e.m.f = Ir + V help

Can anyone help me understand this equation a bit more and when its used for... Finding it hard to get my head around it.

e.m.f = Ir + V

Any help will be much appreciated :biggrin:

Reply 1

walalang

Yeah, I don't understand it either.. It was on the exam paper (Unit 3) and I didn't know how to do it. =/

Reply 2

b_tse

Fingers crossed it doesnt come up in tomorrows unit 2 :confused:

Reply 3

Robob

Basically it sates that the electromotive force is equal to the potential difference across the internal resisitor (well the cell is modelled as being like this) plus the potential difference across the rest of the circuit

Reply 4

Biffy Clyro

I think I speak for most people when I say I hate electricity

Reply 5

Robob

Biffy Clyro

I think I speak for most people when I say I hate electricity

s'alrite electricity hates you too :p:

Reply 6

Duke Flipside

rpotter

What he said.
EMF= Total voltage of the cell
I=Current in the circuit
r=internal resistance of the cell
V= Voltage across the circuit, ie I multiplied by the resistance of the circuit (capital R)

Alternatively, if you're working out the EMF (rather than the internal resistance of the cell) you can just add r to R to get R_T and the use good ol' V=IR...

Reply 7

smurfride

a very simplistic way of putting it is that when energy if given out by the cell, not all of this energy reaches the external circuit. This is because each cell has its own internal resistance-so when charge flows through the cell it picks up energy and then 'loses ' a fraction of this due to the impurities/chemicals inside the cell, especially dry cells, whuich get very hot. So the e.m.f the cell gives off is equal to the Ir of the internal resistance which = V, plus te IR of the external circuit which also equals V.
erm...hope that makes sense, althought it probably doesn't!

Reply 8

insparato

Ir = the "lost" volts V = E - Ir which basically means the terminal p.d. is equal to the Emf take away the voltage provide by the current passing through the internal resistance of a battery. Which means if you rearrange the equation the terminal p.d + the lost volts gives you a value for emf.

Reply 9

chem@uni

https://www.youtube.com/watch?v=UWwIFoP7Uew
For anyone still needing help (aware this is an old post but thought it might help people looking it up now like me :smile:

)

Reply 10

AlphaMolecule

Original post by b_tse

Can anyone help me understand this equation a bit more and when its used for... Finding it hard to get my head around it.

e.m.f = Ir + V

Any help will be much appreciated :biggrin:

The electromotive force is the electrical energy given to electrons in the cell.

The term 'Ir' is significant because of the lower case r, it means the resistance of the cell - the cell is made of metal components, which offer a small amount of resistance.

Therefore, Ir is the potential difference of the cell, and V is the potential difference of components, the sum being the EMF.