# Homework Help: Cicuits and emf

1. Jun 8, 2006

### Lil ze

Hello

Iam stuck on a topic (have been for a while !) about the electro motive force around a circuit (emf). Iam also confused about how internal resistance plays a role in this.
I do vaguely remember a teacher telling me that max current is at max internal resistance (or something around those lines).
I dont do advanced electronics and circuits so I dont think you'll hav to launch into long,advanced explanations :).
Sorry if this is a bit vague. If you're nto sure ill try and reword my problems :).thanks in advance

2. Jun 8, 2006

### Hootenanny

Staff Emeritus
Hi there Lil ze and welcome to PF,

As your inquiry is a little vague I don't really know how deep to go but I'll start with the basics.

Electromotive Force

The electromotive force (emf) of a source is defined as the potential difference across a source when no current flows. This means that the load resistance (not the internal resistance) is infinitly high (or at least very very high for a good approximation). It is also said the the emf is work done per unit charge.

Internal Resistance (r)

Inside a chemical cell (battery), the chemicals present a resistance to the flow of current, this is known as the internal resistance.

There are usually three things to remember when dealing with an internal resistance problem;

• When no current flows in a circuit then emf = pd across source
• When a current flows in a circuit there is a p.d. across the load resistor(R) ($V_{R} = IR$) and a p.d. across the internal resistance(r) ($V_{r} = Ir$).
• From Kichhoff's second law - the current in any part of the circuit is the same, thus emf = sum of the p.d.'s - $emf = IR + Ir$

Last edited: Jun 8, 2006
3. Jun 8, 2006

### Lil ze

thanks a lot ! that seems to hav cleared up some of my problems, ill do some exam Qs and see how it goes :D

thanks hootenanny

4. Jun 8, 2006

### Hootenanny

Staff Emeritus
My pleasure Don't hesitate to come back if you get stuck.