SSC
The SI unit of energy is:
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Answer: Option C — Joule
Detailed Solution
Energy is the capacity to do work. The SI unit of energy, and also of work, is the joule (J), named after the English physicist James Prescott Joule (1818-1889), who showed through careful experiments in the 1840s that heat is a form of energy and that a fixed amount of mechanical work always produces the same amount of heat (the mechanical equivalent of heat). His work helped establish the law of conservation of energy.
One joule is the amount of work done when a force of one newton moves an object through a distance of one metre in the direction of the force: 1 J = 1 N × 1 m = 1 kg m² s⁻². The dimensional formula of energy is [M L² T⁻²]. Energy is a scalar quantity.
Other units of energy and their values in joules:
Calorie (cal): about 4.184 J. It is the heat needed to raise the temperature of 1 gram of water by 1°C. The "Calorie" (with a capital C) used for food energy is actually a kilocalorie (1,000 calories).
Kilowatt-hour (kWh): 3.6 × 10⁶ J; the commercial unit of electrical energy ("unit" on electricity bills).
Electron-volt (eV): 1.6 × 10⁻¹⁹ J; used in atomic and nuclear physics.
Erg: 10⁻⁷ J; the CGS unit of energy.
British Thermal Unit (BTU): about 1,055 J; used for heating and air conditioning in some countries.
Forms of energy include kinetic energy (energy of motion), potential energy (energy due to position or shape), heat (thermal) energy, chemical energy, electrical energy, light energy, sound energy and nuclear energy.
The law of conservation of energy states that energy can neither be created nor destroyed; it can only be converted from one form to another, and the total energy of an isolated system remains constant. For example, in a hydroelectric power plant, the potential energy of water stored in a dam is converted into kinetic energy and then into electrical energy. In a battery, chemical energy is converted into electrical energy.
Why not the other options?
- ✗Watt is the SI unit of power (rate of energy transfer).
- ✗Calorie is a non-SI unit of heat energy.
- ✗Electron-volt is a small non-SI unit of energy used in atomic physics.
Quick tip
Remember: Energy and work = joule (SI, after James Prescott Joule, 1 J = 1 N m); 1 cal = 4.184 J; 1 kWh = 3.6 × 10⁶ J; 1 eV = 1.6 × 10⁻¹⁹ J; power = watt.
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Work, Energy & Power — Preparation
This question tests your understanding of Work, Energy & Power, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers work and its units, kinetic and potential energy, the law of conservation of energy, energy conversions in everyday devices, power and its units, and simple machines. Questions usually ask about energy conversions (a battery converts chemical energy into electrical energy, and an electric motor converts electrical energy into mechanical energy), the unit of power (the watt), and how one horsepower relates to watts (about 746 W). Remember that no work is done when the displacement is zero or perpendicular to the force. Practise the work, energy and power questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.