Power and Efficiency
Average and instantaneous power in mechanical systems. (Physics › Work, Energy and Power, NEET UG syllabus.)
What is Power and Efficiency?
Time rate of doing work, P = W/t, in watts (1 W = 1 J/s); 1 horsepower = 746 W
Key formula / rule: Power
Key points
- Use average and instantaneous power and compute efficiency as useful output over total input
Common exam trap
Confusing power in watts with energy in joules
Definitions
- Name
Power
- Definition
Time rate of doing work, P = W/t, in watts (1 W = 1 J/s); 1 horsepower = 746 W
- Name
Efficiency
- Definition
Ratio of useful output to input, η = Pout/Pin, always below 100 percent because of dissipative losses
- Name
Exam insight
- Definition
At constant speed against resistance the engine power equals resistive force × speed, so maximum speed follows from P = F v
- Name
Worked example
- Definition
Pump raising 500 kg through 10 m in 20 s delivers 2450 W; drawing 3500 W it is 70 percent efficient
Learning objectives
Use average and instantaneous power and compute efficiency as useful output over total input
Formulae
- Name
Power
- Expression
Pavg = W/t; P = F . v = F v cos(θ)
- Name
Efficiency
- Expression
η = (useful output / total input) x 100 percent
Common mistakes
Confusing power in watts with energy in joules
Using average power where the velocity changes
Keywords
power
watt
efficiency
instantaneous power
average power
horsepower
energy
Practice preview
A car moves on a straight road with a constant velocity of 20 m/s. If the resistive force acting on the car is 1000 N, what is the power developed by the engine?…
medium
What is the definition of power in physics?…
easy
A machine does 1200 J of work in 10 seconds. What is its average power?…
medium