Heat, Internal Energy and Work

Introduction

Heat, energy, and work are the core elements of thermodynamics. They all have innumerable uses in our daily lives and various scientific processes. Therefore, it is essential to understand the underlying concepts. There are mainly two types of Energy – Potential Energy and Kinetic Energy. They are further divided into various forms, one of which is Heat Energy. Heat is often misunderstood to be the same as temperature. In the upcoming sections, we will be discussing both heat and temperature and the difference between the two. Apart from that, we’ll also read about thermal expansion caused by the change in heat and temperature, along with internal energy and the work done by a system.

Heat

Heat can be defined as kinetic energy produced by the random motion of matter, i.e., vibratory and rotary motions of molecules. Heat tends to flow from higher to lower temperatures, making it useful in various mechanical and chemical processes. The amount of heat flow is determined by mainly three factors:

• Mass of the substance (m)
• The difference in temperature between two objects (ΔT)
• Nature of the substance

Based on this, we can conclude that Q ∝ mΔT or Q = cmΔT, Where c is proportionality constant, it is known as the specific heat capacity and is determined by the nature of the object. The SI unit of heat is Joule (J).

Temperature

Heat and temperature are often used interchangeably, but this conception is wrong. Heat is the total kinetic and potential energy of the molecules in an object. On the other side, the temperature is defined as the average kinetic energy of the molecules. Therefore, it is denoted by the letter T. It is vital to understand that, unlike heat, it does not tell about the total energy of a thermodynamic system. A body with a higher temperature doesn’t need to have higher heat energy. For instance, an iceberg has a lower temperature than a burning matchstick, but the total heat energy in the iceberg is higher than that in a burning matchstick. Temperature is an intensive property and is not affected by the quantity of the matter in the context.

Thermal Expansion

When an object is exposed to heat or a temperature change, it tends to change its dimensions in terms of shape, size, area, and volume. It is known as thermal expansion. The leading cause of this is the change in the kinetic energy of the molecules. Thermal expansion is broadly divided into three types:

• Linear Expansion

When there is a change in length caused by heat, it is known as linear expansion. The formula for linear expansion is, L/L_o=LT where Lₒ = Original length, L = Expanded length, L = Coefficient of length expansion, ΔT = Difference in temperature and ΔL = Change in length.

• Area Expansion

When there is a change in the area caused by the change in temperature, it is known as area expansion. The formula for area expansion is, A/A_o=AT where Aₒ = Original area, A = Expanded area, A = Coefficient of area expansion, ΔT = Difference in temperature and ΔA = Change in area.

• Volume Expansion

When there is a change in volume due to temperature change, it is known as volume expansion. The formula for volume expansion is, V/V_o=VT, where Vₒ = Original volume, V = Expanded volume, V = Coefficient of volume expansion, ΔT = Difference in temperature and ΔV = Change in volume.

Internal Energy

The sum of total kinetic energy and potential energy present inside a system is its internal energy. It is denoted by the letter U, measured in joules (J). It depends on the state of the molecules and their random movements. We can increase the internal energy by increasing the supply of heat. The formula for internal energy is given as, ΔU = Q + W, where ΔU = Internal Energy of the system, Q = Heat supplied to the system, and W = Work done by the system.

Work

In thermodynamics, everything is divided into two parts – system and surroundings. While the system refers to the object of interest, surroundings refer to everything else. The total energy exchanged between the system and the surroundings is considered the work done. The amount of work done is affected by various factors such as pressure, volume, temperature, etc. The work can be done by the system and on the system, depending upon the path and the system’s initial and final state. The work done on the system is negative, while the work done by the system is positive. The amount of work done is calculated as, W = – ∫ P.dV

Conclusion

As mentioned in the beginning, heat and energy have various applications in our everyday lives and various physical, chemical, and biological processes. Hence, it is essential to study them. Heat and temperature are often confused with each other. However, this lesson explains the two and draws a clear line between them. We also learned about the thermal expansion of objects and the factors affecting them. Furthermore, we got an insight into the internal energy of a system and the work done. You can learn further about the relationship between internal energy and enthalpy.