# Heat Transfer, Conduction, Convection and Radiation

### Transfer of Heat:

Heat is a form of energy. It cannot be lost according to the first law of thermodynamics; rather it will be transferred. The first and second law of thermodynamics states that:

Energy can neither be created nor destroyed but can be transformed or converted from one form to another

1st Law of thermodynamics

In the course of energy transfer, some energy is lost in the form of heat to the surroundings

2nd Law of Thermodynamics, in respect to food chain in biology

Heat transfer indicates how heat flows between two systems of varying temperature. Heat will always be transferred from the region of higher temperature to the region of lower temperature. Please read our article on the concept of heat and temperature here.

Now, let’s understand how the particles of matter move before we delve into the transfer of heat.

Matter is composed of particles. Their constituent’s particles can either vibrate in a fixed position or translate (move from one point to another) or rotate. This movement of the particles of matter is a function of the kinetic energy present in them. (Think of kinetic energy as energy in motion). We can thus define temperature as the measure of the average kinetic energy possessed by the particles of matter. An increase in particles vibration, translation or rotation will imply an increase in temperature while a decrease in particles vibration, translation or rotation will mean a decrease in temperature. You may read our article on matter here.

Heat can be transferred through:

1. conduction
2. convection

• ### Conduction

Let's explain heat transfer by conduction through this simple experiment which you may carry out at home.

Step 1: Boil 3 cups of water in a pot. When the water is very hot, put out the gas.

Step 2: Get another pot and fill it with 3 cups of cold water.

Step 3: Get an iron rod and make both ends of its edge contact both the hot water and the cold water.

Observation: When we carry out the above procedure, it will be observed that the temperature of the iron rod will increase and heat will be transferred into the cold water from the iron rod. Recall that iron is a good conductor of heat and electricity. Please read our article on properties of metal here. The movement of heat energy will follow the path below.

Hot Water >> Iron Rod >> Cold Water

Note: It is only the transfer of heat energy that brought about an increased temperature in the iron rod and then, the cold water; nothing more, nothing less. Below are some notable points:

• Heat is the only factor that escaped from the warm water.
• Heat is the only factor that was gained by the iron rod.
• Heat is the only factor that was gained by the cold water.
• With regards to the iron rod, the heat gain will increase its particles vibrational kinetic energy.
• With regards to the cold water, the heat gain will increase its particle translational kinetic energy.

This heat transfer will continue until equilibrium is achieved; that is, both of the water and the iron rod will eventually have the same temperature.

• ### Convection

Convection is a process of heat transfer from one point to another through the movement of fluids. A conductor or metal is not involved here. Convection explains the transfer of heat in liquids and gases. The boiling process of water occurs through convection. Initially, the water molecules inside the pot possesses equal kinetic energy. When placed on the gas cooker, the water molecules closest to the bottom and margins of the pot begin to move or translate as a result of heat transfer by conduction from the metallic pot (since the pot is a solid and a conductor). As a result, the water molecules (closest to the bottom and margins of the pot) becomes hot. They expand as a result and become even lighter, thus gaining an increased ability to move (an increase in their average kinetic energy). They move to the top and center of the pot displacing the water molecules that were initially there to the bottom as a result of the difference in density of both molecules.

Note: The water molecules at the top and center of the pot have a higher density since they are farther away from the bottom and margins pot. Also, they have a lower kinetic energy when compared to those at the bottom and edges. They are therefore displaced downwards (since they have higher density and lower kinetic energy) by the water molecules at the bottom and margin of the pot which had now translated upwards and inwards. This continues until the entire water molecules attain the same kinetic energy. At this point, equilibrium is said to be achieved. If the water molecules are still left inside the pot, all will be converted to a gaseous state as they eventually evaporate. (The pot becomes empty by now).

In a liquid, the heat energy produced by the particles of higher kinetic energy stimulates the liquid particles of lower kinetic energy (to an increased kinetic energy or translational motion) until equilibrium is achieved.

It is important to understand that convection is the main method of heat transfer in water and air.

With regards to convection in air particles; if will put on the heater (assuming) it’s on the floor, the kinetic energy of the air particles around the floor region (closest to the heater) will be increased. The particles of this hot air move upwards and sideways, mixing with the cooler air in these regions. The net result of this rising hot air is the transfer of heat energy from a higher region to a lower region.

Note: Convection is achieved by the translational and rotational movement of matter (translational and rotational movements are involved in liquids and gases respectively); which results in the transfer of heat energy. Conduction is NOT usually achieved by the vibration movement that is present in solids.

The term radiate in English mean to “to spread out from a central point”. For instance, the light of the sun radiates to the earth. Also, a lighted bulb will radiate its light in a dark room. One reason why some people put out their bulbs before they go out is to simply conserve energy. Leaving on the bulb will bring an extra heat into the room where it’s present. Recall that the bulb will possess a large amount of heat energy (becomes hot) if left on for a long period of time. Since energy cannot be lost, it will be radiated to the room where it’s present. The energy transfer goes thus:

Electrical Energy >> Light Energy >> Heat Energy

Note: In radiation, the heat energy is transferred by electromagnetic waves. In fact, the heat energy which the earth receives from the sun is as a result of electromagnetic waves travelling through space. Please read our article on layers and composition of the atmosphere here.

All objects (televisions, phones, phones, computer and any other object) radiate energy in the form of electromagnetic waves. The rate at which this energy is released is directly proportional to the Kelvin temperature (T) raised to the fourth power.

• ##### Radiation rate = k•T4

The hotter an object is, the more its ability to radiate heat energy. The radiated energy is a collection of a range of wavelength referred to as emission spectrum. The wavelengths within the spectra of the emitted radiation will decrease when the temperature of an object increases; and as a result, hotter objects will usually emit a shorter wavelength at a higher frequency.

Alfred Ajibola is a Medical Biochemist, a passionate Academician with over 7 years of experience, a Versatile Writer, a Web Developer, a Cisco Certified Network Associate and a Cisco CyberOps Associate.