Thermal Energy Transfer

Conduction, Convection and Radiation

Radiation: heat transfer via radiant energy

Radiant energy is in the form of electromagnetic waves

All substances, at any temperature above absolute zero, emit radiant energy.

We don’t run out of energy because:○ Energy can neither be created nor destroyed○ All things also absorb energy

Dark objects absorb a lot of energy, but also radiate a lot.

If a surface is hotter than its surroundings it will be a net emitter and will cool.

If a surface is cooler than its surroundings it will be a net absorber and will warm.

The Sun’s Radiant Energy The sun has a very high temperature

and emits a high frequency of radiant energy.

Energy comes from nuclear reactions

Terrestrial Radiation: the earth’s radiation

The earth has a comparatively low temperature and emits a lower frequency.

Energy comes from radioactivity in the earth’s interior

Conduction: heat transfer via particle (e-) collision

Solids are good conductors because particles are closer together

Metals are the best conductors of thermal energy

○ Metal atoms have loosely held electrons

Poor conductors are called insulators

○ Insulators have tightly held electrons

Convection:heat transfer via movement of a fluid

As fluids near the bottom are heats they begin top move faster. This causes them to spread apart and become less dense.

Less dense water rises away from the heat source and cooler fluid moves down towards it.

Convection in Nature During the day the land is warmer than the water

During the night the water is warmer than the air.

Phase ChangesEvaporation, Condensation, Boiling, Melting and Freezing

Energy is Needed for a Phase Change

Whenever a substance changes phase, an energy transfer occurs.

○ Heat of Fusion: amount of energy needed to change a substance from a solid to a liquid (melt)

○ Heat of Vaporization: amount of energy required to change a substance from a liquid to a gas (boil)

SolidLiquid Gas

Energy is absorbed when change of phase is in this direction

Energy is released when change of phase is in this direction

Practice Problems:The heat of fusion of water is 335 kj/kg.

How much energy must be absorbed to melt 1 kg of ice?

335 kj How much energy must be released to

freeze the water back into ice?335 kj

There is no temperature change during a phase change!

Energy is going into the change in phase

not into eating up the substance!