How transfer of heat happens

In our previous "The World of Science" page, we wrote about some different types of thermometers, which are all used to measure temperature. In order to measure our body temperature, our body heat must transfer to the thermometer bulb. So, how does this heat transfer happen?

For the transfer of heat to be possible in a system, there should be a temperature difference in the system; heat is always moving from high to low temperatures. The place you find the higher temperature is known as the heat source. The area where the temperature is lower is the heat sink.

When examining systems, scientists measure a number called the temperature gradient. It is the change in temperature divided by the distance. The units are degrees per centimetre. If the temperature is dropping over a specific distance, the gradient is a negative value. If the temperature goes up, it is a positive value.

When you touch a surface, how can you tell whether it is hot or cold? The temperature of that surface must first transfer to your skin. Then you can say what you feel. When your mother is cooking, if the heat does not transfer to the pots, how can she warm them up and cook? Of course, it is all thanks to the transfer of heat. Do you know the ways in which heat is transferred?

Convection

Convection is the way heat is transferred from one area to another when there is a "large movement of matter". It is the movement of huge amounts of an object, taking the heat from one area and placing it in another. Warm air rises and cold air replaces it. The heat has moved.

Therefore, it is the transfer of heat by motion of objects. When an area of hot water rises to the top of a pot and gives off energy, that is convection. Another example is, warm air in the atmosphere rising and giving off energy. The thing to remember is that the object moves.

Radiation

Heat transfer by radiation happens when a temperature gradient exists and the transfer of energy happens without a conductive medium. That means, there is no matter there for the heat to pass through.

Radiation is the energy carried by electromagnetic waves (light). Those waves could be radio waves, infrared, visible light, UV or Gamma rays. Radiation is usually seen in the red and infrared sections of the EM spectrum.

In addition, if the temperature of an object doubles (in Kelvin), the thermal radiation increases 16 times. Therefore, if it goes up four times, it increases to 32 times the original level.

Scientists have also discovered that objects that are good at giving off thermal radiation are also good at absorbing the same energy. Usually the amount of radiation given off by an object depends on the temperature. The rate at which you absorb energy depends on the energy surrounding you.

Conduction

This is a situation where the heat source and heat sink are connected. As we discussed before, the heat flows from the source, down the temperature gradient, to the sink. It is different from convection because there is no movement of large amounts of matter. The source and the sink are connected.

Conduction is special as it needs more free energy than the others, for transferring thermal energy. If you touch an ice-cream cone, the ice-cream heats up because you are a warmer body. If you lie on a hot sidewalk, the energy moves directly to your body by conduction.

When scientists studied good thermal radiators, they discovered that good thermal conductors are suitable for conducting electricity. So when you think of a good conductor, think of gold, silver and platinum.

Janani Amarasekara