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My attempt to define following terms as per my understanding. I am currently at high school.

  1. Electromotive force (EMF): Potential between two terminals when open circuited.

Wikipedia's version of EMF for electromagnetic induction: In electromagnetic induction, EMF can be defined around a closed loop of conductor as the electromagnetic work that would be done on an electric charge (an electron in this instance) if it travels once around the loop. Here closed circuit has been used for defining EMF, which further confuses me.

  1. Potential: Amount of work done per unit charge to move it from one point to another.

  2. Potential Difference: Difference in electric potential between two reference points.

  3. Voltage: Another term for potential.

  4. Voltage Difference: Another term for potential difference.

I never understood these terms to the level I wanted to. How can we describe these terms in more detail using relatively simple language and maths (up to calculus level) and have our concept clear?

Community
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Alisha Yadav
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  • Hi Alisha! Welcome to Physics SE. :) I am voting to close this question as it is unclear what you are asking. If you want to ask about a specific concept then kindly edit your question accordingly. –  Mar 11 '19 at 00:55
  • I am editing my question. – Alisha Yadav Mar 11 '19 at 00:56
  • There are some concepts that just cannot be extrapolated from past experience. Electricity and magnetism concepts fall into that category. You need to carefully read your book, and learn to work example problems the same way that the author works them. Then, after thinking about such problems for some time, you should start getting more comfortable with new terminology and concepts. – David White Mar 11 '19 at 01:34
  • This sounds counter intuitive, but if you want to understand the concepts more deeply the way is to use more complex words (and math), not simpler. At some point, ideas cannot be simply described with our everyday language. – Javier Mar 11 '19 at 01:56
  • Please be more specific. You should ask about a specific topic or doubt. Try and find out. That's your achievement. – user8718165 Mar 11 '19 at 03:54
  • I don't think you can define EMF in terms of potential, because EMF is actually the more general concept. The electrostatic potential is only defined under certain simplifying assumptions, particularly that there are no significant time-varying fields (although we can fudge and handle time-varying fields if they are restricted to individual components such as capacitors and inductors). EMF can be defined even when the electric field is non-conservative (i.e. when there are induced EMFs per Faraday's law). – The Photon Mar 11 '19 at 05:11
  • I probably haven't stated my argument strictly correctly, which is why I give it as a comment rather than an answer. – The Photon Mar 11 '19 at 05:12
  • This question has already been asked more than once. Not all the answers you'll find are correct but at this link ( https://physics.stackexchange.com/questions/15402/what-is-the-difference-between-electric-potential-potential-difference-pd-vo ) you may find good answers (those with positive evaluation). Then, if you'll have further questions on the same subject, you may ask another, more focused question. – GiorgioP-DoomsdayClockIsAt-90 Mar 11 '19 at 06:06
  • Possible duplicate of What is the difference between electric potential, potential difference (PD), voltage and electromotive force (EMF)? – GiorgioP-DoomsdayClockIsAt-90 Mar 11 '19 at 06:07

3 Answers3

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Many people have difficulty with electrical concepts because they feel those concepts are, for want of a better term, abstract. For me, it works best to try and describe those concepts with analogies that people can understand. As you said, voltage or potential difference is defined as:

"The potential difference V between two points is the work per unit charge required to move the charge between the points". (This is from the NCEE reference handbook for the PE FE exam)

Consider the following, unofficial, definition in the case of gravity (I just made it up).

"The gravitational potential difference between two points is the work required per unit mass to move the mass between the points".

One is a consequence of an electric field. The other the consequence of a gravitational field.

Hope this helps.

Bob D
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EMF at a simple level can be reduced to say 2 atoms, let's say a chlorine one and a sodium one. As we bring these atoms closer together we can measure a stronger and stronger force. At any point we can measure this force, this is an EMF force. When we put enough chemical in a battery the exact same force is present, whether yo have big Li battery or small one the EMF per cell is still 4V ( or 4V even for one molecule). In a real battery there is internal resistance and that's why people say "open circuit" EMF because you can measure the true potential this way. Even a weak battery ( only a few atoms left to react) will show the 4V if you use a very high resistance voltmeter. All of the above terms are equivalent, in an ideal closed circuit you can measure the EMF, ideal is always used in circuit problems but in reality there is no such thing as ideal.

PhysicsDave
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In layman terms:

  • Potential energy $U$: A term for "how strongly" a charge wants to move if it could. Put a negative charge close to the negative battery terminal, and it is repelled strongly. The electrical potential energy stored in such setup is huge. At different points throughout a circuit, a charge would be "pulled/pushed in" differently, so different points correspond to different amounts of potential energy.

Potential energy is in general defined as the amount of work that can be done. When strongly repelled or attracted, a huge amount of work can be done on the charge (giving it a huge amount of kinetic energy). So, how far/fast it will move if released, gives an idea of the amount of potential energy stored.

  • Potential $V$: This is just potential energy per charge, $V=U/q$. It is easier to compare points in a circuit by how they would affect the same amount of charge; therefore this term has been invented.

  • Potential difference $\Delta V$: This is just the difference in potential between two points. As when placing a ball on a high shelf, the ball will not want to start rolling sideways. The gravitational potential energy is the same at other points on the same shelf. It will only roll to a place of lower gravitational potential energy. Only the difference matters.

Since only the difference matters, we never care about the actual value of potential $V$ at a point. In fact, we can't ever know the true value. We only care about the difference between the potential at that point and another point. When someone says that "there is a potential of 2 V at this point", then they are actually comparing it to something else, usually to ground (which is at 0 V).

  • Voltage $\Delta V$: Another term for potential difference.

Often, the symbols are a bit mixed up here. Often $V$ is used for voltage and can be confused with potential. You just must be clear on the meaning of a symbol in a specific context.

  • Electromotive force (EMF): Think of EMF as "supplied" voltage. There is a voltage across the terminals of a battery, because the battery generates this voltage. It "supplies" this voltage. EMF is another word invented for such "supplied" voltage. Voltage can also be "supplied" due to magnetic induction and in other ways. In all such cases, the term EMF might be used. But it means nothing new.

The word "force" in the EMF term is a bit misleading. It is not a force. It is a "supply" of energy. But since energy differences (voltages) directly reflect how and whereto charges want to move due to repulsions/attractions in the circuit, "supplied" energy is sometimes thought of as a "driving force" for the flow of charges.

Steeven
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