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Electric potential difference vs. voltage drop

  1. Mar 3, 2014 #1
    Are these the same thing? Or does the electrical potential difference refer to the voltage source only?
     
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  3. Mar 3, 2014 #2

    Simon Bridge

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    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. Mar 4, 2014 #3
    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. Mar 4, 2014 #4

    NascentOxygen

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    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. Mar 5, 2014 #5

    Simon Bridge

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    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?
     
  7. Dec 29, 2015 #6
    I want a correct difference between voltage and potential difference in practical
     
  8. Dec 29, 2015 #7

    NascentOxygen

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    Hi sandhya. :welcome:

    There is no difference between voltage and potential. There is no difference between voltage difference and potential difference.
     
  9. Dec 29, 2015 #8

    sophiecentaur

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    Fwiw, I would say that the term "Voltage Drop" always implies dissipation or transfer of energy from the circuit. Energy sources 'add' PD to a circuit, they don't 'drop' it.
     
  10. Dec 30, 2015 #9

    anorlunda

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    I believe that the source of confusion for many beginners is trying to imagine voltage "at a point" as an absolute. If we remind them it always takes two points to measure or define voltage or potential differences, that is the best way to explain it.

    All motion is relative. All voltages are differences.
     
  11. Dec 30, 2015 #10
    POTENTIAL refers to the energy of an electric charge in an electric field (or a mass in a gravitational field)..these are physics CONCEPTS.
    The VOLT is a unit of measurement of electric potential. I VOLT means 1 Joule/coulomb (no special name for the Gravitational unit but it is 1 Joule/kg)
    A potential difference (pd) refers to dissipation of energy, ie in a resistor.
    A emf refers to a source of energy, i.e a battery etc
    Energy sources do not 'add pd to a circuit'..(there is no pd if there is no flow of charge)
    Energy sources provide an emf.
     
  12. Dec 30, 2015 #11

    epenguin

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    Wouldn't it be true to say that 'voltage' is basically slang for potential? Although the slang became very widely accepted, in fact many people don't realize that it is that. Although 'amperage' has been very frowned on and 'wattage' not quite as much? That 'voltage' is being slowly pushed out of scientific use, but we can't get rid of it that fast because it would be dangerous to the public if we substituted Danger High Voltage signs with High Potential ones?

    Then the difference between potential and emf has always baffled me. Would emf be the same in many contexts as 'open circuit voltage' - er, potential?
     
  13. Dec 30, 2015 #12
    "Then the difference between potential and emf has always baffled me"..... I hope that, for the good of physics, you are not confused about the difference between emf and pd !!. For the general public what is wrong with 'DANGER KEEP OUT' ... no physics knowledge required to understand this message !!
     
  14. Dec 30, 2015 #13
    I do agree with the reference to 'amperage'....who would ever talk about 'amerage difference'....AMP.and VOLT are units..why refer to 'voltage difference'.....unless you are very confident ...you would never refer to 'amperage difference'...would you ??
    What is wrong with'Potential difference in volts' !!!!!
    EMF in volts ????
     
  15. Dec 30, 2015 #14

    epenguin

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    I am probably beyond the stage where I can do much harm to physics or wreak the harm done to me.
    I checked with wiki and it said emf and Open circuit potential are the same thing so I seem to have worked something out nevertheless. But perhaps you woud not say 'open circuit potential' in all circumstances where you say emf?
    Signage has improved and you now have skulls and crossbones and lightening flashes but there will surely be people who don't get these. Well not the first time and there isn't a second.

    At school we were told 'amperage' was very bad but no one turned a hair at 'voltage' - that was the term used in lessons mostly (capacitors were still condensers then mynah mynah). I am sure you will find old textbooks, especially for electricians and technicians, and engineering journals full of this terminology, devices like voltage stabilisers, laws like KCL and KVL not KPL, there is even quite a lot of this at this site. And it has always been admitted that emf is not a good term really! Open circuit potential' OTOH is really suggestive and tells you what is meant.
     
  16. Dec 30, 2015 #15

    jim hardy

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    Language gets so confusing..... because we all take shotcuts


    I was taught in high school that voltage is potential difference
    but what was potential ?
    The concept of work done moving against a field just didnt seem to relate .

    What clarified it for me was when concept of absolute potential clicked in place -
    ' the work done in bringing a unit of charge from infinity to wherever you are.....'
    i could just imagine myself traveling from Alpha Centauri to Miami Central High School with a bucketful of charge and tabulating the force encountered every centimeter...

    Nobody is going to go out to infinity and hold a voltmeter lead there for you to make that measurement

    so

    we have to settle for measuring the difference in absolute potentials between two points closer to us, and we call that voltage,
    We do not know the absolute potential of either point and cannot measure it
    but their difference is readily measured by a voltmeter with two leads of finite length.

    'Difference in absolute potentials' shortens to potential difference , aka 'voltage'.

    So
    In my world, voltage is always a potential difference between two points.
    We get sloppy in our communication and do not always define those two points,
    one of them is usually a 'circuit common' of some sort which is often called 'ground'
    though it may be or may not be connected to earth .

    If that potential difference exists because of current flowing through a resistance, that's a voltage drop.
    If that potential difference exists because of, let's say the chemistry going on inside a battery, that's not a voltage drop it's just a voltage (or maybe a voltage rise).

    Sorry to be so verbose
    but the price of clear communication is excruciating attention to language.

    Hope mine was clear .. exaggeration is often a useful tool.

    old jim
     
  17. Dec 31, 2015 #16

    Svein

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    A bit of the problem lies with the English language - you tend to re-use words. What you call "voltage" and associate with the unit "Volt", other languages use very different words for the two concepts. Some words for "voltage" in other languages:
    • German: "Spannung"
    • Scandinavian: "Spenning"
    • French: "Tension"
    • Spanish: "Tensión"
    All these words translate back in English as "tension".
     
  18. Dec 31, 2015 #17

    epenguin

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    But Italians nationalistically say 'voltaggio'. :oldbiggrin:
     
  19. Jan 1, 2016 #18
    You are absolutely right Jim...we measure potential difference before we ever know what potential means...'excruciating attention to language' is essential for students. You must know what is meant by 'emf'...'pd' before you can sink into the mire of sloppy language...High tension !!....we all use sloppy language and should be aware of that when we profess to be giving guidance to students.
     
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