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# emf = IR + Ir watch

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1. can someone explain to me how this equation is derived. there was a question to prove this equation. tnx
2. (Original post by vp03)
can someone explain to me how this equation is derived. there was a question to prove this equation. tnx
I don't know the derivation but i E = I (R+r) is the same as V=IR.
3. It comes from the fact that the sum of voltages across components is equal to the supply voltage. Using V=IR for two resistances r and R gives the equation.
4. You just use Kirchoff's 2nd law (sum of EMFs around a closed loop = sum of p.ds around closed loop).
5. (Original post by vp03)
can someone explain to me how this equation is derived. there was a question to prove this equation. tnx
Kirchoff's 2nd law: sum of emf around a loop=sum of pds around loop

Edit: oops got beaten by nylex and co
6. the revision thread might help. ive put kirchoffs' laws on it aswell

http://www.uk-learning.net/t47596.html
7. THis seems like a long way rouind, but this is what I would write:
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8. (Original post by vp03)
can someone explain to me how this equation is derived. there was a question to prove this equation. tnx
If you are talking about the internal resistance of a cell, then you assume that the cell is made up of a 'perfect cell' with no internal resistance, and a resistor with resistance r.

Now imagine a complete circuit with a cell and resistor.

Using Ohm's law,

e.m.f. of cell = I(R + r)

where R is the resistance of the external resistor and r is the internal resistance of the cell

Hence,

e.m.f. = IR + Ir

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