about emf and voltage

Discussion in 'General Electronics Chat' started by uzair, Dec 30, 2007.

  1. uzair

    Thread Starter Active Member

    Dec 26, 2007
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    Its a silly question but it is confusing me. So, i want to ask that what is the differnce between emf and voltage. And also tell why is the voltmeter used, to measure emf or to measure voltage?
     
  2. acecom7

    New Member

    Dec 30, 2007
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    EMF stands for electromotive force and as for voltage they are one in the same. Technically speaking, voltage is just a form of electron flow and current pushes those electrons across wires; hence the force. Also, Voltmeters are used to measure voltage to make sure that any major components you have on a board are not over-exposed to voltage because your components will burn so you have to take precaution when connecting any type of board to a power supply.
     
  3. BlueDevry

    Member

    Dec 26, 2007
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    Voltage is another name for potential difference and is the cause of electron flow. EMF or electromotive force is the same as voltage but is somtimes used to describe a voltage source on a schematic .
     
  4. uzair

    Thread Starter Active Member

    Dec 26, 2007
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    If i call voltage as potential difference then what do you say about it.And if they are "one in the same" then why do we use different names for them????
     
  5. Dave

    Retired Moderator

    Nov 17, 2003
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    In all essence they are one and the same thing, EMF is the energy per unit charge or in other words the voltage. In context it typically refers to the "force" induced by a device as a voltage, for example you would say a battery is a source of EMF.

    Some sources cite EMF as being a touch outdated.

    Dave
     
  6. Dave

    Retired Moderator

    Nov 17, 2003
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    Beaten to the answer!

    It is mostly historic. Voltage is the potential difference, whereas your wouldn't describe it as something that acts as an EMF (a source of voltage), although if you consider Maxwell's work, he claimed that potential differences can act as EMFs.

    Dave
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    Isn't it true that Volts is a unit of measure used when referring to an emf. A volt is defined as the emf developed across 1 Ohm with 1 Amp of current flowing through it.

    hgmjr
     
  8. uzair

    Thread Starter Active Member

    Dec 26, 2007
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    "Potential difference is relative between two points", it is quoted it in the first chapter i think.Keeping this in view can we say the same for emf? And if emf is just related to a source,then is it right to say that voltmeter measures emf?
     
  9. Dave

    Retired Moderator

    Nov 17, 2003
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    Sure is, an EMF source is measured in volts.

    Yes, but remember to keep it in context. If you put the VM in parallel around the EMF source you will measure the voltage of the source.

    Dave
     
  10. BlueDevry

    Member

    Dec 26, 2007
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    You could also say that 1 volt = 1 Joule/1 Coulumb. which is another way of saying emf.
     
  11. uzair

    Thread Starter Active Member

    Dec 26, 2007
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    Sir i was not beaten to your reply.My internet has a frustratingly slow speed.
     
  12. Dave

    Retired Moderator

    Nov 17, 2003
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    Sorry, I meant someone answered your question before I did (I was too slow in giving an answer!)

    Dave
     
  13. thingmaker3

    Retired Moderator

    May 16, 2005
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    EMF is indeed relative between two points. A voltmeter always measures EMF between the two points the leads are connected to.

    If we use a meter to measure the EMF across each resistor in the following: http://www.allaboutcircuits.com/vol_1/chpt_5/2.html

    ...we will read 9V of EMF across the battery, 1.5V of EMF across R1, 5V of EMF across R2, and 2.5V of EMF across R3. We would read 6.5V of EMF across R1 and R2 together, and 7.5V of EMF across R2 and R3 together.
     
  14. uzair

    Thread Starter Active Member

    Dec 26, 2007
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    Its ok sir.Thanks to all for clearing the point.
    Cheers!
     
  15. Dave

    Retired Moderator

    Nov 17, 2003
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    Not to make this more confusing, but the question arises: why is the electromotive force not measured, like other forces, in Newtons - strangely it obeys Newtonian Laws of Physics. As an interesting point if you look at Maxwell's 1865 paper, where he introduced his infamous equations, he referred to the electromotive force as what we now call the electric-field strength measured in Newtons/Couloumb (Force/Charge) - perhaps this would have been a better convention to follow other than the one we currently have.

    Dave
     
  16. studiot

    AAC Fanatic!

    Nov 9, 2007
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    You can’t understand these terms in isolation as they are intimately bound up with each other.

    A conductor, which has no source of electrical energy of its own, can acquire energy by virtue of its geometric position in space or its connection to an electrical circuit.
    We measure this as Potential Difference (PD) in voltage between the conductor and some reference point in the circuit or source. If we change the circuit or the geometry the potential acquired by the conductor will change.
    For example a metal conductor, placed between two charged plates will acquire a potential difference between itself and one or other of the plates. Change the charge on the plates or move the conductor and its PD will alter. There is no requirement for charge (current ) to actually flow.
    In electronics we refer to devices that have no energy source as passive and the voltage across them as the PD across them. If we took the circuit away the PD would decay to zero. A humble resistor is a good example of such a component.

    An EMF is a physical source of electrical energy. It is measured in volts and may be AC or DC.
    As such it contains its own internal source of electrical energy that is independent of the rest of the circuit or of geometry.
    We say that there is a potential difference of whatever volts across the EMF, or that the EMF supplies a voltage of whatever volts.
    For example a battery can be deployed as an EMF. The voltage between the battery terminals does not depend upon their distance apart, but upon the internal workings of the battery. Equally a 1.5 volts battery will always have a potential difference of 1.5 volts across it, regardless of how we connect it within the circuit. If we took the circuit away the battery voltage (PD) would remain at 1.5 volts.
     
  17. recca02

    Senior Member

    Apr 2, 2007
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    Though people tend to think of EMF and voltage as different its something i dont think we even need to bother about. Till now i have been doing fine referring to all sources such as
    generator, batteries as voltage source and rating them in the same units besides i dont remember ever mentioning 'EMF measured by voltmeter'(even if i was i measuring EMF).

    the term Electromotive force is a sort of misnomer..its not a force, its an energy IIRC.
     
  18. Dave

    Retired Moderator

    Nov 17, 2003
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    Since EMF is measured in Joules per Coulomb or energy per unit charge, then yes I would agree - Maxwell certainly did!

    Dave
     
  19. uzair

    Thread Starter Active Member

    Dec 26, 2007
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    I am now able to conclude the discussion as:The emf is same as voltage but it is used in difeferent sense.Thy have same units and we must not confuse both terms.We should have a mind-set like a voltmeter (because it views both as same and equally measures them).Am I right?
     
  20. thingmaker3

    Retired Moderator

    May 16, 2005
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    By that logic... would mass be energy? - measured in Joules per square V?

    E = 1/2 MV^2 ergo M = 1/2 E / V^2
     
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