Thermal PhysicsThermal Physics

12/20/0712/20/07

Outline

Topic 3 Thermal physics [11hr]

• 3.1 Thermal concepts

• 3.2 Thermal properties of matter

• 3.3 Ideal gases (covered in next PowerPoint)

3.1 Thermal Concepts

Objectives:• Understand how a temperature scale is constructed.• Understand heat is energy that is exchanged by systems

at different temperatures.• Understand internal energy.• Understand absolute temperature is a measure of

average kinetic energy.• State the meaning of the mole and the Avogadro

constant• Describe mechanisms by which thermal energy is

transferred.

3.1 Temperature

• Temperature – Useful idea to compare two bodies internal energy– This is measured using a

thermal property of a substance like:

• Linear expansion (mercury)• Electrical resistance (

thermocouple)• Emitted radiation (Infrared)

3.1 Temperature Scales

• Fahrenheit - Daniel Gabriel Fahrenheit (1686 - 1736) 0o is the temperature of a ice, water, salt mixture, 32o is freezing water, 96o was body temperature.

• Celsius - Anders Celsius (1701 - 1744) 0o is freezing water, 100o was the boiling point of water at 1 ATM. Later related to Kelvin scale.

• Kelvin - William Thomson, 1st Baron Kelvin (1824 - 1907) defined by two points: absolute zero, and the triple point of specially prepared water.

3.1 Heat & Internal Energy

• James Prescott Joule (1818 -1889) determined the mechanical equivalent of heat experimentally.– Heat is defined as energy transfer

resulting in a temperature difference.– Internal energy – the total kinetic

energy of the molecules of a substance, plus any potential energy between the molecules.

3.1 Absolute Temperature

• Measure of the average kinetic energy of the molecule in a substance.

• This is directly proportional to the Kelvin temperature.

• Sometimes thermal energy is used in reference to internal energy.

3.1 The Atomic Model

Three phase of ordinary matter.

• Solid: High density with molecules in a fixed position.

• Liquid: Lower density with molecules further apart and free to change position.

• Gas: Lowest density with molecules even further apart and free to move.

Water at triple point

3.1 Moles

• Avogadro’s number tells the molar mass

• Hydrogen has a molar mass of 2 g mol-1, so 2 g of hydrogen represents 1 mole

• In other words, there are 6.02 x 1023 atoms of hydrogen.

Example

How many grams are there in a quantity of oxygen containing 1.20 x 1025 molecules?

Solution:

• The number of moles is 1.20 x 1025 / 6.02 x 1023 = 19.93 mol

• Since the molar mass is 32 (periodic table) 19.93 x 32 = 638 g = 0.638 kg

3.1 Heat Transfer

There are three ways energy is transferred:• Conduction is movement of molecular

kinetic energy through collisions between molecules.

• Good conductors of heat are usually good electric conductors.

• This includes most metal as their valance electron are free to move and “bump” into things.

3.1 Conduction Equation

• Also called “heat current”• Q / t is the energy per unit

time, Joules sec-1

• – thermal conductivity• A – cross sectional area• T – temperature• x – length of material

x

TA

t

Q

Example

Two rods of the same length and cross-sectional area are joined together. The left rod has a higher k than the rod at right. The ends of the rod are kept at a fixed temperature as shown. In which rod is the rate of heat transfer the largest? Is the temperature at the joining point lower or higher than 50 oC?

100 oC 0 oC

Solution

Imagine a vertical line through any part of the rod. The heat entering the line must be equal to the heat leaving it (conservation of energy). Hence, the rate of heat transfer is the same everywhere. A much larger temperature difference can be maintained across a bad conductor of heat. Thus, the temperature at the joint will be higher than 50 oC.

3.1 Convection

Since fluid molecules are free to move “hotter” fluid is less dense than “colder” fluid. In hot fluids the molecules are further apart, thus less dense. This less dense fluid rises in the presence of colder fluid and create a “convection current.”

3.1 Radiation

• Radiation requires no medium (matter) to transfer energy.

• Every body at some absolute temperature T radiates away energy as electromagnetic waves.

• P (power) AT4

IB Note: this equation is not tested Infrared picture of house

3.1 Radiation

• Good emitters of radiation are dark and dull, think charcoal.

• At low temperatures emission is small but increase by T4 as temperature rises.

• These are also good absorbers, think black shirt on a hot summer day.

• Shiny surfaces are reflectors, therefore are poor absorbers and emitters.

• AT room temperature objects emit wavelengths in the infrared spectrum.

Cross Sectional AreaCross section

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