Thermal Physics How do we define temperature Definition

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