How does heat travels

Convection is the process of heat transfer from one location to the next by the movement of fluids. The moving fluid carries energy with it. The fluid flows from a high temperature location to a low temperature location. To understand convection in fluids, let's consider the heat transfer through the water that is being heated in a pot on a stove. Of course the source of the heat is the stove burner.

The metal pot that holds the water is heated by the stove burner. As the metal becomes hot, it begins to conduct heat to the water. The water at the boundary with the metal pan becomes hot. Fluids expand when heated and become less dense. So as the water at the bottom of the pot becomes hot, its density decreases. Differences in water density between the bottom of the pot and the top of the pot results in the gradual formation of circulation currents.

Hot water begins to rise to the top of the pot displacing the colder water that was originally there. And the colder water that was present at the top of the pot moves towards the bottom of the pot where it is heated and begins to rise. These circulation currents slowly develop over time, providing the pathway for heated water to transfer energy from the bottom of the pot to the surface. Convection also explains how an electric heater placed on the floor of a cold room warms up the air in the room.

Air present near the coils of the heater warm up. As the air warms up, it expands, becomes less dense and begins to rise. As the hot air rises, it pushes some of the cold air near the top of the room out of the way. The cold air moves towards the bottom of the room to replace the hot air that has risen. As the colder air approaches the heater at the bottom of the room, it becomes warmed by the heater and begins to rise.

Once more, convection currents are slowly formed. Air travels along these pathways, carrying energy with it from the heater throughout the room. Convection is the main method of heat transfer in fluids such as water and air. It is often said that heat rises in these situations. The more appropriate explanation is to say that heated fluid rises. For instance, as the heated air rises from the heater on a floor, it carries more energetic particles with it.

As the more energetic particles of the heated air mix with the cooler air near the ceiling, the average kinetic energy of the air near the top of the room increases. This increase in the average kinetic energy corresponds to an increase in temperature. The net result of the rising hot fluid is the transfer of heat from one location to another location. The convection method of heat transfer always involves the transfer of heat by the movement of matter. This is not to be confused with the caloric theory discussed earlier in this lesson.

In caloric theory, heat was the fluid and the fluid that moved was the heat. Our model of convection considers heat to be energy transfer that is simply the result of the movement of more energetic particles.

The two examples of convection discussed here - heating water in a pot and heating air in a room - are examples of natural convection.

The driving force of the circulation of fluid is natural - differences in density between two locations as the result of fluid being heated at some source. Some sources introduce the concept of buoyant forces to explain why the heated fluids rise.

We will not pursue such explanations here. Natural convection is common in nature. The earth's oceans and atmosphere are heated by natural convection. In contrast to natural convection, forced convection involves fluid being forced from one location to another by fans, pumps and other devices. Many home heating systems involve force air heating. Air is heated at a furnace and blown by fans through ductwork and released into rooms at vent locations.

This is an example of forced convection. The movement of the fluid from the hot location near the furnace to the cool location the rooms throughout the house is driven or forced by a fan. Some ovens are forced convection ovens; they have fans that blow heated air from a heat source into the oven. Some fireplaces enhance the heating ability of the fire by blowing heated air from the fireplace unit into the adjacent room. This is another example of forced convection.

A final method of heat transfer involves radiation. Radiation is the transfer of heat by means of electromagnetic waves. To radiate means to send out or spread from a central location. Whether it is light, sound, waves, rays, flower petals, wheel spokes or pain, if something radiates then it protrudes or spreads outward from an origin.

The transfer of heat by radiation involves the carrying of energy from an origin to the space surrounding it. The energy is carried by electromagnetic waves and does not involve the movement or the interaction of matter. Its kinetic energy increases.

But where did the wheelbarrow get its kinetic energy? It came from the person exerting the force, who used chemical energy stored in the food they ate to move the wheelbarrow. In other words, when the person did work on the wheelbarrow, they transferred a certain amount of chemical energy to the wheelbarrow, causing it to move. As a result, the person's store of chemical energy decreases and the wheelbarrow's kinetic energy increases. Heat is the transfer of energy from a warmer object to a cooler object.

For example, a lighted match higher temperature object will transfer heat to a large pan filled with lukewarm water lower temperature object.

Note that the actual amount of thermal energy each object has doesn't matter, as the pan of lukewarm water might have more thermal energy than the match. What is needed for heat transfer to take place is a difference in temperature between two objects. Without this difference, no heat transfer can take place.

This process is called convection. When a lava lamp is switched on, the bottom of the lamp becomes hot. This causes convection in the liquid of the lamp, making blobs of wax inside move up and down. The lump of colored wax inside the glass container moves and changes shape, like volcanic lava, as the lamp heats up and cools down.

Hot objects give off heat as invisible waves, called infrared, in a process called radiation. These waves can move through air or space. The glowing hot element of a toaster radiates heat when it is switched on.

It can move through materials — or in the absence of them. This is radiation. Consider visible light, a form of radiation. It passes through some types of glass and plastic. X-rays, another form of radiation, readily pass through flesh but are largely blocked by bone.

Radio waves pass through the walls of your home to reach the antenna on your stereo. Infrared radiation, or heat, passes through the air from fireplaces and light bulbs. Light, X-rays, infrared waves and radio waves all travel to Earth from the far reaches of the universe. Those forms of radiation will pass through plenty of empty space along the way. X-rays, visible light, infrared radiation, radio waves are all different forms of electromagnetic radiation.

Each type of radiation falls into a particular band of wavelengths. Those types differ in the amount of energy they have. In general, the longer the wavelength, the lower the frequency of a particular type of radiation and the less energy it will carry.