SSC
What is the SI unit of power?
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Answer: Option C — Watt
Detailed Solution
Power is the rate of doing work, or the rate at which energy is transferred or converted. It tells us how fast work is done.
Power (P) = Work done (W) / Time taken (t), or P = E/t.
The SI unit of power is the watt (W), named after the Scottish engineer James Watt (1736-1819), who greatly improved the steam engine in the 1760s-1770s, a key invention of the Industrial Revolution. One watt is the power of an agent that does one joule of work in one second: 1 W = 1 J/s. Power is a scalar quantity. Its dimensional formula is [M L² T⁻³].
Larger units are often used:
- 1 kilowatt (kW) = 1,000 W.
- 1 megawatt (MW) = 10⁶ W, used for power stations.
- 1 gigawatt (GW) = 10⁹ W.
Horsepower (hp) is an older unit of power, still used for engines of vehicles and pumps. It was introduced by James Watt to compare the power of steam engines with that of horses. 1 hp ≈ 746 watts.
Power can also be expressed as force multiplied by velocity: P = F × v.
In electricity, power is calculated as P = V × I (voltage × current), or P = I²R, or P = V²/R. Electrical appliances are rated by their power: for example, a 60 W bulb, a 1,000 W iron or a 1.5-tonne air conditioner of about 1,500-2,000 W.
It is important not to confuse power with energy. The kilowatt-hour (kWh), used on electricity bills and commonly called a "unit", is a unit of energy, not power. 1 kWh is the energy consumed by a 1,000-watt appliance running for one hour: 1 kWh = 1,000 W × 3,600 s = 3.6 × 10⁶ joules.
For example, a 100 W bulb used for 10 hours consumes 1 kWh (1 unit) of electrical energy.
Why not the other options?
- ✗Joule is the SI unit of work and energy.
- ✗Newton is the SI unit of force.
- ✗Horsepower is a non-SI unit of power (about 746 W).
Quick tip
Remember: Power = work/time, SI unit watt (1 W = 1 J/s, after James Watt); 1 hp = 746 W; kWh (unit on electricity bill) = energy = 3.6 × 10⁶ J; electrical power P = VI.
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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.