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What is the difference between voltage drop and wattage?

  1. Oct 11, 2009 #1
    Voltage drop is how much energy a device uses. Wattage is the rate of energy transferred into the device per unit of time. Aren't both essentially the same thing? Or am I just confused?
  2. jcsd
  3. Oct 11, 2009 #2


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    Staff: Mentor

    Voltage drop is determined by the resistance of the device (not addressing complex impedance, which is not part of your question). It is totally determined by the resistance of the device, and how much current is passing through it:

    V = IR

    Voltage drop is not energy per se. Voltage is a measure of energy stored, given some storage medium, like capacitors for example:

    E = 1/2 C*V^2

    But in general, voltage is like a potential energy, with the effects of how that energy is expended depending on the delivery and consumer mechanisms.

    Not sure that helps, though. Maybe post a follow-up question?
  4. Oct 12, 2009 #3


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    Gold Member

    The voltage drop across a resistor multiplied by the current through a resistor will tell you the power consumption of that resistor. Wattage is the units of power:

    P = VI

    So V alone is not enough to tell you the wattage.
  5. Oct 12, 2009 #4
    Another way to think about the difference between voltage and power (wattage) is to consider that across ANY size resistor plugged into a household outlet, for example, the voltage drop across that resistor will be the same... about 120v in the US.

    But the power consumed varies considerably according to P = IE = E2/R

    So when you double the size of the resistor you half the power consumed...yet the voltage drop remains fixed.....
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