Heat - Part IV

How heat is transmitted : We have seen earlier that heat is transmitted from a hotter body to a colder body, till a thermal equilibrium is reached.

In general, heat flows from a hotter to a colder body by one of the three processes :  

1. Conduction
2. Convection
3. Radiation

The net flow of heat can sometimes be through more than one of these processes. Now we will see in details, these processes.

Conduction :  Take a candle and light it. Hold a stainless steel rod or spoon to the flame. You will see that the end of the rod/spoon where you are holding is getting hot. This is the transmission of heat by conduction. The heat is transmitted by one neighboring atom to another, from the hotter end to the colder end. What actually happens is this :  The end of the spoon that touches the flame, the atoms or molecules get heated up and start vibrating more. The electrons with high kinetic energy get dislodged and start moving around. The vibration is transmitted or transferred to the neighboring atoms or molecules. This phenomena is transferred to subsequent atoms (or molecules). More electrons are released which move towards the colder end. Thus the heat is ìconductedî.

Conduction of heat is defined as the mode of transmission of heat, through a medium, without actual migration of the atoms or molecules of the medium.

For conduction of heat, a material medium is necessary. Conduction occurs mostly in solids. This can be understood if we see how solids, liquids and gases are bound to their respective atoms or molecules. Solids are tightly bound. The atoms or molecules cannot migrate from one site of the solid to the other. Although,  the electrons can do so. In liquids and gases, the atoms or molecules are not that tightly bound, so when the heat has to be transferred or transmitted, the atoms and molecules themselves move. This is called convection of heat, which we will see in the next section.

In the experiment of heating a spoon, if instead you had used a glass rod, you will notice that the end of the glass rod that you are holding, does not become hot at all. This shows that heat cannot readily flow in glass. Glass is a bad conductor of heat. Other bad conductors of heat are rubber, wood, cork, paper, bakelite, etc. You will notice that the handles on cooking pans are made of a plastic called bakelite. That is why you can hold a hot pan. Water is also a bad conductor of heat. Most of the metals are good conductors of heat. This is because they loose electrons which can freely move around and conduct heat (as well as electricity). Most of the ionic compounds are relatively good conductors of heat. Most of the covalently bonded compounds are bad conductors of heat.

Radiation :  Keep a vessel of water for boiling. Hold your hand close to the vessel. You will feel the warmth of the vessel. Similarly, if we sit in the sun, we feel warm. How is the heat transmitted in this case? The process cannot be conduction, as there is no direct contact. The process cannot be convection because there are no material particles moving from the heat source towards the hand.  The invisible way of transmission of heat is known as radiation.

Radiation is defined as the mode of transfer of heat in the form of ëelectromagnetic wavesí[1].  For this no material medium is required.

Heat from the sun is the best example of radiation. The sunís rays reach the earth without having any material medium in between the earth and the sun. The electromagnetic rays carry energy from the sunís surface to the earth and heat up the earth and its atmosphere.

[1] Light is a form of electromagnetic wave. When electrons are excited to higher orbitals, they can return to lower orbitals by emission of electromagnetic waves. This we will study later. Suffice to know that electromagnetic waves carry energy and do not need any medium for traveling from one place to the other.

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