Be it cooking, use of insulators, temperature measurement by thermometer, protection of body from cold, hot fomentation or any other such phenomenon, it all involves transfer of heat.
Heat is also transferred in boiling water or by a room heater to the surrounding air or by sun’s rays to our body. But, in these cases, heat is transferred by different ways which we will discuss in next blog.
Now, what is heat?
Heat is a form of energy. It makes the substance hotter. When heat is removed from the substance, the substance cools down in sometime, or we can say that its temperature becomes lower or it decreases.
Now, how we can define temperature?
Can we make out correctly always, by touching a substance, whether the substance is hot or cold? Most of the times we can. But sometimes, the sense of touch can be misinforming us, like when we dip our hand in very cold water and then dip it in lukewarm water, we will feel the lukewarm water to be quite hot and vice versa.
So, temperature can be defined as being an authentic or dependable measure of hotness or coldness of a substance.
Man has devised an instrument for measuring temperature called THERMOMETER. Thermometer gives an exact degree of hotness or coldness ie. the temperature of a substance.
Mercury, which is a liquid metal, is used in thermometer because heat is transferred quickly from the substance to the mercury by touching the mercury bulb in thermometer, where the level of mercury shows the temperature of the substance on the graduated scale (Celsius/Fahrenheit scale) encompassing the mercury.
So, by now, we know that when heat is transferred (given to a substance or taken away from the substance) a substance becomes hot or cold.
So, how does this transfer of heat occur?
Let’s take an example
We dip a room temperature stainless steel (metal) spoon in a cup of hot coffee. After sometime, we find that the steel spoon also becomes hot ie. its temperature also rises. So, it’s obvious that heat contained in hot coffee has been transferred to cold spoon.
Similarly, when we keep a steel saucepan (metal) on flame, the pan becomes hot. So, heat from hot flame has been transferred to the cold pan.
Now, if we remove the hot pan from flame and keep it aside, the pan cools down slowly. This shows that heat has been transferred from hot pan to the colder surroundings.
So, we can say that heat flows from hot substance to cold substance or from a substance at high temperature to a substance at lower temperature.
Here, it is important to know that heat may be transferred from one substance to another or within the same substance.
Example:- When we boil an egg in water, the heat is transferred in the following manner:-
Hot flame ➡️ saucepan ➡️ water ➡️ outer portion of egg ➡️ inner and innermost portion of egg
The transfer of heat within the inner portions of egg is the heat transfer within the same substance.
Is the heat transferred in all substances – solid, liquid and gases, by the same phenomenon?
The transfer of heat from a hot solid substance to a cold solid substance or within the same solid substance, without the movement of substance, is called Conduction. So, we can say, that the heat is transferred in solids by the phenomenon called CONDUCTION.
In liquids and gases, this transfer of heat takes place by the phenomenon called Convection and in the absence of medium (vacuum), this occurs by the phenomenon called Radiation (these two phenomena will be discussed in my next blog).
Now, a very tickling question- how the heat is transferred in solids without the movement of substance?
We know, from our previous knowledge that in solids, particles (atoms/ molecules) are very tightly packed and they remain fixed at their position ie. they do not move.
When these particles get heat energy, they vibrate or wiggle at their position (there is no actual movement of particles from hotter end to colder end during heat transfer by conduction).
Due to this vibration, the particles possess kinetic energy (KE). The more the heat energy given, the more is the vibration of particles and more is the kinetic energy that they possess and so, consequently more is the temperature that the particles attain.
The vibrating particle causes the neighbouring particle to vibrate and the chain continues causing heat energy to flow from particle to particle. The heat is transferred to another substance (solid) till a thermal equilibrium is attained ie. both substances (solids) acquire same temperature.
Good conductors and bad conductors
The substances that conduct heat easily and quickly are called good conductors or simply CONDUCTORS. All metals and their alloys are good conductors of heat. Metals like copper, silver, aluminium, and metal alloys like brass, steel, stainless steel are very good conductors.
The substances that do not conduct heat easily are called bad or poor conductors or INSULATORS. Plastic, cloth, wool, wood, paper, clay, cork etc are poor conductors of heat.
Also, in general, liquids are poor conductors and air is a very poor conductor of heat.
Uses of good and poor conductors of heat
1. The cooking metal utensils, being good conductors, transfer heat very quickly to the food being cooked.
2. Plastic or wooden handles of utensils like saucepan, frying pan, electric iron are poor conductors, so help in easy handling of these hot objects.
3. Woollen clothes keep us warm because the fibres of wool trap air, so stop flow of heat from our body warm body to cold surroundings. Since our body doesn’t lose its heat, so we feel warm.
4. Fur of animals and feathers of birds trap air and keep them warm.
5. Use of hollow bricks in construction of houses traps air in the walls and so protects from extreme cold or heat.
Some Questions For Readers
Q1. Which will keep us warmer- wearing a single thick layer of clothing or wearing more thin layers of clothing?
Q2. Which will feel colder in winters- a metal utensil or wooden utensil and why?
Q3. Why copper base below stainless steel utensil is better?
Q4. Why roasting a chicken becomes difficult if we take care that outside doesn’t get overcooked?
Q5. Is food a poor conductor of heat?