thermal concepts notes

8/13/2019 Thermal Concepts Notes

1/12

Topics 3.1 & 3.2.9-3.2.12

Chapters 13.1-3,11; 14.1-2,7-9


2/12

At a microscopic level, temperature is regarded as a

measure of the average kinetic energy per molecule

associated with its movement in the substance

Temperature

Internal Energy Materials have internal energy (U)

Internal energy of a body is the total energy associated with

the thermal motions of the particles

Can be comprised of both Ekand Epassociated with

particle motion

Ekcomes from the translational and rotational motion

of molecules

Epcomes from the forces between the molecules

(bonding, intermolecular forces, etc.)


3/12

The term thermal energy or heat represents non-

mechanical energytransfer

between a system andits surroundings due to a temperature difference

Occurs from higher to lower temperature regions

Incorrect to refer to the thermal energy of/in an

object

Occurs between TWO objects

Heat is the flow of thermal energy from one object

to another Increases the internal energy of the receiver

Decreases the internal energy of the donor

Thermal Energy/Heat


4/12

Heat can be transferred from one body to another

by: Conduction (contact)

Convection (through a fluid)

Radiation (from electromagnetic radiation, photons)

Methods of Heat Transfer


5/12

Conduction An exchange of energy between microscopic particles by

collisions Less energetic particles gain energy during collisions with

more energetic particles

Rate of conduction depends upon the characteristics of the

substance In general, metals are good conductors

Contain large numbers of electrons that are relatively

free to move through the metal

Transport energy from one region to another

Conduction can occur only if there is a difference in

temperature between two parts of the conducting medium


6/12

When two bodies are placed in contact, heat flows from the

warmer body to the colder body until the two objects reach

the same temperature

At thermal equilibrium, NET energy flow is zero

Energy flow has not stopped

Thermal Equilibrium

Thermometers Devices used to measure the temperature of an object or a

system

Make use of physical properties that change withtemperature

Many physical properties can be used


7/12

Thermometers can be calibrated by placing them in thermal contact

with an environment that remains at constant temperature

Celsius Scale

The upper fixed point is the boiling point of pure water at

atmospheric pressure (set to 100 oC)

The lower fixed point is the melting point of pure water at

atmospheric pressure (set to 0 oC) Scale between them was divided by 100 to give individual degrees

Kelvin Scale

Based on atomci level (no negative Kelvin temperature)

When the pressure of a gas goes to zero, its temperature is273.15 oC This temperature is called absolute zero

Absloute zero corresponds to no motion at the atomic level

To convert: TK = TC+ 273.15

Temperature Scales


8/12

Amount of a substance which contains the same

number of elementary entities as there are in 12grams of carbon-12

1mole = 6.02x1023particles/molecules/atoms/ions

Denoted By NAand called Avogadro constant

The Mole

Molar MassMolar mass is the mass of 1 mole of the substance

SI unit: kg mol-1


9/12

Concept that matter is made up of atoms which are

in continual random motion is called kinetic theoryThis leads to the idea of pressure

Particles collide with the walls of the container

Particles change their momentum and exert a force

on the wall

This creates pressure

Collisions of the molecules with the container (not

with each other) create the pressure

Kinetic Theory


10/12

Ideal gas is a theoretical gas that obeys the gas laws and

thus fit the ideal gas equation exactly Real gases conform to the gas laws under certain limited

conditions

But they condense to liquids and then solidify if the

temperature is lowered Furthermore, there are relatively small forces of

attraction between particles of a real gas

Ideal Gas vs. Real Gas


11/12

To make a real gas obey ideal gas laws, the following

assumptions are made about he molecules in a gas:

1. Gases consist of a very large number of tiny particles

called atoms or molecules moving in random directions

with a variety of speeds

2. On average, molecules are far apart from one another(much larger than their diameter) so that the range of

intermolecular forces is small compared with the distance

between them

3. No forces act between particles except during collisions4. Molecules are assumed to obey Newtons Laws and are

assumed to interact only when they collide

5. All collisions are assumed to be perfectly elastic

Kinetic Model of Ideal Gas


12/12

The kinetic theory relates the macroscopic behavior

of an ideal gas to the microscopic behavior of itsmolecules or atoms

HOMEWORK:

1. How are pressure and temperature related? Explain

using kinetic theory.

2. How are pressure and volume be related? Explain

using kinetic theory.3. How are volume and temperature related? Explain

using kinetic theory.

Kinetic Theory

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