B What is electromotive force? What's its relationship to Voltage?

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Electromotive force (EMF) is defined as the potential difference between two points before they are connected in a circuit. When the circuit is closed, there are two scenarios: if there is no resistance, EMF equals voltage, which is only achievable in ideal circuits. In practical circuits, EMF is greater than voltage due to internal resistance and energy loss. Voltage can never exceed EMF, as EMF represents the potential difference in an open circuit, while voltage reflects the result of applying EMF in a closed circuit. The voltage at the output terminals of a power source matches the EMF only in ideal conditions; otherwise, it is affected by internal resistance and the load's current draw.
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New poster has been reminded to post their whole question at PF and not just link to external websites.
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This was a question that I posted on the Physics stack exchange, but I'm posting it here to have a higher probability of receiving an adequate answer.
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It is fine to post the question on both sites, but not this way. The question should be self contained on each site so that people can understand what you are asking on each site directly.
 
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Likes Vanadium 50
I will answer your question but only the one here on PF.

Electromotive force is basically the potential difference between two points before they are connected .

Now there are two cases after connection is made between the points of 2 different potentials

Case 1: EMF = Voltage
This happens when there is no resistance and no loss of energy (only possible in ideal circuits) and there is no internal resistance in the voltage source

Case 2: EMF > Voltage
This happens in practical circuits we see in daily life . Here the components have resistances , charges leak , voltage source has internal resistance and energy is lost as heat/light .

Voltage can never be greater than EMF as EMF is a perfect tendency or a perfect potential difference(EMF is in a open circuit ) but voltage is the the result of applying EMF in a closed circuit
 
In circuit analysis, voltage at a the output terminals of a power source that is open circuit is taken as the same thing as the EMF inside the power source. This is case 1 above.

When the power source is connected to something with finite resistance the voltage drops according to both (1) the amount of internal resistance in the power source prior to the output terminals, and (2) the amount of current drawn by the load. This is case 2 above. Zero internal resistance, or infinite external load, would mean that the terminal voltage is the same as the EMF, just as in case 1 above.
 
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