is electromotive force always constant??

[streudels]

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i've been cross-referencing different sources as to the exact definition of the electromotive force, and i keep getting the following:

emf = terminal voltage

but i know terminal voltage changes due to the voltage drop that occurs when an electrical circuit is closed. what i just need to absolutely confirm is that emf is practically constant (as based on what kind of voltage source is being used, e.g. battery, or a generator), given the equation:

terminal V = emf - IR

where the ONLY time emf = terminal V = open-circuit voltage is when the electric circuit is open. And that open-circuit voltage is ALWAYS equal to emf and that open-circuit voltage = terminal voltage ONLY when the circuit is open.

moreover, what exactly is the role of emf once the circuit is closed? I know it's responsible for driving the current, but if this is the case, what's the role of the terminal V then if not the same thing?

Otherwise:

1) during discharge: terminal V < emf, due to the voltage drop from internal resistance
2) during recharge: terminal V > emf, due to restoration of potential difference across the terminals of the battery (i.e. voltage source)

 

[FIREitUP]

i've been cross-referencing different sources as to the exact definition of the electromotive force, and i keep getting the following:

emf = terminal voltage

but i know terminal voltage changes due to the voltage drop that occurs when an electrical circuit is closed. what i just need to absolutely confirm is that emf is practically constant (as based on what kind of voltage source is being used, e.g. battery, or a generator), given the equation:

terminal V = emf - IR

where the ONLY time emf = terminal V = open-circuit voltage is when the electric circuit is open. And that open-circuit voltage is ALWAYS equal to emf and that open-circuit voltage = terminal voltage ONLY when the circuit is open.

moreover, what exactly is the role of emf once the circuit is closed? I know it's responsible for driving the current, but if this is the case, what's the role of the terminal V then if not the same thing?

Otherwise:

1) during discharge: terminal V < emf, due to the voltage drop from internal resistance
2) during recharge: terminal V > emf, due to restoration of potential difference across the terminals of the battery (i.e. voltage source)

bear with me because I haven't gone over this in a little while.

But, EMF is basically a misnomer for potential difference. You are correct in stating that V=EMF-Ir, but on the MCAT internal resistance is almost always ignored, unless otherwise stated. So basically think of EMF of another way of saying potential difference (voltage).

I think you are definitely over-thinking things here.

Since we established that EMF and Voltage are the same for all intents and purposes, then EMF can't be constant in a circuit where there is a voltage drop due to resistors.

 

[CaptainSSO]

Your post is pretty convoluted IMO, but for the MCAT, "electromotive force" is just another term for voltage (J/C).

I think you are making all of this more complicated than it needs to be (in terms of the MCAT -- obviously there is nothing wrong in general with curiosity and a desire to learn, that's one of my major driving forces in life).

But basically just know the voltage (emf) of a battery is constant (up until just before it dies). Also, and you already seem to understand this, the voltage available to the actual circuit the battery is hooked up to will be less than the actual emf of the battery due to internal resistance.

I haven't actually heard the term "terminal voltage" before, but from the equation you provided the reason it is not always equivalent to the emf of the battery is just because of the voltage lost due to resistance of the battery itself.

 

[streudels]

thanks for the responses! it definitely makes a lot more sense just getting the essentials. i would get one piece of info from one book, which would make me ask questions about the info i got from another review book, and it got crazy from there. i probably already know more about batteries than i would've liked, but at least i got my information down with regards to that.