The SI unit of power is?

Question

Examoogle · Accepted Answer

J/s (joules per second)

Explanation: Power is defined as the rate of doing work or the rate of energy transfer. In the International System of Units (SI), the unit of work/energy is the Joule (J) and the unit of time is the second (s), making the SI unit of power, the Watt (W), equivalent to Joules per second (J/s).

Working Principle: Power represents the derivative of work with respect to time ($P = \frac{dW}{dt}$). When 1 Joule of energy is expended or transferred in an interval of 1 second, the resulting power is 1 Watt. This relationship holds for mechanical, electrical, and thermal systems, maintaining dimensional consistency across physics.

Key Formulas: $P = \frac{W}{t}$ — Definition of power as work per unit time; $P = V 	imes I$ — Electrical power in terms of voltage and current

Key Points: The Watt is a derived SI unit named after James Watt.; 1 Watt is equal to 1 Joule per second.; Power is a scalar quantity.; In electrical systems, 1 Watt is also equivalent to 1 Volt-Ampere (in DC circuits).

Advantages: Standardized unit for all forms of energy transfer.; Allows direct comparison between mechanical and electrical power ratings.

Applications: Rating electrical appliances and devices.; Measuring the output of mechanical engines and turbines.

Additional Info: kW (kilo-watt) is a decimal multiple (1000 W), not the base SI unit.. Ws (watt-second) is a unit of energy, not power, because it is the product of power and time.. J/h is a unit of power but is not the standard SI unit due to the use of hours rather than seconds.

Answer: B is correct — The SI unit of power is the Watt (W), which is dimensionally defined as one Joule per second (J/s).

Exam Tip: Always verify if a unit is for energy (product of power and time) or power (rate of energy) to avoid common pitfalls in electrical circuit calculations.

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