Thermal Energy Transfer

How is Thermal Energy Transferred Between Different Objects?

Conduction, convection and thermal radiation are the primary mechanisms of thermal energy transfer.

Conduction occurs when two solids of different temperatures touch.
Thermal energy is transferred from the hotter object to the cooler object through a process called conduction.

Thermal conduction occurs because o the transfer of kinetic energy between particles.
When atoms of one portion of a material are in contact with vibrating atoms of another portion, the vibration is transferred from atom to atom.
Since we associate motion with temperature, high T portions vibrate more than low T portions, so we can imagine the vibration "impulse" to travel through the material from high T to low T.
- While an imperfect analogy , it helps to imagine a Newton's cradle, where one ball bumps against another and transfers part of its energy to it, which transfers that energy to the next ball.
- In the case of conduction, particles behave like the balls in a Newton's cradle.

Metals are good heat conductors because they have lots of free electrons.
- This is the same reason they are good electrical conductors.

The rate of transfer of thermal energy by conduction can be found through the following equation:

The distance from the heated end, L, may also be written as x.
The rate at which ΔQ/Δt at which heat energy is transferred depends directly on the cross-sectional area A and inversely with the length L of the conductor.
The rate, ΔQ/Δt, is also proportional to the difference in temperature, ΔT, between the two ends.

Thermal convection is the transfer of thermal energy due to the movement of a fluid due to changes in density.
Convection requires a fluid as a medium of heat transfer.
- For example, hot air is less dense than cold air, so it rises.
- But as hot air rises it cools, and becomes denser and sinks.
- This leads to a cycle, which forms a circulation called a convection current.

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