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How do measurement units affect electrical devices?

  1. Mar 16, 2014 #1
    1.What do watts do and how do they affect an electrical device like a motor?
    2. what do amps do and how do they affect an electrical device like a motor?
    3. what do volts do and how do they affect an electrical device like a motor?
    I understand what these measurment are and how to solve for them but I am not sure what they do. For example, what measurement would determine how long the device can run if it was on a battery?
  2. jcsd
  3. Mar 16, 2014 #2


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    Hi Jewish_Vulcan! May you live to be 120, and prosper! :smile:
    What do those electric units measure? :wink:
    A battery has only a certain amount of charge, sometimes marked on the battery in Ah or mAh (milliamp-hours). That will help you work out how long it will run a particular device.
  4. Mar 16, 2014 #3
    what do you mean by measure? volts measure current times resistance or the potential difference(pressure), watts measure current times volts or how much work can be done over a certain amount of time, amps measure voltage divided by resistance and an amp is the amount of charge in the circuit but I do not know what they do to devices in circuits. For example what would be the different effect of a 12volt battery at 5watts to a 12volt battery at 10 watts on an electric motor. I am asking for some actual examples of what those measurments do.
  5. Mar 16, 2014 #4


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    It is all to do with energy. The unit of energy is the joule.
    An “engine” converts energy between different forms.
    A “motor” is an engine that causes movement.

    The unit of charge is the coulomb. One coulomb = 6.241×1018 electrons.
    The chemistry and capacity of a chemical battery determines how many electrons are available.
    One amp of electric current is defined as a flow of one coulomb of charge per second.

    The unit of power is the watt.
    One watt is defined as an energy flow of one joule per second.

    A motor rated at, say 1000 W, is capable of converting 1000 joules of energy per second into movement.
    The actual power conversion, (consumption), of a motor should not be greater than it's rated power.

    Electric power is the product of voltage by current.
    The ratio of voltage to current is impedance, it shares the unit ohm with resistance.

    Relatively speaking:
    The more current a motor can handle the thicker is the wire and the less winding turns are needed.
    The more voltage a motor can handle, the thicker the insulation and the more winding turns are needed.
  6. Mar 16, 2014 #5


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    I fully do not understand the question. You say you know how to measeure these things, but don't understand what "they do"

    What do amps do? Amps is a measurement of current flow. Do you understand current flow?
    Watts is a measurement of power consumption. How much current was consummed at what voltage. Do you understand power?

    As baluncore said, it all gets down to energy, but you need to understand circuit elements and relationships better to be able to make useful energy calculations. I know that is what you are asking, but there is no simple answer.

    For example, a battery is rated in ma-hours. How many hours will it last for a given ma load. For example, a 1200mah battery will last 10 hours supplying 120ma. The voltage of the battery determines how much power is being delivered at 120ma. For example, a 3.7V battery supplying 120ma is providing 0.44 watts of power.
  7. Mar 18, 2014 #6

    jim hardy

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    @ J_V:
    There's the physics definitions which can get math intensive
    and there's the simplified analogies we use to enable us to become competent at handling electrical devices.

    Get these basic quantities straight in your mind in this order:

    1. What is Charge? Understand what's a coulomb, and what's the charge of an electron.
    2. What is Current ? Understand what's an ampere, and what's the difference between rate of charge movement and rate of electron drift.
    3. What is Voltage? Understand what is difference between an electron volt and a joule.
    4. What is Impedance? Understand difference between QV cross B , Q1Q2/r2 , and IR.
    5. What is a Watt? Understand difference between a watt and a joule and a watt-hour.

    When you've done above you'll see why the British refer to electron tubes as "valves", and voltage as "pressure".
    It's no longer considered academic to use those old fluid mechanics analogies but they do help one stumble up the learning curve.

    When you've done above you have achieved a basic level of competence.
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