Let’s Get Physics-l: Potential Difference and e.m.f

E.M.F or electromotive force of a supply (Cell, battery etc.) is the energy gained per unit charge by charges passing through the supply. It is measured in Volts (V) or Joules per Coulomb (JC^-1). Basically, the e.m.f is the transfer of energy supply in the power supply from which chemical energy (If using a battery) is transferred to the charge carriers (Electrons in a circuit) as electrical energy.

Potential difference, on the other hand, is the transfer of energy from the charge carriers to the components in the circuit, from electrical to whatever form of energy is required for the component to operate. This is also measured in Volts (V) or Joules per Coulomb (JC^-1).

If we return to my train example I used way back in the Conventional Current VS Electron Flow post here. If we consider a simple looped track with a train station and a factory. The train carriages are the charge carriers. The train pulls up to the station, its carriages empty. Passengers board the train till it’s at full capacity. The train then goes around to the factory where the passengers disembark from the train to go and work in the factory. The train goes back to the station and the process repeats.

Now, in reality, current doesn’t stop to let energy be transferred and it happens fluidly with the current flowing continuously. However, it does help to picture what goes in a circuit. The train station or power supply has lots of awaiting passengers or joules. The passengers then get on the train, transferring from the platform to the carriages in a similar way that e.m.f operates. The train then takes the passengers to the factory or component where they dismount and transfer from the carriages to the factory to work, in a similar way to potential difference.

Potential difference and e.m.f are very similar in what they accomplish but there are subtle differences that set them apart. First, e.m.f occurs in the power supply whilst p.d. (Short hand for potential difference) occurs across components. In addition, e.m.f is the energy transferred per unit charge whilst potential difference is the work done per unit charge.

We can use a series of equations to quantify e.m.f and p.d. These are:

ε = W/Q

  • ε – e.m.f (V or JC^-1)
  • W – Energy Transferred to charge (J)
  • Q – Charge (C)

V = W/Q

  • V – Potential Difference (V or JC^-1)
  • W – Work Done (J)
  • Q – Charge (C)

As is quite apparent, their is a definite link between both p.d and e.m.f so ensure that you are aware of the differences.


Summary

  • e.m.f – The transfer of chemical energy to electrical energy, per unit charge, inside the power supply. Measured in V or JC^-1.
  • p.d – The transfer of electrical energy to another form of energy, per unit charge, across a component. Measured in V or JC^-1.
  • ε = W/Q
    • ε – e.m.f (V or JC^-1)
    • W – Energy Transferred to charge (J)
    • Q – Charge (C)
  • V = W/Q
    • V – Potential Difference (V or JC^-1)
    • W – Work Done (J)
    • Q – Charge (C)

That’s potential difference and e.m.f for you. As ever, if you have any thoughts, feedback, requests or questions please leave them in the comments.

 

 

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