emf = IR + Ir

vp03

can someone explain to me how this equation is derived. there was a question to prove this equation. tnx

Reply 1

nofronting

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

A0307

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

Nylex

You just use Kirchoff's 2nd law (sum of EMFs around a closed loop = sum of p.ds around closed loop).

Reply 4

hihihihi

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

chats

the revision thread might help. ive put kirchoffs' laws on it aswell

http://www.uk-learning.net/t47596.html

Reply 6

john !!

THis seems like a long way rouind, but this is what I would write:

eqeqeqeq.JPG27.8KB

Reply 7

hornblower

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