conduction, convection and radiation. radiation: heat transfer via radiant energy radiant energy is...
DESCRIPTION
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.TRANSCRIPT
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Thermal Energy Transfer
Conduction, Convection and Radiation
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Radiation: heat transfer via radiant energy
Radiant energy is in the form of electromagnetic waves
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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.
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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
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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
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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
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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.
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Convection in Nature During the day the land is warmer than the water
During the night the water is warmer than the air.
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Phase ChangesEvaporation, Condensation, Boiling, Melting and Freezing
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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
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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
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There is no temperature change during a phase change!
Energy is going into the change in phase
not into eating up the substance!