The Student Room Group
Reply 1
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.
Reply 2
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.
Reply 3
You just use Kirchoff's 2nd law (sum of EMFs around a closed loop = sum of p.ds around closed loop).
Reply 4
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
Reply 5
the revision thread might help. ive put kirchoffs' laws on it aswell

http://www.uk-learning.net/t47596.html
Reply 6
THis seems like a long way rouind, but this is what I would write:
Reply 7
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