18.11:
Mechanisms of Heat Transfer I
There are countless examples of heat transfer, such as burning skin by holding a hot pan, boiling milk, or heating food in a microwave.
When there is a temperature gradient, heat transfer can occur in three ways: conduction, convection, and radiation.
Conduction is the process in which heat is transferred from a hot region to a cold region within an object, or between two objects in physical contact.
For example, if a pan is placed above a fire, heat will transfer to the pan, and eventually, it becomes hot.
Further, if one end of a steel spoon is placed above a fire, the other end becomes hot due to heat transfer.
In conduction, the molecules of the hot side have higher kinetic energy. They collide with the molecules of the colder region and transfer energy.
The rate of heat transfer through conduction is given by an equation involving temperature gradient, surface area, and thickness of the material that heat flows through.
'k' is the material's thermal conductivity, which is high for conductors and low for insulators.
18.11:
Mechanisms of Heat Transfer I
Just as interesting as the effects of heat transfer on a system are the methods by which the heat transfer occur. Whenever there is a temperature difference, heat transfer occurs. It may occur rapidly, such as through a cooking pan, or slowly, such as through the walls of a picnic ice box. So many processes involve heat transfer that it is hard to imagine a situation where no heat transfer occurs. Yet, every heat transfer takes place by only three methods: conduction, convection, and radiation.
For example, heat transfer occurs in a fireplace by all three methods: conduction, convection, and radiation. Radiation is responsible for most of the heat transferred into the room. Heat transfer also occurs through conduction into the room, but much more slowly. Heat transfer by convection also occurs through cold air entering the room around the windows and hot air leaving the room by rising up the chimney.
Conduction is heat transfer through stationary matter by physical contact. The matter is stationary only on a macroscopic scale as thermal motion of the atoms and molecules occurs at any temperature above absolute zero. Heat transferred from a stove’s burner through the bottom of a pan to the food in the pan is transferred by conduction.
Suggested Reading
- OpenStax. (2019). University Physics Vol. 2. [Web version]. Retrieved from https://openstax.org/details/books/university-physics-volume-2; pp 35-36