Units of Power and Energy

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Energy and Power
  • Power is the rate of doing work or, equivalently, the rate of using or producing energy. 
    • Power is Energy or Work divided by Time.
    • Power can also be expressed as Force times Velocity,:
      • Power = (F x D) / T = F x (D/T) = F x V.
  • Sentences referring to energy and work:
    • We used 500 Kilowatt-hours of electrical energy last month
    • It takes 600 foot-pounds of work to lift a 100 pound weight 6 feet in the air
  • Sentences referring to power:
    • This 10 watt LED bulb uses electrical energy at the rate of 10 joules per second
    • My portable electric generator has a power output of 1600 watts, that is, 1600 joules per second.
  • Analogies for distinction between energy and power
    • How far you ran (energy) vs how fast you ran (power)
    • How much you ate (energy) vs how fast you ate (power)
    • Energy is stuff.  Power is how fast you make or use the stuff.
Units of Energy
  • Joule (J)
    • force x distance (N x m)
      • 1 joule = 1 newton x 1 meter
    • mass x velocity2
      • 1 joule = 1 kg x 1 (m/s)
  • Foot-pound (ft-lb)
    • force x distance = (lbf x ft)
      • 1  ft-lb = 1 pound-force x 1 foot = 1.356 joules
  • Kilowatt-hour (kW⋅h)
    • power x time = (energy / time) x time = (kW x h)
      • 1 kW⋅h = 1000 watts x (60 x 60) seconds = 3.6 million joules
  • Electron volt (eV)
    • volt x electric charge =  (energy / charge) x charge
Units of Power
  • Watt (W )
    • Energy / time (J/s)
      • 1 W = 1 joule of work done per second
      • 1 W = 1 joule of energy produced or used per second
  • Horsepower (hp)
    • Energy / time (ft-lb/minute)
      • 1 hp = 33,000 foot-pounds of work per minute
      • 1 hp = 745.7 watts
Calculators