Heat Transfer: Conduction, Convection, and Radiation in Everyday Experiences
Heat transfer is a fundamental concept in physics that describes the transfer of thermal energy from one body to another. There are three primary modes of heat transfer: conduction, convection, and radiation. In this article, we will delve into a brief explanation of each phenomenon (no math involved) and provide examples of how they can be experienced in everyday life.
Conduction
Conduction is the transfer of heat energy through direct contact between objects. This process occurs when there is a temperature difference between two materials in physical contact. The rate of heat transfer depends on the temperature difference, the area of contact, and the thermal conductivity of the materials involved.
Convection
Convection is the transfer of heat energy through the movement of fluids. Be it hot air or a liquid, heat is transferred from a moving fluid that carries heat from one place to another. That movement can occur through the change in buoyancy of the heated fluid (such as hot air rising) or by forcing the fluid over another object. Those types of convection are called natural and forced convection, respectively.
Example:
Regarding everyday examples of conduction and convection, here’s a good example that combines both methods into one. Imagine you’re a small child holding your hand over the kitchen stove that’s just been used. You can feel the warm air rising from the stove. You probably didn’t think of it as air rising, but just that it feels warm. As you lowered your hand, moving it closer to the stove, it got a little hotter and hotter. “Hey, that’s neat,” your young, inquisitive mind thought. You brought your hand ever closer until you accidentally touched the stove and “Ouch, that hurts”, throwing your hand away from the stove, after quite possibly having burned yourself. What you experienced was convection initially, followed by conduction. That conduction was instantly painful and demonstrated how much faster conductive heat transfer occurs than convective. You probably didn’t think of it that way at the time, but you experienced two types of heat transfer in very short order.
Radiation
Radiation is a different animal altogether and doesn’t rely of molecular movement like conduction or convection do. It consists of the transfer of heat energy through electromagnetic waves which occur when a body emits thermal radiation. The rate of heat transfer depends on the temperature of the body and its emissivity, which is a function of the surface properties of the material emitting and absorbing the radiation. If you want to get technical, everything emits radiation that is above absolute zero (the coldest temperature there is [0 K, -273.15° C or -459.67° F]). Even a piece of dry ice emits thermal radiation, but the net radiation it receives is positive because it receives a lot more radiation from its surroundings than it emits. Electromagnetic radiation is common to all of us. Everything we see gets to us via electromagnetic radiation (light). Radio and TV waves are electromagnetic radiation but at different wavelengths. X-rays, ultraviolet radiation, gamma rays and microwaves (that you heat food with) are all forms of electromagnetic radiation too. Collectively they make up the electromagnetic spectrum. The only difference between them all is that they are at different wavelengths. The wavelength not mentioned yet is the infrared, which exists in the wavelength range of 700 nm (nanometers) to 1 mm (millimeter). The infrared range starts where the color red in the visible spectrum ends.
Here’s the odd part: none of the wavelengths in the electromagnetic spectrum need anything to ‘carry’ them from one place to another. They are perfectly at home in a vacuum. As the name suggests, the electromagnetic spectrum consists of electrical waves and magnetic waves fluctuating perpendicularly to each other in the direction of travel. It’s a good thing they work in a vacuum, otherwise, heat from the sun would never reach the earth and provide the heat we need to live. Standing outside and feeling bright sunlight hit your face is an example of radiation heat transfer. Step into the shade and you can feel the temperature drop immediately, which is because you’ve greatly reduced the amount of infrared radiation you’re receiving.
It gets more interesting with the electromagnetic spectrum. All those types of radiation get from one place to the other at the speed of light. Light rays (obviously), radio and TV signals, X-Rays, microwaves and all the others travel at the speed of light, including infrared.
Example:
The best example of this is when the author was attending a stunt show at Universal Studios Hollywood. The show consisted of several large fiery explosions. It was amazing that as soon as you could see a big explosion, you instantly felt the heat wash across your face and body. You saw and felt them at the same time. The image of the explosion and the infrared heat both traveled at the speed of light.
To wrap it all up, conduction, convection and radiation are the three types of heat transfer and you can feel them in everyday activities if you know what to look for.
