Heat is being moved from the hot finish of this rod come the cold end via conduction, but the hot finish of the stick is additionally radiating warmth via that orange glow.

You are watching: What is the direction of heat flow


Throughout the universe, it’s herbal for energy to circulation from one location to another. And unless civilization interfere, thermal power — or warmth — normally flows in one direction only: from hot toward cold.

Heat moves naturally by any type of of three means. The procedures are recognized as conduction, convection and radiation. Sometimes an ext than one may occur at the very same time.

First, a tiny background. All matter is do from atoms — either single ones or those bonded in teams known together molecules. These atoms and also molecules are always in motion. If they have actually the very same mass, warm atoms and molecules move, ~ above average, much faster than cold ones. Also if atoms are locked in a solid, they still vibrate ago and forth about some typical position.


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In a liquid, atoms and molecules are complimentary to flow from place to place. In ~ a gas, lock are also more cost-free to move and will fully spread the end within the volume in i beg your pardon they room trapped.

Some the the many easily interpreted examples the heat circulation occur in her kitchen.


Put a pan top top a stovetop and also turn top top the heat. The steel sitting end the burner will be the an initial part the the pan to get hot. Atom in the pan’s bottom will begin to vibrate faster as castle warm. They also vibrate farther back and soon from their mean position. As they bump right into their neighbors, lock share v that neighbor several of their energy. (Think that this together a really tiny variation of a cue sphere slamming into other balls during a video game of billiards. The target balls, previously sitting still, gain some the the cue ball’s energy and move.)

As a result of collisions through their warmer neighbors, atom start moving faster. In other words, castle are now warming. These atoms, in turn, transfer some of their increased energy to neighbors even farther from the original resource of heat. This conduction of warm through a solid steel is exactly how the manage of a pan gets hot also though it may be nowhere close to the source of heat.


Convection occurs when a product is complimentary to move, such together a liquid or a gas. Again, take into consideration a pan top top the stove. Put water in the pan, then revolve on the heat. Together the pan it s okay hot, some of that warmth transfers to the molecule of water sit on the bottom the the pan via conduction. That increases the movement of those water molecule — they room warming.

Lava lamps illustrate warm transfer via convection: Waxy blobs obtain warmed at the base and also expand. This renders them much less dense, so they climb to the top. There, they offer off your heat, cool and then sink to complete the circulation.Bernardojbp/iStockphoto

As the water warms, the now starts to expand. That provides it much less dense. The rises above denser water, carrying away heat native the bottom the the pan. Cooler water flows under to take it its place next come the hot bottom of the pan. As this water warms, it expands and rises, ferrying that is newly-gained energy with it. In quick order, a circular flow of rising warmth water and falling cooler water set up. This circular pattern of heat transfer is well-known as convection.

It’s additionally what mostly warms food in an oven. Air it is warmed by a heating facet or gas flames at the height or bottom the the range carries that heat to the central zone whereby the food sits.

Air that’s warmed in ~ Earth’s surface expands and rises just like the water in the pan top top the stove. Big birds such together frigate bird (and human flyers riding engineless gliders) often ride this thermals — climbing blobs of wait — to obtain altitude there is no using any type of energy of your own. In the ocean, convection caused by heating and also cooling helps to drive s currents. These currents move water roughly the globe.


The third form of energy transfer is in some ways the many unusual. It deserve to move through products — or in the lack of them. This is radiation.

Radiation, such as the electromagnetic energy spewing from the sunlight (seen right here at two ultraviolet wavelengths) is the only form of energy transfer that works throughout empty space.NASA

Consider visible light, a kind of radiation. That passes v some species of glass and also plastic. X-rays, another form of radiation, readily pass with flesh but are greatly blocked by bone. Radio tide pass with the wall surfaces of your house to reach the antenna on her stereo. Infrared radiation, or heat, passes with the wait from fireplaces and light bulbs. But unlike conduction and also convection, radiation doesn’t require a material to move its energy. Light, X-rays, infrared waves and radio tide all travel to planet from the far reaches that the universe. Those forms of radiation will pass with plenty the empty an are along the way.

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X-rays, visible light, infrared radiation, radio waves room all different forms the electromagnetic radiation. Each type of radiation drops into a certain band of wavelengths. Those species differ in the amount of energy they have. In general, the much longer the wavelength, the lower the frequency of a particular form of radiation and also the less power it will certainly carry.

To complicate things, it’s essential to keep in mind that an ext than one type of warm transfer may take place at the very same time. A stove’s burner not just heats a pan but additionally the surrounding air and makes it less dense. The carries warmth increase via convection. However the burner likewise radiates warmth as infrared waves, making things surrounding warm up. And if you’re utilizing a cast-iron skillet to chef a tasty meal, be sure to seize the take care of with a potholder: it’s gonna be hot, many thanks to conduction!