Heat (Energy) Transfer Mechanisms

Due to the microscopic motion of the atoms/molecules comprising the substance (solids and liquids). Atoms/molecules vibrate passing on some of this energy to their neighbours, thus heat (energy) can flow from one part of an object to the other.

For a rod, cross section A and length experimentally we find that the rate of heat transfer (dQ/dt) is given by,

where

is the change in temperature and k is the thermal conductivity of the material, measured in Watts/m.⁰C.
Note that the negative sign is a result of the fact that heat flows in the direction of decreasing x.

For the Steady State situation (when the temperature at each point on the sample is no longer changing with time), we can write,

where, T_H and T_C are the temperatures at the hot and cold ends of the rod, respectively.

Caused by the macroscopic motion of the substance, only possible when the atoms/molecules can migrate through the sample as in gases and liquids (fluids).

Hot fluid expands, becomes less dense and rises due to buoyancy forces
Cold fluid contracts, becomes more dense and sinks due to gravity

Natural convection needs gravity and a hot reservoir below a cold reservoir, e.g. weather patterns (convection can be forced by the use of fans).

RADIATION

All bodies emit electromagnetic radiation which carries away heat (energy).

The resultant direction of heat flow depends upon the relative temperatures of the object and its environment (heat always flows from the hot to cold).
Note that the type of radiation is dependent on the temperature of the object, e.g. the human body emits radiation primarily in the infra-red, hence the usefulness of heat sensing binoculars.
Also, electromagnetic radiation can be transmitted through a vacuum. Isolating a thermodynamic system from the effects of radiation is essentially impossible.
Physics 299 will address the topic of radiation in more detail.