa system plus the potential energy associated with the molecular forces.


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Temperature

Intuitively: concept of hotness or coldness of asubstance with respect to something else.

To measure the temperature of a body weneed to find a property of the body thatchanges as the hotness changes.

In 1742, Andreas Celsius created the

temperature scale that is known by his name.

0C Freezing point of water100C Boiling point of water


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Thermal Equilibrium

Two or more bodies are in thermalequilibrium when they have thesame temperature.

Body A and body B are each inthermal equilibrium with body C.Therefore they are in thermalequilibrium with each other andhence have the same temperature(Zeroth Law of Thermodynamics).


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Absolute temperature scale

0 K = -273.15 C or 0C = 273.15 K

T = TC + 273.15

TC= T -273.15

For practical purposes:

T = TC + 273

TC= T -273

The temperature scale used in Physics isthe absolute temperature scale or Kelvinscale. Its unit is the kelvin(K)

Temperature has a lower limit0 K or absolute zero


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Heat As Energy

Heat is energy that is transferred from onebody and into another as a result of adifference of temperature.

Thus, when a hot object is brought in contactwith a colder body, heat will be transferred tothe colder body and increase its temperature.We say that the colder body has beenheated.

Heat and work, unlike temperature, pressure, and volume, are notintrinsic properties of a system.


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Internal Energy

All substances consist of molecules inconstant motion. They therefore havekinetic energy.

In addition, there are forcesbetween molecules

(electrical in nature).Increasing the averageseparation of twomolecules requieres workto be done. This work goesinto potential energy

associated withintermolecular forces.

Internal energy is the total kinetic energy of

the molecules of a sustance, plus anypotential energy associated with forcesbetween the molecules.

The heat that is transferred from a hot to a cold body increases the internal

energy of the cold body (and decreases the internal energy of the hot bodyby the same amount


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Temperature, again

The absolute temperature is a measure of the average kineticenergy of the molecules of a substance. The average kineticenergy of the molecules is directly proportional to the absolutetemperature in kelvin.

We therefore have a relationship between amicroscopic concept and a macroscopic concept.


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A hot body is brought into contact with a colder body until

their temperatures are the same. Assume that no otherbodies are around. Is the heat lost by one body equal to theheat gained by the other? Is the temperature drop of onebody equal to the temperature increase by the other?

A body at a given uniform temperature of 300 K and internalenergy 8 x 106J is split into two equal halves.

a) Has any heat been exchanged?b) What is the temperature of each half?c) What is the internal energy of each half?


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The giant hornet Vespamandarinia japonica preys onJapanese bees. However, if oneof the hornets attempts toinvade a bee hive, severalhundred of the bees quickly form

a compact ball around thehornet to stop it. After 20minutes the hornet is dead,although the bees do not sting,bite, crush, or suffocate it.

Why, then, does the hornet die?


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Confusion around the concept

of thermal energy


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Heat transfer (conduction)

Thermal conductionis the process by which atemperature difference causes the transfer o thermalenergy from the hotter region of the body to thecolder region by particle collision without there beingany net movement of the substance itself.

Conduction can occur in solids,liquids and gases.

Gases: Due to the collision between fast and slow moving particles wherekinetic energy is tranfered from the fast to the slow particle.

Liquids: Due to increased vibrational energy. Because the majority of theparticles are coupled to other particles they also begin to vibrate moreenergetically.

Solids: Two ways. Similarly to liquids or by mobile electrons.


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Heat transfer (convection)

Thermal convectionis the process in which a temperature difference causesthe mass movement of fluid particles from areas of high thermal energy toareas of low thermal energy (the colder region).

Liquids and gases can transfer heatreadily by convection.


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The mole, molar mass and

Avogadrosnumber

The mass of an atom is exceedingly small. Forexample, the isotope carbon-12 is 1.99 x10-23g.

In 1961 the International Union of Pure and

Applied Chemistry defined the masses ofatoms relative to carbon-12 that wasassigned a value of 12.0. Therefore, therelative atomic mass is defined as the massof an atom when compared with 1/12 themass of carbon-12 atom.

Mass of carbon-12 = 12.000 uMass of oxygen = 16.000 u = 16/12 Mass of carbon-12


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Mole

The SI fundamental unit for the amount of a substance is themole (mol).

The mole is the amount of substance that contains as many particles(atoms, molecules) as there are in 12 g of carbon-12

Avogadro found that equal volumes of gasesat the same temperature and pressurecontained the same number of particles. Onemole of any gas contains the Avogadronumber of particles NA=6.02 x 10

23, and it

occupies 22.4 dm3 at 0C and 101.3kPapressure (STP).

Molar mass of carbon-12 = 12 g/molMolar mass of oxygen = 16 g/ mol


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Molar mass

M = m/n

Where M is the molar mass (g/mol) , m isthe mass (g), and n is the amount ofsubstance in moles.

n = N/NAWhere N is the total number ofparticles

Important note: When using the mole, the atoms or molecules should be clearlystipulated. For example, one mole of copper atoms contains 6.02 x 1023cooperatoms. One mole of nitrogen molecules (N2) contains 6.02 x 10

23of nitrogenmolecules and 12.04 x 1023nitrogen atoms.


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Example

1. Calculate the number of moles of oxygen molecules contained in 64 gof oxygen gas, O2.

2. Calculate the number of oxygen molecules in part 1 of this example.

3. Determine the volume of oxygen gas that would be present at STP.

4. Calculate the mass in 0.75 mol of carbon dioxide gas.

thermal concepts

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