What is the SI unit of power?
- A.Joule
- B.Watt
- C.Newton second
- D.Kilowatt hour
Correct answer
Explanation
The correct answer is B, watt. Power is the rate of doing work, that is work divided by the time taken, so its unit is the joule per second, which is named the watt after James Watt. A machine of one kilowatt does one thousand joule of work every second.
Option A, the joule, measures work or energy, not the rate at which it is delivered. Option C, the newton second, is the unit of impulse and of momentum. Option D, the kilowatt hour, is a unit of energy in spite of its name, because it is a power multiplied by a time; it is the commercial unit of electricity and equals 3.6 million joule. The confusion between the watt and the kilowatt hour is deliberate in many papers, so keep the distinction of rate against quantity clear.
Read the full article: Work, Energy and Power: Formulas, Units and Conversions
Practice Questions
View allWhat is the SI unit of work?
- A.Newton
- B.Joule
- C.Watt
- D.Pascal
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Work is the product of force and displacement, so its unit is the newton metre, which is given the name joule; one joule of work is done when a force of one newton moves a body through one metre in its own direction.
Read the full explanation →The work done by a force is zero when the angle between the force and the displacement is
- A.0 degrees
- B.45 degrees
- C.90 degrees
- D.180 degrees
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Work equals force into displacement into the cosine of the angle between them, and the cosine of ninety degrees is zero, so a force at right angles to the motion does no work at all.
Read the full explanation →The kinetic energy of a body of mass m moving with speed v is given by
- A.mass into g into height
- B.half of mass into the square of the speed
- C.mass into the speed
- D.mass into the square of the speed
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Kinetic energy is the energy a body has because of its motion, and it grows with the square of the speed, so a body moving twice as fast carries four times the kinetic energy.
Read the full explanation →One horsepower is approximately equal to
- A.100 watt
- B.500 watt
- C.746 watt
- D.1000 watt
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The horsepower is an older unit of power that survives in the rating of motors and vehicles, and one horsepower is taken as about 746 watt, that is roughly three quarters of a kilowatt.
Read the full explanation →The commercial unit of electrical energy is the
- A.Joule
- B.Watt
- C.Kilowatt hour
- D.Erg
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Household electricity is billed in these units, and one kilowatt hour, popularly called one unit, is the energy consumed when an appliance of one kilowatt runs for one hour; in SI terms it equals 3.6 million joule.
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