topic 3: thermal physics

7/24/2019 Topic 3: Thermal Physics

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Topic 3: Thermal physics

3.1 – Thermal concepts

Molecular theory of solids, liquids and gases

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Molecules are held together by intermolecular forces.The fourth state of plasma is not required to be learnt.

Temperature and absolute temperatureTemperatures describe how hot or cold an ob(ect is and determines the direction ofheat flow between two bodies.Thermal energy transfers from an ob(ect with higher temperature to another ob(ectwith lower temperature. The energy transfer is called heat )the flow of energy due totemperature difference*. +b(ects which are in thermal equilibrium hae the sametemperature.

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Temperatures in 0elin can be calculated by deducting the temperatures in 1elsius



by 2$3.!.

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The absolute temperature of a body in the 0elin scale is directly proportional to the

aerage inetic energy per molecule inside the body. bsolute 4ero is "0 or -2$3 degrees 1elsius.Temperatures cannot be lower than absolute 4ero. 5t is the temperature whereparticles hae 4ero aerage inetic energy )no random motion*.

5nternal energy5nternal energy is the sum of total inetic energy )total thermal energy* and totalpotential energy.0inetic energy is energy associated with the random/translational rotational motionsof molecules.6otential energy is associated with forces between molecules.

7pecific heat capacityThe specific heat capacity of a substance is gien by

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school.net/Malaysia/8niersityand1ollege/76M/reisioncard/physics/heat/images/heatcapacityformula.pngand is defined by the amount of heat needed to raise the temperature of g of thesubstance by 0.9ifferent substances hae different specific heat capacities because of differentdensities and physical properties.

'5The thermal capacity of an ob(ect is gien by ;<c= )or ;<c>T* and is defined by theamount of heat needed to raise an ob(ect?s temperature by 0.

6hase change

1hange of phase 6rocess 0inetic energy 6otential energy

7olid to liquid Melting 8nchanged 5ncreases

@iquid to solid 'ree4ing 8nchanged 9ecreases

@iquid to gas %oiling 8nchanged 5ncreases

Aas to liquid 1ondensation 8nchanged 9ecreases

9uring a phase change, temperature and inetic energy remain constant andpotential energy changes )which increases as molecules spread out and ice ersa*.

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Boiling: +nly occurs at the boiling point )a specific temperature depending onsubstance* and occurs throughout the liquid.Evaporation: 1an occur at any temperature and only occurs at the surface of the



liquid. The fastest moing particles eaporate, decreasing the aerage 0B of theremaining particles. s a result, temperature drops.

7pecific latent heatThe specific latent heat of a substance is gien by

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Specific latent heat of fusion: The amount of heat required to change g of asubstance from solid to liquid without any change in temperature.Specific latent heat of vaporization: The amount of heat required to change g ofa substance from liquid to gas without any change in temperature.

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3.2 – Modelling a gas

6ressure

6ressure is defined as the normal force per unit area and is gien by the equation

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Bquation of state for an ideal gas



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0inetic model of an ideal gas ssumptions: The collisions between molecules are perfectly elastic.

The molecules are identical spheres.

The olume of molecules is negligible compared to the olume of the gas. Molecules do not interact with each other ecept when they are in constant.

5mplications: bsolute temperature is directly proportional to the aerage 0B and aerage speedof the molecules of an ideal gas.

Mole, molar mass and the ogadro constantMole: @ie the word Gdo4onG, a mole is a unit of quantity. 5t is used to measure the

number of atoms or molecules.

mole of any material contains ."22H"I23 atoms or molecules. The alue."22H"I23 is called the ogadro constant.

The number of moles of a substance can be calculated by diiding the number

of molecules of that substance by the ogadro constant.

Molar Mass: The molar mass is the mass of mole of any element or compound.

9ifferent materials/elements hae different molar masses which can be found as

the Mr in the periodic table.

vogadro constant:."22H"I23

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9ifferences between real and ideal gasesThe ideal gas is based on a list of assumptions stated preiously. Coweer, in realgases, such assumptions may not be true. 'orces eist between gas molecules in real gases )intermolecular forces*.

The olume of molecules is not negligible compared to the olume of gas in real

gases.

Deal gases may behae similarly to ideal gases under high temperatures and lowpressure.

topic 3: thermal physics

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