Concept of Energy

Energy is a fundamental concept in science that governs every activity in the universe. It is defined as the capacity of a body or system to perform work. Energy manifests in various forms, such as mechanical, heat, light, sound, chemical, and nuclear energy, and it plays a critical role in both natural processes and human-made systems.

The ability to store and utilise energy is central to all physical phenomena. In the SI system, energy is measured in joules, while the CGS system uses ergs. Among the various forms of energy, mechanical energy is of special interest in many practical applications. Mechanical energy can be broadly classified into two types:

1. Potential Energy (PE): The energy a body possesses due to its position or configuration.
2. Kinetic Energy (KE): The energy a body possesses due to its motion.

The unit joule is too small and hence is inconvenient to express large quantities of energy. We use a bigger unit of energy called a kilowatt hour (kWh).

Let us say we have a machine that uses 1000 J of energy every second. If this machine is used continuously for one hour, it will consume 1 kWh of energy. Thus, 1 kWh is the energy used in one hour at the rate of 1000 J^s–1 (or 1 kW).

1 kW h = 1 kW × 1 h

= 1000 W × 3600 s

= 3600000 J 1 kW h

= 3.6 × 10⁶ J.

The energy used in households, industries and commercial establishments is usually expressed in kilowatt hour. For example, electrical energy used during a month is expressed in terms of ‘units’. Here, 1 ‘unit’ means 1 kilowatt hour.

The energy of a body is its capacity to do work.

• Energy and work are directly related — when work is done, energy is transferred; doing work decreases energy, and receiving work increases it.
• No energy transfer occurs when the applied force is perpendicular to the displacement (e.g., centripetal force in circular motion).
• Units of energy are the same as those of work:
    - S.I. unit: Joule (J)
    - C.G.S. unit: Erg
    - 1 J = 10⁷ erg
• Practical units of energy:
    - 1 Wh = 3600 J = 3.6 kJ
    - 1 kWh = 3.6 × 10⁶ J = 3.6 MJ
    - 1 calorie = 4.18 J, 1 kilocalorie = 4180 J
• Energy in atomic-scale processes is measured in electron volt (eV), where 1 eV = 1.6 × 10⁻¹⁹ J