Heat, Temperature & Thermal Expansion

Heat & Temperature are extremely common words that are used in our day to day lives and yet they are the most misunderstood and confused terms where we end up talking of heat like temperature and temperature like heat. They are interconnected and yet they are different. When heat is added to a substance, its temperature increases while if you remove heat from a substance, its temperature decreases. It is extremely important to realise the difference between temperature & heat. A clear understanding of the concepts can lead to a better understanding of the world around us. 

Heat and Temperature

The total energy of motion of the molecules within a substance is defined as heat. Temperature is defined as the average measure of the energy of the motion of the particles in the substance. Heat is dependent on factors such as the size and speed of the particles in motion and also the number of particles that are energised. Temperature on the other hand is independent of these factors and is only a measure of the rate of exchange of heat energy. It is a number that relates the kinetic energy the molecules of a substance possess, which is directly linked to the energy possessed by the molecules of the substance. Thus, the kinetic energy is directly proportional to the temperature of the substance. Hence if the temperature of the body is doubled, it implies that the energy possessed by the substance has doubled. 

In simpler terms, heat is a form of energy while temperature is the rate of change of heat energy. Say, you have a cup and a thermos full of tea at the same temperature. By the virtue of the number of particles due to the higher volume of the thermos, the heat energy possessed by the thermos is larger than that possessed by the cup even though both of them are at the same temperatures. 

SI Units

Heat is a measure of energy and is measured in Joule. It can also be measured in calories but the SI unit of heat is Joule. It is symbolically represented by the letter Q. 

Temperature is the rate of change of heat energy and its SI unit is kelvin. Though more commonly it is measured in degree Celsius & degree Fahrenheit. It is symbolically represented by the letter T

What Happens when Heat is added?

Let us have a close look at the phenomenon that occurs around heat and temperature. There are two phenomena that occur when we add heat to a substance: 

1. The kinetic energy of the molecules of the substance increases causing a corresponding increase in the temperature of the substance. In simpler terms, heat energy is transferred to the molecules as vibrational kinetic energy which causes the molecules to vibrate with a higher speed implying an increase in temperature which is defined as the rate of change of energy. 
2. The substance can also undergo a phase change, i.e change from solid to liquid phase or liquid to gaseous phase and vice versa. For example, at 100 ° C when more heat is added to water it absorbs the Latent heat of vaporisation and changes its phase to gaseous phase at 100 ° C without a difference in temperature. This is different from the phenomenon when heat is being supplied to water, say 50℃ and the temperature increases to 100℃. This heat absorbed is defined as the specific heat capacity of water. It is extremely important to understand that heat is a form of energy that is absorbed or released while temperature is the measure of the rate of change of this energy.

When an object is heated or cooled the length changes by an amount that is proportional to the original length of the object and the change in temperature of the object. According to Merriam-Webster, thermal expansion is the increase in linear dimensions of a solid or in the volume of a fluid because of change in temperature.

For example, take a steel that just passes through a ring. Now heat the steel ball and raise its temperature. You will observe that the steel ball increases in size and does not pass through the ring. Now let the steel ball cool for some time, you will notice that the steel ball passes through the ring again. This phenomenon is known as Thermal Expansion. 

It can affect the following measurements:

• Pressure (changes in volume and area)
• Torque (changes in radius)
• Flow (changes in volume and area)
• Speed/Velocity (changes in length/distance)
• Energy (changes in length/distance)
• Volume (changes in length, width, and height)
• Area (changes in length and width)

Thermal Expansion occurs in Linear dimensions, in area and in volume.