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What makes heat rise?

Category: Physics
Published: July 10, 2014

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University

Heat does not rise, hot air does (usually). Heat can spread out in all directions. There are three main ways for heat to travel:

candle flame
Public Domain Image, source: Christopher S. Baird.
  1. Radiation: All wavelengths of electromagnetic waves, including light, carry energy. When the electromagnetic waves strike an object, they are partially absorbed and the energy that the waves carried is converted to heat in the object. Also, hot objects emit electromagnetic waves ("thermal radiation") that carry away energy and can heat up other objects that they hit. In a loose sense, you can think of electromagnetic waves as transferring heat from one object to the next. Although, strictly speaking, when the waves are traveling they are only carrying electromagnetic field energy and not heat. The heat is generated when the waves are absorbed by matter.
  2. Conduction: When one hot object is in direct contact with another object, the heat can pass directly from the one object to the other through the touching surfaces.
  3. Convection. When a fluid such as air or water touches a hot object, it can heat up and then move in bulk as a fluid, thereby carrying the heat quickly to new locations. Hot air rising is a common example of heat convection. For this reason, "heat" and "hot air" tend to be confused with each other.

Thermal radiation tends to spread out in all directions and not just up. When you are standing a few steps back from a large campfire, most of the heat you are receiving is being delivered to you via thermal radiation. Although the hot air of the campfire's flame travels mostly upwards, the thermal radiation has no problem coming out sideways and hitting you. The thermal radiation of a campfire spreads out in all directions, so that you can feel it heating you no matter where you stand (as long as you are close enough). Sunlight heating you up is another example of thermal radiation. The sunlight has no problem traveling out in all directions through space and coming down through earth's atmosphere to hit you.

Heat traveling by conduction can travel in all directions as well. Conducted heat tends to travel the most in the direction where there is the largest temperature gradient, and in the direction where the material has the highest thermal conductivity. In other words, heat that is conducted travels most strongly to regions that are the coldest, along paths where the heat meets the least resistance. If you lay a long metal rod down diagonally, so that its top end it situated in a flame, and its bottom end is on the ground, the heat from the flame will have no problem traveling down the rod to the bottom end via conduction.

Heat traveling by convection can also move in all directions, but it does tend to move mostly sideways and upwards if natural convection currents are allowed to form in the fluid. Fluids such as air and water typically become less dense when they are heated, causing them to be pushed sideways and upwards by the colder, more-dense fluid around them that is being pulled more strongly down by gravity. But this is not always the case. When water stays below 4° Celsius, it actually gets more dense as it heats up. This means that in a cold winter pond, the warmer water sinks down to the bottom. So even for something as simple as water, heat traveling under convection does not always travel up.

Also, convection can be driven by more than just gravity. In a rotating reference frame such as a centrifuge or a turntable, centrifugal force can become the dominant force. When this is the case, the less dense fluids (typically the warmer ones) will convect towards the center of rotation under the influence of the centrifugal force, and the more dense fluids (typically the colder ones) will convect away from the center of rotation. This situation can be easily verified. Place a lit candle in an open glass jar (to keep it from blowing out) and place them on top of turntable. As the turntable turns, you will see the candle's flame point towards the center of rotation instead of upwards. As another example, convection can be forced by fans and pumps. Hot air has no problem going down if there is a fan blowing it that direction.

In summary, heat can travel in all directions. The direction that heat is traveling depends strongly on the situation. Furthermore, even hot air can travel in all sorts of directions and not just up. Hot air only travels up when gravity is the dominant force at work.

Topics: conduction, convection, electromagnetism, energy, heat, hot air, light, radiation, thermal radiation, thermodynamics