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I'm confused about power and voltage.

  1. Sep 26, 2009 #1
    I know power is what causes devices to function in a circuit. What role does the voltage have in a circuit?
  2. jcsd
  3. Sep 27, 2009 #2
    Voltage is like the Force you give the electrons to move through the circuit (hence the name EMF = electro motive force). Technically this is not true, as we now know that the voltage source (i.e. your battery) creates an electric fileld, which leaves the wire and then at your resistors the electric electric field re enters the circuit. You'll learn about this in an electromagnetism course.

    A model that my teacher gave me to understand circuit theory is: consider your wires to be pipes - and inside these pipes you have water and little tiny balls. These tiny balls are your electrons. Your battery source (i.e. your voltage source) is like a pump which increases the pressure that the the water and the little balls experience as they flow through your circuit (pipe) system. Your current is a measure of how many of these little balls whizz through the system - like the flow rate in your pipes. Resistances are like drops in the pressue (you could think that the pipe gets smaller so its harder for the balls to squeaze through).

    More mathematically the current is dQ/dt - the rate of change of charge carriers with time.

    Your Power is like how much pressure you apply to the number of little balls flowing in your fluid. So you could apply a small ammount of pressure to a large ammount of balls (corresponding to a low voltage but a high current) and this would have the same power as applying a large ammount of pressure to a small number of balls (corresponding to high voltage but low current).

    More mathematically the power at a point in the circuit is equall to the current multiplied by the voltage drop or voltage gain:
    i.e: P = VI

  4. Sep 27, 2009 #3


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    Voltage tells you how much work a single electron can do. Electric current tells you how many electrons you use per second. The product (voltage * electric current) tells you how much work per second you can do, which is also called power.

    Water pipe analogy:
    Voltage = height difference between two points of the pipe
    Electric current = amount of water that flows per second
    Power = Voltage * Electric current
  5. Sep 27, 2009 #4


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    And Power is related to the Square of the Voltage.

    P=V 2 /R

    R is the resistance.
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