Electric potential difference vs. voltage drop

  1. Are these the same thing? Or does the electrical potential difference refer to the voltage source only?
  2. jcsd
  3. Simon Bridge

    Simon Bridge 15,474
    Science Advisor
    Homework Helper
    Gold Member

    All voltage drops are potential differences but not all potential differences are voltage drops.

    If you have a voltage ##V_a## at point ##A##
    ...then that is the potential difference between point ##A## and some reference - usually the ground, the negative terminal of the power supply, or "infinity", or something like that, but you are free to pick any reference point you like as long as you are consistent.

    If you also have a voltage ##V_b## at some point ##B: B\neq A##,
    ... then there can be a potential difference between them - just pick one to be the reference point.
    ... usually you put the negative terminal of the voltmeter on the reference point.

    If a positive charge moves from A to B
    ... then it passes through a potential difference of ##\Delta V = V_b-V_a##
    ... if ##V_a > V_b## then ##\Delta V < 0## and it has experienced a voltage drop.

    It is easy to get turned around with this stuff - the terms are sometimes used interchangeably and you are supposed to work out what is intended by context. Normally we will talk about the PD across a component and the "voltage of" a power supply - which may also be called the EMF.

    If the PD across a component is given as a "voltage drop" then the voltage drop is in the direction of the current through the component.
  4. OK. So if we are given a simple circuit with a battery and three resistors, a potential difference could be from the negative to the positive on a battery or across each resistor (in either direction) and a voltage drop will have nothing to do with the battery, but can be taken in only one direction across resistors. Correct?
  5. NascentOxygen

    Staff: Mentor

    I don't think you should try to nail down these terms too rigidly. Suppose you have a battery with a potential difference of 1.5v across its terminals, and you connect the battery along with other components to form the branch of a circuit. The contribution the battery makes to the potential difference across that branch can be regarded as a voltage drop or a voltage rise, depending on the orientation of the battery.

    In casual conversation we usually speak of the voltage drop across a resistor. But in calculations, if you are working your way along a branch towards an increasing voltage level, we could speak of the voltage rise across that resistor and treat it accordingly.
  6. Simon Bridge

    Simon Bridge 15,474
    Science Advisor
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    Gold Member

    I agree with Nascent here:
    Trying to nail down the terms will only lead to confusion later.
    The meaning depends on the context as already described.

    Treat "voltage drop" as an informal useage with no strict definition.
    "potential difference" is more precise - provided the reference point is made clear.
    Expect to have to use metadata and context to work out what people are talking about.
    Do not expect everyone to use precise language.

    Have you done any work on "Kirchoff's Laws" yet?
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