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Wednesday, December 2, 2009

Welcome to the blog of Pranav and Chaitanya! Our blog will educate you about the three types of heat transfer.

Have you ever roasted a marshmallow over a campfire? Have you smelt the smoke coming from the roasted marshmallow? Or did you feel warmth from the fire? In each of these situations, you experienced the work of the three types of heat transfer: conduction, convection, and radiation. Conduction, or direct heat, occurs when the faster moving molecules of the fire collide with the cooler molecules of the marshmallow, causing them to move faster and become hot.

Diagram 1

The diagram (#1) above shows the metal rod conducting heat to the person's hand. Some solids, unlike the metal rod, are poor conductors of heat, making them excellent for insulating heat. These solids are called insulators. Some examples of insulators would include wood or plastic, which is often used for making handles for pots and pans. Conduction is also found in movie theaters. Popcorn machines used radiation (another type of heat transfer), and convection. Convection is heat transfer by air, or liquid. the popcorn is heated by the air surrounding it. The light inside the popcorn machine produces infrared rays, which proves that popcorn machine produces radiation. You may not know it, but just the scent of popcorn is a form of convection. Back home, let us say that you are cooking something on the stove. This would be a form of conduction. The hot molecules from the stove collide with the cool molecules of the food. This is called conduction or direct heat.

Diagram 2

The picture above shows a thermos, another example of an insulator. People use the thermos for keeping water the same temperature it originally was for a considerable amount of time. The thermos creates a vacuum which keeps the speed of the molecules the same. Instead letting the warm and cool molecules collide, the thermos doesn't let any air in or out. When heat is transferred through liquids such as water, or gases, this is called convection. Typical examples include breathing hot air on your hands during a cold day. The heat from your mouth warms up your hands. This transfer of heat by air is convection.

When liquids such as water are heated, their molecules move faster and farther apart. This decreases their density and makes the molecules lighter. Now, the heavier surrounding molecules sin down, pushing the lighter molecules upward, creating a convection current. One may have never realized this, yet there are convection currents everywhere. Convection currents tell us how wind currents are formed, how houses are heated, how birds can fly, and even why smoke rises.

Diagram 3

Diagram 3, above shows how warm air rises and cool air sinks in the air. This is so, since the heated warm molecules are pushed upwards by the heavier cool air molecules. This is why warm air rises and cool air sinks. This doesn't happen so just in the air, but in our homes as well. But why does hot air rise? This is because of the Principle of Buoyancy, where denser molecules sink, and the less dense molecules, such as the hot air molecules, rise. Take a cork, for example. If one would drop the cork into a jar of water, it would float, since it is less dense. Water has a density of 1. Therefore, the cork has a density less than one since it floats. What would happen if you dropped a coin in to the jar? It would sink as it is denser. The Principle of Buoyancy does not only apply to liquids, but to gasses as well.

Diagram 4

The diagram above shows a house whose heating system relies on the principle of buoyancy. Most houses have a cycle of hot air and cool air, where the furnace heats up the air, sending the hot air up to the second level, where the hot air cools, and sinks back to the heating furnace. The following video helps one understand more about convection and how heating in your house works. (the video talks about heating in houses around 2:00)

Let's go back to Diagram 2, with the picture of a thermos. Since it created a vacuum to stop the molecules from outside to enter and the molecules inside exiting, conduction and convection did not occur. Unlike conduction or convection, radiative heat energy can be transferred through a vacuum in the form of electromagnetic waves by radiation. This is similar to the way light and radio waves travel. These heat waves, also known as infrared waves, travel from heat sources such as the sun, a light bulb, or even a flashlight!

Diagram 5

Diagram 5 (above) shows a great example of radiation playing a role in our daily lives. Ninety-three million miles away, the sun sends out infrared heat waves through space, to the Earth. The rays enter the atmosphere, and soon reach the clouds, where the rays reflect off the clouds. Not all of the infrared rays are reflected off the clouds; some are absorbed by water, dust, and 03. While infrared rays are being emitted by the sun, infrared radiation is being emitted by the surface of the Earth. These rays are being sent through sky, out of the atmosphere, and into space. Radiation not only occurs in outer space, but also in our every day lives. Infrared waves can be sent through light bulbs. There are several different types of light bulbs- incandescent, flourescent, halogen, LED. Incandescent bulbs produce 98% heat energy. If you touch an incandescent light bulb, you would experience conduction. If you keep your hands close to the bulb, you would feel warm air. This is convection.  

Congratulations! You have learnt all about the three types of heat transfer! Now you can test your skills in the quiz (it's actually a poll) located in the upper right hand corner and the top of our blog. Good luck!

The molecules in a solid are tightly packed together, and they move slightly, vibrating in a fixed position as illustrated in an ice cube. When heat energy is added , the molecule move faster and break free from their fixed position changing from a solid into a liquid state. This process is called "melting". When an object melts, the molecules in the solid move farther and farther apart until they reach a liquid state. You can watch the following videos to learn more about atoms and molecules.

There is more space between the molecules in a liquid allowing them to move more freely. When heat energy is added to water, its molecules move faster, colliding with each other until they move freely into space , changing into a gaseous state called "steam". The process of changing from a liquid to a gas is called "evaporation". While transitioning from the state of liquid to gas, the particles in the matter move farther and farther apart until they have reached the state of gas.

Molecules in a gas are farthest apart and move freely in space. When a liquid is heated and changes quickly into a gas, as in boiling waster, that is called "rapid evaporation".