transfer of thermal energy

2.3.1 | conduction

in conduction, thermal energy is transferred through a material without the material itself moving. this occurs when vibrating particles collide and transfer kinetic energy to neighboring particles. conduction is good in solids, as the particles are close together, but bad in liquids and gases, since the particles are further apart.

conduction can also occur by delocalized electrons in metals, where electrons move freely and carry thermal energy. they move freely through the metal lattice, transferring energy without moving the atoms themselves.

there are thermal conductors and insulators. thermal conductors conduct thermal energy well, while thermal insulators do not.

2.3.2 | convection

in convection, thermal energy is transferred by the movement of heated particles. this occurs in fluids (liquids and gases) where particles are free to move. when a fluid is heated, it expands and becomes less dense, causing it to rise. cooler, denser fluid then moves in to replace it, creating a convection current.

convection is important in natural phenomena like wind patterns and ocean currents, as well as in everyday applications like heating systems and cooking.

convection cannot occur in solids because the particles are fixed in place and cannot move freely, while it is the main method of thermal energy transfer in fluids.

due to convection, air conditioners are kept at the top of the room, while heaters are placed at the bottom. this is because warm air rises and cool air sinks, so placing the air conditioner at the top allows it to cool the air that rises, and placing the heater at the bottom allows it to heat the air that sinks.

2.3.3 | radiation

in radiation, thermal energy is transferred by infrared waves. this occurs without the need for a medium, meaning it can happen in a vacuum. all objects emit thermal radiation, and the amount depends on their temperature.

radiation is important in many natural and artificial processes, such as the sun heating the earth, and in applications like infrared heating and cooking.

the rate of thermal radiation emitted by an object depends on its temperature and surface area. hotter objects emit more radiation than cooler ones.

good and bad radiation emitters/absorbers

• good emitters/absorbers: dark, matte, rough surfaces (e.g., black paint, charcoal.) these are bad reflectors.
• bad emitters/absorbers: light, shiny, smooth surfaces (e.g., silver, polished metal). these are also good reflectors.

receiving and emitting energy

• an object that absorbs more radiation than it emits will increase in temperature.
• an object that emits more radiation than it absorbs will decrease in temperature.
• an object that absorbs and emits radiation at the same rate will maintain a constant temperature.

factors affecting radiation

• temperature: hotter objects emit more radiation.
• surface area: larger surface areas emit more radiation.
• surface properties: dark, matte surfaces are better emitters/absorbers than light, shiny surfaces.