Thermal Energy

Heat

• Heat: transfer of thermal energy from one object to another because of temperature differences– Heat flows spontaneously from hot

objects to cold objects–Why does a cold drink become warm?

Temperature

• Temperature: measure of how hot or cold an object is compared to a reference point– Reference points for Celsius are freezing

and boiling points of water– Reference point for Kelvin is absolute

zero which equals 0 K

Temperature

• Temperature is related to average kinetic energy of particles– KE of particles is due

to random motion through space

– As an object heats up the particles move faster = higher KE of particles = higher temperature

Thermal Energy

• Thermal energy = total potential and kinetic energy of all the particles in an object– Depends on the mass, temperature and

phase (solid,liquid,gas) of an object

Thermal Energy Example

• Cup of tea– Temp of 110°F– KE of particles 80 J– Thermal Energy

lower

• Tea pot full of tea– Temp of 110°F– KE of particles 80 J– Thermal energy

higher

Thermal Contraction and Expansion

• Thermal Contraction– As temp decreases

the particles move slower causing less collisions• Balloon outside on a

cold day

• Thermal Expansion– As temp increases

the particles move faster and spread apart• Balloon brought in

from cold into warm room

•Gases expand more than liquids and liquids expand more than solids•Thermal expansion is used in thermometers to show temperature

Measuring Heat Changes

• Calorimeter is used to measure changes in thermal energy– Uses the principle

that heat flows from hotter to cooler objects until they reach the same temperature

Heat Transfers

• Conduction• Convection• Radiation

Conduction

• Conduction: Occurs within a material or between materials that are touching– Collisions between particles transfer

thermal energy without any overall transfer of matter

– Conduction is slower in gases than in liquids or solids because the particles are farther apart• Conduction is faster in metals because the

electrons are free to move around

Conduction

• Thermal conductors: material that conducts thermal energy well–Wire rack in oven– Pots and pans• Aluminum and copper

– Not all thermal conductors are hot• Tile floors

– Feel cold because they are transferring thermal energy rapidly away from your skin

Conduction

• Thermal insulators: material that conducts thermal energy poorly– Air–Wooden spoon–Wool– Plastic foam cups

Use trapped air to slow down conduction

Convection

• Convection: transfer of thermal energy when particles of a fluid move from one place to another– Fluids = liquids or

gases– Convection currents:

fluid circulates in a loop as it heats up and cools down

Radiation

• Radiation: transfer of energy by waves moving through space– All objects radiate

energy. As the temperature increases, the rate at which it radiates energy increases

Thermodynamics• Study of conversions between

thermal energy and other forms of energy

• Zeroth law: two systems in thermal equilibrium with a third system are in thermal equilibrium with each other

Thermodynamics

• 1st law: Energy is conserved• 2nd law: Thermal energy can flow

from colder objects to hotter objects only if work is done on the system

• 3rd law: Absolute zero cannot be reached