Potential Difference vs. Electromotive Force – A Level Physics

Published on May 15, 2015 by

This is where it gets tricky! You may previously have used the term 'voltage' but now you need to understand the difference between the e.m.f. of a supply and the p.d. across a component. This also shows you where to put a voltmeter in a circuit and the definitions of all key terms. Please note that this analogy is not perfect, it doesn't really explain how energy can be transferred almost instantaneously or how alternating current works but it's a start.

Electromotive Force: The energy gained per unit charge by charges passing through a supply (from chemical to electrical).

Potential Difference: The energy lost per unit charge by charges passing through a component (from electrical to other forms).

Thanks for watching,



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  • bara shraim 7 years ago

    Hello Sir, 
    Thanks for this helpful video. 
    But I did not understand what you said between 5:00 and 5:03 where you say ” and in order for it to “something” part of the circuit ” . What is the word you said mean and what is it?

    • Tobias 5 years ago

      ‘basically have no’ ^^

  • Physics Online 7 years ago

    Between 5.00 and 5.03 I said it must go in parallel with the component in the circuit.

    • azoul ali 4 years ago


    • Joshua Arnold 3 years ago

      You should pin this comment. And your videos are great 🙂

  • Peter 7 years ago

    Great videos. Please keep making them.

    • Physics Online 7 years ago

      +ModdersApprentice Thanks – hope they’re useful.

  • wayne maracle 7 years ago

    very good

  • Arshmah Siddiqui 7 years ago

    Absolutely wonderful video. Easy to understand analogy. Clears my concept very much. Thank you!

  • Asmit Bhantana 7 years ago

    nice wow!!!!!!!!!!!!!!!!!!

  • Willson Basyal 6 years ago

    i just love your” little green men”… Keep making videos!!

  • Fahad Al-Khater 6 years ago

    This explanation is 100x better than my high school teacher

  • Raghad Ayman 6 years ago

    your explanation are clear and so helpful; thank you so much I don’t know what would i have done without them.

  • H.Bannai 6 years ago

    really helpful

    • Physics Online 6 years ago

      +KeeegoHD Thanks

  • JakkuWolf Insomnia 6 years ago

    If electromotive force by definition is the rate of energy transferred from chemical to electrical energy per unit charge. Why is it not considered in terms of being a force?

  • Samuel Jones 6 years ago

    amazing vids for revision! keep it up dude

  • Youssef Amr 6 years ago


  • salbts 6 years ago

    I wanna ask a question. When we measure potential difference, the positive end of battery is connected to positive end of voltmeter, in measuremnet of current, positive of battery is connected to positive of ammeter but in emf it is opposite? Can u plz explain this. shouldn’t positive ends be connected to negative for flow of charge???

  • Aakash Sawant 6 years ago

    good attempt to explain the concept

  • Bella Wilson 6 years ago


    • Physics Online 6 years ago


  • Kundan Dahal 6 years ago

    can u say why the value of emf and pd are different in this interpretation.?

  • BMX Philosophy 6 years ago

    Awesome video. Thanks a lot, your explanations are very clear ^.^

  • Kexx Petit 6 years ago

    Does the fact that that particle is moving from positive to negative mean that the particle is positive? Because it will be attracted to the negative end.

    • Aswathy Mahesh 6 years ago

      Kexx Petit u mean charge carriers.?…they are electrons…..they move from the negative to positive side…thats why an electric field is set up and due it it….there is flow of current from positive to negative in the direction of electric field

  • Anish Tiwari 5 years ago

    Can you start a biology and chemistry channel please.

    • Physics Online 5 years ago

      If only I had time…

  • Flarex 5 years ago

    I needed this.

  • Mossboy16 5 years ago

    Your channel ad came up before this video 😂

  • Yunguncletracks Inc. 5 years ago

    thank you this was a great help been stressing for a while and you helped me out !!!!!!!!!!!!

  • Mirsab Naduthodi 5 years ago

    very well.explained


  • KelstonCosta 5 years ago

    Bro thank u so much great explanation 😍

  • Jamie Kirkhouse 5 years ago

    Does this mean that if there is no component then there is no PD? Only EMF?

  • Semih Chelik 5 years ago

    very good explanation! keep going…

  • Alex Hamzelou 5 years ago

    these videos are absoluetly brilliant

    • Physics Online 5 years ago

      Thank you – there are now over 500!

  • Rahul Mawa 5 years ago

    really loved the way you showed it with the help of lego,you made it easier and way more polite(in some ways)

  • SP prem sarimella 5 years ago

    Very interesting

  • Nikita Chaudhari 5 years ago

    This was great. Thank you so much!

  • the medium cheese 5 years ago

    Wouldn’t it be more correct to say that the electrical energy becomes vibrational kinetic energy in the atoms, which causes it to radiate thermal energy and light?

  • Pritika Shrestha 5 years ago

    Thank you so much for this innovative idea.

  • Joseph Woodward 5 years ago

    Love physics but some suff is just boring, can’t wait till I get into astro physics

    • Physics Online 5 years ago

      The PotatoCraft I’ve the full set of astrophysics videos on my website

  • safwan Khandaker 5 years ago

    hey sir could u solve some mathematical problems from electricity
    this chapter really boggles my mind and confuses me a lot
    #IAL #unit2

  • Onim 5 years ago

    Current is the flow of charge per second, but what is charge? I mean i know it’s Q= IT, but what is this charge? and If current is the flow of charge then what causes the current i mean what is that thing that carries the charge?
    are the charge carries some actual particle that carries the charge?

    • Physics Online 5 years ago

      Onim Dip Charge is a property that some particles have, just like most particles have a property called mass.

  • ᄋᄌ 5 years ago

    Excellent explanation!!

  • Umar Taher 5 years ago

    Bro I want some help in solving past paper qurations could you make some videos explaining how to solve questions offered by Cambridge

  • Qwerty Azerty 5 years ago

    Thank you sir for the video! I was thinking that the electromotive force was the force necessary to push the electrons in the circuit, but I wonder now how the electrons can keep moving after releasing all the energy to the bulb (in this example). I don’t understand.

    • Suicidal Ducks 4 years ago

      Electrons aren’t moved by energy they move because they are attracted to the positive terminal of each component in the circuit. This is current – the flow of charge carriers from one terminal to another of opposite charge.

  • Freddie Rahman 5 years ago

    Can we say that the electromotive force transfers other forms of energy to electrical or do we have to state that it’s chemical

  • Adnan 4 years ago

    Magnificent effort for better grasping………1000 salutes and of course thank you…

  • Dayle Donne 4 years ago

    These videos are superb, keep them coming!

  • Ned Owen 4 years ago

    Mate, thankyou

  • arunDir 4 years ago

    Wow great vid and explanation. Thanks

  • Sherry 4 years ago

    i was really confused about it and was mixing them up thank you for your effort of explaining

  • Abdullah Raheel 4 years ago

    it was an awesome video but can some one explain at what happens at the atomic level ? i mean does electrons get excited or something?


    British English is wonderful

  • Siddiki 4 years ago

    Very nice explanation. Thanks for the video!!

  • Liu Peter 4 years ago

    Videos are excellent.Can you make a summary for all the a level physics definitions that need to be memorized, thank you very much.Wish your channel to be better and better.

  • Our Nature Perspective 4 years ago

    Sir, why we say that electron must be in parrell in voltmeter to measure it ,why not in series or what will happen if in series ?

  • Lucky Cat 4 years ago

    Best loved it

  • Lucky Cat 4 years ago

    Very very nice electrical physics was never this much interesting

  • bereket aklilu 3 years ago

    I liked your creativity really did but the lego thing took all my concentration away I had to go through the video 3 times, not that helpful.

  • Vandebharat 18 3 years ago

    Respect from india, bombay.

  • Muhammad Mirsab 3 years ago

    How could I ever thank you enough, I finally understood Emf. and P.d.
    And I’m 19, been through school and so much, but I guess all it required was some unorthodox explanation using Legos 😂👌🏼

  • Aya Alblassy 3 years ago

    I am 14 and This was so helpful thanks a lot

  • Aya Alblassy 3 years ago

    So wait emf is the energy picked up by the Lego guy and of us the energy dropped off?

  • Uzair Bhat 3 years ago

    SIR, As per definition current is flow of charge that means positive and negative charges, but only protons carry positive charge but protons are bound in nucleus Then how they can move?

    • Mokashfa Saeed 3 years ago

      I think the whole nucleus move

  • Gamemaster8167 3 years ago

    Just had the biggest click I ever had to understand Potential diffference, while watching this. Thank you so much

    • Physics Online 3 years ago

      Not a problem – it’s great when those moments happen and all becomes clear,

  • Rakshith Kakunje 3 years ago

    Thank you so much this helped me understand this concept better than 4 other teachers who had taught me this.

    • Physics Online 3 years ago

      Thank you!

  • M. Ayub 3 years ago

    I would really love if you could explain the development of higher and lower potentials inside the source of emf. Thank u and really helpful explanation.

  • The Naked Wizard 3 years ago

    Thank you

  • rhythmandacoustics 3 years ago

    Does the distinction actually matter? One is just voltage source and the other the voltage drop. By definition the potential difference between two points is the voltage. The source anode and cathod has voltage and when the circuit is completed the full voltage of the source goes to the load.

  • エレナL. 3 years ago

    I wish my teacher dont whack me if i send tis to his email….and suggest him to teach like you did😂😂

  • The Best 3 years ago

    Where I live, it’s very hard to find a good a level physics teacher. People like u make our life easier. Saying thank you is not enough to appreciate ur efforts to teach us. 💙💙💙💙💙💙💙💙💙

  • farhaad farhaad 3 years ago

    Hi dear Lewis . Thank you for your video . As far as I have learned , electrons ( charge carriers ) do not move so far through a wire. In fact electrons move a very short distance . They actually transfer their electrical field to one another and they move only few centimeters. No electron leaves the conductor . For example in a wire of 1 meter long it may never happen that an electron reaches from one head to the other , because they have a back and forth movement . Is this right ? thanks again for your video .

  • farhaad farhaad 3 years ago

    Let me ask another question I’ve always had. If we connect only one (positive) head of a power supply to a conductor ,What will happen? I mean what exactly do electrons do ?. Do they still start moving toward the other end of the wire ? Do they accumulate somewhere id. on one end of the wire or what ? Do they receive the electrical field from the power supply and keep transferring it to each other ? thank you

  • Aryzz12343 3 years ago

    Thank youuuuuuuuuu you are saving my life right nowwwwwwww

  • Shivani Singhal 3 years ago

    your way of explanation is unique
    very nice video

  • Nab'O 3 years ago

    I do Leaving Cert Physics in Ireland and this is similar to what we learn found it hard to grasp but it’s way easier now thanks!!

  • Bunty 2 years ago

    Very understanding way to explain.
    Got the idea. The major difference is that the emf is always higher than the potential difference in the ckt what i came to understand from this video.
    Thanks a lot

  • ibrahm mansoor 2 years ago

    the almighty lego master

  • Γιώργος Παπαντωνίου 2 years ago

    You are talking about “charged particles” movement and “current flow”, but as i know the only particles that move are electrons making the current flow. So what are these “charged particles” ?

    • Physics Online 2 years ago


  • Abdulaziz Alharbi 2 years ago

    What a great explanation, the problem is, people always treat EMF and Potential Difference as one thing (Voltage), which makes electricity hard to understand and you feel there is a contradiction. (Ex: they always say that when we have a close circuit we have hight current + voltage is the thing that makes electrons move which means if there is no voltage so there is no current, but in a close circuit as I mentioned the current is very hight but they say the voltage will be 0 !! how we will have current then? but what they mean is we still have EMF the electrons are loaded with energy, but the electrons with a close circuit do not drop this energy since there is no load (which means there is no potential difference, potential difference = 0) and they use Voltage here instead of potential difference.

  • hehehee 2 years ago

    Words simply cannot express my gratitude. Thanks for your wonderful analogy and succinct explanation.

    • Physics Online 2 years ago

      You’re very welcome

  • Rufaida Kunbargi 2 years ago

    This is so good! Thank you.

  • Steffi Mendes 2 years ago

    All I could think about the entire video is how similar you sound to Josh from the Sidemen. Great video!

  • annonymous annonymous 2 years ago


  • SageStation 2 years ago

    Isn’t voltage potential difference? Like that’s its definition lol?

Comments are closed.