Convection Notes

Consider a hot object in a vacuum. It can't transfer thermal energy by conduction.
Radiation is possible, but we'll focus on a hot object surrounded by a fluid (gas or liquid).
The hot object heats the surrounding fluid via conduction.
In a weightless environment, further conduction would heat the rest of the fluid.
However, in a normal environment, as the fluid's temperature increases, it expands.
The fluid closest to the heat source warms up first, expanding before the fluid farther away.
The expanded fluid is less dense than the surrounding fluid and experiences an upward buoyant force.
As the warmed fluid flows upward, cooler fluid moves in to take its place, contacting the hot object.
More fluid is heated and rises, continuing the process until all the fluid is heated.
Convection: Heat transfer by currents (gross motion) of particles in an unevenly heated fluid.
Convection involves motion of mass, unlike conduction (successive collisions) and radiation (electromagnetic waves).
Convection is driven by gravitational force; it wouldn't work in an orbiting space shuttle.

The hot object cools down as the fluid warms up.
Convection cools a hot object using a fluid.
If a warm body is placed in cold air:
- The body warms the air, and the air cools the body by conduction.
- Convection causes the warmed air to rise, replaced by cool air.
- The body loses energy to heat the air, which is then replaced by more cool air.
Wearing a jacket provides insulation:
- The jacket's outside is close to the outdoor temperature.
- The jacket's inside is close to body temperature.
- This reduces conduction and subsequent convection.

Devices called radiators warm rooms and cool car engines.
Car engines generate a lot of internal energy and get very hot.
- Fluid is passed near the hottest parts and pumped to the radiator.
- The radiator is a metal reservoir near the front of the engine.
- Cool air blows past the radiator and picks up heat by conduction.
- Cooling occurs by conduction, not radiation.
- This type of radiator should be called a conductor.
A room-heating radiator is a set of metal pipes with steam or hot water.
- The pipes become hot and drive convection currents in the room.
- Radiators are often painted with shiny metallic paint.
- This reduces radiation, maintaining a high temperature to drive convection.
- This type of radiator should be called a convector.

Radiant energy from the sun heats the Earth unevenly.
Land masses heat up faster than water.
Air in the atmosphere is warmed unevenly at the surface, creating convection patterns.
Areas of high pressure, low pressure, winds, and the water cycle are driven by convection.