Thermal Physics

How do we define temperature?

• “Definition:” An object’s temperature is related to the average kinetic energy of its molecules• True, but NOT a definition

• Definition: An object’s temperature is a measure of its tendency to spontaneously give up energy to its surroundings• Three ‘standard’ temperature scales:

• USA: Fahrenheit• Everywhere else: Celsius• Physics/chemistry: Kelvin

Tell us more about molecular energy…• An important form of non-mechanical energy is thermal energy, the

energy related to temperature• The source of thermal energy is the kinetic energy of the object’s

internal molecules• We see the effects of molecular motion when we study diffusion• This relates the temperature of an object to the speed of its molecules, which

tells us that there’s a lowest temperature (absolute zero)

What is heat?

• Definition: Heat is the energy TRANSFERRED between bodies in thermal contact• Heat moves from high energy to low energy• When something ‘feels hot,’ it is ADDING energy to you• When something ‘feels cold,’ it is TAKING energy from you

• Method of heat exchange depends on the materials involved• Heat is transferred until objects reach thermal equilibrium (“same

temperature”)• Eureka! Heat vs. Temperature

Conduction

• Conduction transfers energy through molecular collisions• Good conductors have lots of ‘free’ electrons, so good thermal conductors

also tend to be good electrical conductors• When SOLIDS are in contact, they transfer by conduction

• Eureka! Conduction

Convection

• Convection transfers energy by moving matter around• Convection occurs in fluids (liquids AND gases), which tend to be poor

conductors• Currents in the fluid transfer the hotter material until thermal equilibrium is

reached

• Eureka! Convection

Radiation

• Radiation transfers energy in the form of electromagnetic waves• If enough energy is transferred, some of it will be visible• Objects that are good emitters are also good absorbers• Objects that are good emitters/absorbers are POOR reflectors

• Eureka! Radiation Waves