EMF and PD Watch

Adrian Ma
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Can someone help me with the difference between EMF and PD?

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Hamoody
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(Original post by Adrian Ma)
Hi

Can someone help me with the difference between EMF and PD?

Thanks
Electromotive force (emf) refers to the ability of a circuit to do work on electrically charged particles. Since this work is done by the electromagnetic force, the force exerted on the charge (and hence the total work) will be proportional to the magnitude of the charge. The proportionality constant is what we call the emf. Thus, the total work done is the product of the emf and the particle's charge.

When there is an electric potential difference between two points on a circuit, an emf results, because electric field lines point from high potential to low potential, and the electrostatic force can do work on charges. When there are only electrostatic forces, the emf is equivalent in magnitude to the electric potential difference, because the electric potential difference is defined to be the work done in moving a unit test charge from one point to another.

However, emf is a more general concept. Suppose a loop of wire is held stationary while we turn on a magnetic field perpendicular to the plane of the loop. By Faraday's law, this induces an electric field in the loop, which tends to cause electrons to circulate through the loop. This can't be described in terms of potential differences, because an electron can go all the way around the loop, come back to its initial position, and have gained kinetic energy in the process (although, in a real circuit, the kinetic energy will be converted into heat by electrical resistance). Even though electric potential is no longer well-defined in such a circuit, the emf around the loop is perfectly well-defined: we just take the force on an electron by the induced electric field, integrate it all the way around the loop, then divide by the force on the electron.
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Kyx
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(Original post by Adrian Ma)
Hi

Can someone help me with the difference between EMF and PD?

Thanks
(Original post by Hamoody96)
Electromotive force (emf) refers to the ability of a circuit to do work on electrically charged particles. Since this work is done by the electromagnetic force, the force exerted on the charge (and hence the total work) will be proportional to the magnitude of the charge. The proportionality constant is what we call the emf. Thus, the total work done is the product of the emf and the particle's charge.

When there is an electric potential difference between two points on a circuit, an emf results, because electric field lines point from high potential to low potential, and the electrostatic force can do work on charges. When there are only electrostatic forces, the emf is equivalent in magnitude to the electric potential difference, because the electric potential difference is defined to be the work done in moving a unit test charge from one point to another.

However, emf is a more general concept. Suppose a loop of wire is held stationary while we turn on a magnetic field perpendicular to the plane of the loop. By Faraday's law, this induces an electric field in the loop, which tends to cause electrons to circulate through the loop. This can't be described in terms of potential differences, because an electron can go all the way around the loop, come back to its initial position, and have gained kinetic energy in the process (although, in a real circuit, the kinetic energy will be converted into heat by electrical resistance). Even though electric potential is no longer well-defined in such a circuit, the emf around the loop is perfectly well-defined: we just take the force on an electron by the induced electric field, integrate it all the way around the loop, then divide by the force on the electron.
Might want to put a TL;DR on that one
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Konanabanana
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(Original post by Adrian Ma)
Hi

Can someone help me with the difference between EMF and PD?

Thanks
Emf is the voltage supply by the power supply, whereas p.d is the difference, before and after. You can have an EMF of 15v and a p.d of 12 around the power supply that's it where the internal resistance equation come sin. :d
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