Editing Electromotive force (section)

== Distinction with potential difference ==
Although an [[Voltage|electrical potential difference (voltage)]] is sometimes called an emf,<ref name=Fogiel>
{{cite book |title=Basic Electricity
 |page=76
 |url=https://books.google.com/books?id=_DapslzANfwC&pg=PA76
 |first= M.|last=Fogiel
 |isbn=978-0-87891-420-3
 |year=2002
 |publisher= Research & Education Association
 }}</ref><ref name="Halliday">
{{cite book
 |title=Fundamentals of Physics
 |edition=6th
 |first1=David|last1=Halliday |first2=Robert|last2=Resnick |first3=Jearl|last3=Walker 
 |url=https://books.google.com/books?id=VXlEQlznCO0C&pg=PA638
 |page=638
 |isbn= 978-0-471-75801-3
 |publisher=Wiley
 |year=2008
 }}</ref><ref name=Freeman>
{{cite book
 |title=Fundamentals of Telecommunications
 |first=Roger L| last=Freeman 
 |url=https://books.google.com/books?id=6_yQ-dEGc5wC&pg=PA576
 |page=576 |isbn=978-0-471-71045-5
 |publisher=Wiley
 |year=2005
 |edition=2nd
 }}</ref><ref name=Croft>
{{cite book
 |title=Practical Electricity
 |year=1917
 |first=Terrell|last=Croft
 |page=[https://archive.org/details/practicalelectr01crofgoog/page/n551 533]
 |publisher=McGraw-Hill
 |url=https://archive.org/details/practicalelectr01crofgoog
 }}</ref><ref name=Loeb>
{{cite book
 |title=Fundamentals of Electricity and Magnetism
 |first=Leonard B.|last=Loeb
 |page =86
 |url=https://books.google.com/books?id=zw-3icfx9qAC&pg=PA86
 |isbn=978-1-4067-0733-5
 |publisher=Read Books
 |edition=Reprint of Wiley 1947 3rd
 |year=2007
 }}</ref> they are formally distinct concepts:

* Potential difference is a more general term that includes emf.
* Emf is the cause of a potential difference. 
* In a circuit of a voltage source and a resistor, the sum of the source's applied voltage plus the ohmic voltage drop through the resistor is zero. But the resistor provides no emf, only the voltage source does:
** For a circuit using a battery source, the emf is due solely to the chemical forces in the battery.
** For a circuit using an electric generator, the emf is due solely to a time-varying magnetic forces within the generator.
* Both a 1 volt emf and a 1 volt potential difference correspond to 1 joule per coulomb of charge.

In the case of an open circuit, the electric charge that has been separated by the mechanism generating the emf creates an electric field opposing the separation mechanism. For example, the chemical reaction in a voltaic cell stops when the opposing electric field at each electrode is strong enough to arrest the reactions. A larger opposing field can reverse the reactions in what are called ''reversible'' cells.<ref name=Peters>
{{cite book
 |title=Concise Chemical Thermodynamics
 |first1=J. R. W.|last1=Warn |first2=A. P. H.|last2=Peters |page=123
 |url=https://books.google.com/books?id=oCTRVcJ1mqYC&pg=PA123
 |isbn=978-0-7487-4445-9
 |year=1996
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 |publisher=CRC Press
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{{cite book
 |title=Thermodynamics for Chemists
 |first=Samuel|last=Glasstone
 |url=https://books.google.com/books?id=oW5XqmTSXyEC&pg=RA1-PA301
 |page=301
 |isbn=978-1-4067-7322-4
 |publisher=Read Books
 |year=2007
 |edition= Reprint of D. Van Nostrand Co (1964)
 }}</ref>

The electric charge that has been separated creates an electric [[potential difference]] that can (in many cases) be measured with a [[voltmeter]] between the terminals of the device, when not connected to a load. The magnitude of the emf for the battery (or other source) is the value of this open-circuit voltage. 
When the battery is charging or discharging, the emf itself cannot be measured directly using the external voltage because some voltage is lost inside the source.<ref name="Halliday"/>
It can, however, be inferred from a measurement of the current <math>I</math> and potential difference <math>V</math>, provided that the internal resistance <math>R</math> already has been measured: ''<math>\mathcal{E} = V_\text{load} + IR \, .</math>''

"Potential difference" is not the same as "induced emf" (often called "induced voltage"). 
The potential difference (difference in the electric scalar potential) between two points A and B is independent of the path we take from ''A'' to ''B''.  
If a voltmeter always measured the potential difference between ''A'' and ''B'', then the position of the voltmeter would make no difference. 
However, it is quite possible for the measurement by a voltmeter between points ''A'' and ''B'' to depend on the position of the voltmeter, if a time-dependent magnetic field is present. 
For example, consider an infinitely long [[solenoid]] using an [[Alternating current|AC current]] to generate a varying flux in the interior of the solenoid. 
Outside the solenoid we have two resistors connected in a ring around the solenoid. 
The resistor on the left is 100 Ω and the one on the right is 200 Ω, they are connected at the top and bottom at points ''A'' and ''B''. 
The induced voltage, by Faraday's law is <math>V</math>, so the current <math>I = V/(100+200).</math> Therefore, the voltage across the 100 Ω resistor is <math>100 \ I</math> and the voltage across the 200 Ω resistor is <math>200 \ I</math>, yet the two resistors are connected on both ends, but <math>V_{AB}</math> measured with the voltmeter to the left of the solenoid is not the same as <math>V_{AB}</math> measured with the voltmeter to the right of the solenoid.<ref name=Shadowitz>{{cite book
 | title = The Electromagnetic Field
 | first = Albert|last=Shadowitz
 | publisher = McGraw-Hill Book Company
 | year = 1975
 | isbn = 0-07-056368-3
 | pages = 396–398
 | edition = 1st
 }}</ref>
<ref name=McDonald2>
{{cite web 
|last1=McDonald |first1=Kirk T. 
|title=Lewin's Circuit Paradox 
|url=http://kirkmcd.princeton.edu/examples/lewin.pdf 
|website=Physics Examples 
|publisher=Princeton University
|date=2010
 }}</ref>