# difference between emf and voltage

Discussion in 'MCAT Study Question Q&A' started by destroyMCAT, Aug 14, 2011.

1. ### destroyMCAT

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"emf = V = IR. This relationship is saying that as a charge moves through the circuit, the potential increase in the emf is equal to the potential drop as that charge moves through the resistor"

Can someone explain that phrase. What the difference between emf and voltage? Please and thank you.

2. ### indianjatt 7+ Year Member

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Same thing. Essentially what the equation is saying is that when you go in a loop (loop rule), the total change in V-IR = 0. Therefore the emf or voltage must lose all of its potential to IR in a circuit when going through a loop (final and initial point are the same).

3. ### liveoak

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same. do watch out for terminal voltage vs emf. terminal voltage takes into account the internal resistance of the voltage source.

4. OP

### destroyMCAT

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ah, i see. thanks! So, what's terminal voltage then?

I'm looking at a diagram in the book and trying to understand what is going on. Emf, capacitor and resistor in a simple loop. The current (positive charge) moves from the positive end of the voltage source to the capacitor. As a result, the capacitor becomes charged + and negative.. Between the capacitor is an insulator (air), how does current flow from the negative end of the capacitor to the rest of the loop?

5. ### liveoak

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Once the cap is charged, no more current flows. Why? you now have two 6 volt sources, and they cancel out (no more potential difference).

The positive charge collects on the positive plate (current) negative on negative plate (electrons).

terminal voltage is the voltage supplied to the circuit by the battery when the internal resistance of the battery is taken into account. Thus, terminal voltage is always a bit lower than the naked voltage, or emf.

6. OP

### destroyMCAT

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Oh, I see. So both positive and negative charges at the same time to charge the capacitor. I forgot that there was negative charge in the wire too.

As for terminal voltage.. I somewhat understand it. Ok, so the battery has an emf, and as it goes through the circuit (if there's resistance), voltage would be dissipated across each one and the voltage would end up to be zero at the end.. therefore, wouldn't the emf = terminal voltage. I know that's wrong, but I don't completely get the reasoning. Thanks!

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