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Power loss and Energy loss

Discussion in 'General Electronics Discussion' started by Integrator741, Oct 9, 2013.

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  1. Integrator741


    Jun 16, 2013

    I was doing some examples in my electronics book and one showed me how to calculate efficiency of a motor. Output power/Input power. When they found it, they calculated the power loss and energy loss per minute. Can somebody please explain me the difference between "Power loss" and "Energy loss"

    Thank you :)
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The difference is that one is an amount and the other is a rate.

    Power is energy per unit time.
  3. Integrator741


    Jun 16, 2013
    "The power loss in the motor is 24.6 - 20.944 = 3.656 kW and energy loss per minute = 3.656*60 = 219.36 kW s"

    So the amount of lost power is 3.656 kW and my rate of loosing the power is 219.36 kW per sec?

    Does that mean I loose 219.36 kW every second when i use that motor? Or I misunderstand something?
  4. BobK


    Jan 5, 2010
    Watts is a measure of power. Which is energy per second. You do not lose Killowatts every second, you lose KiloJoules every second. If you are losing power at a rate of 1W then you are losing 1 Joule of energy every second.

  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Power (watts) is a measure of the rate of energy flow. When you turn on a 60W light bulb, it converts energy from electrical energy to light and heat at a rate of 60 joules per second. Every second that the light bulb is turned on, 60 joules of energy is converted from one form to the other. So the wattage tells you how quickly energy is converted; it measures the rate of energy flow.

    I'll try a few examples.

    A 1 kW electric heater converts electical energy into heat energy (and a bit of light) at a rate of 1000 joules per second. If you run it for one hour (which is 3600 seconds), you have used 1 kWh (one kilowatt-hour) of energy, which is 3,600,000 joules, or 3600 kJ (kilojoules). Here where I live, 1 kWh (3600 kJ) of electrical energy costs about 25 cents.

    So a 1 kW electric heater costs 20 cents per hour to run; it uses energy at a rate of 3600 kJ per hour.

    If you have a 100W motor that's 90% efficient, that means it consumes 100 joules of energy for every second that it runs, and converts 90 of those joules into mechanical energy. The remaining 10 joules that it consumes every second are converted into other forms of energy - heat, mainly, and vibration/sound. So it is wasting 10 joules per second (10 watts) of energy.
  6. Integrator741


    Jun 16, 2013
    Thank you! Now i get it :)
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