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If there is a temperature difference in a system, heat will always move
from high to low temperatures. The place you find the higher temperature,
that place is the HEAT SOURCE. In the area where the temperature is lower,
that area is the HEAT SINK. When examining systems, scientists measure
a number called the TEMPERATURE GRADIENT. It's the change in temperature
divided by the distance. The units are degrees per centimeter. If the
temperature is dropping over a specific distance, the gradient is a negative
value. If the temperature goes up . . . positive.
CONVECTION
Convection is the way heat is transferred from one area to another when
there is a "bulk movement of matter." It's the movement of huge
amounts of an object, taking the heat from one area and placing it in
another. Warm air rises and cold air replaces it. The heat has moved.
So it is the transfer of heat by motion of objects. When an area of hot
water rises to the top of a pot and it gives off energy, that is convection.
Another example is warm air in the atmosphere rises and gives off energy,
that's convection too. The thing to remember is that the object moves.
RADIATION
The definition of heat being transferred by radiation happens when a temperature
gradient exists and the transfer of energy happens without a CONDUCTIVE
MEDIUM. That means there is no matter there for the heat to pass through.
Radiation is the energy carried by electromagnetic waves (light). Those
waves could be radio waves, infrared, visible light, UV or Gamma rays.
Radiation is usually in the red and infrared sections of the EM spectrum.
And get this, if the temperature of an object doubles (in Kelvin), the
thermal radiation increases 16 times. So if it goes up four times, it
increases to 32 times the original level.
Scientists have also discovered that objects that are good at giving off
thermal radiation are also good at absorbing the same energy. Usually
the amount of radiation given off by an object depends on the temperature.
The rate you absorb the energy depends on the energy of the surrounding
you.
CONDUCTION
This is a situation where the heat source and heat sink are connected.
As we discussed before, the heat flows from the source, down the temperature
gradient, to the sink. It's different from convection because there is
no movement of large amounts of matter. The source and the sink are connected.
Conduction is special in that it needs more FREE ENERGY than the other
ways of transferring thermal energy. If you touch an ice cream cone, the
ice cream heats up because you are a warmer body. If you lay on a hot
sidewalk, the energy moves directly to your body by conduction.
When scientists studied good thermal radiators, they discovered that good
thermal conductors are good at conducting electricity. So when you think
of a good conductor, think about Gold, Silver and Platinum.
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