Question

What is Internal resistanc ?established the relation r= (E_v/v)R

Answer 1

EMF and Potential Difference

In any circuit there are components that put energy in to the circuit and components that take energy out. From now on, we will say that any device putting energy into a circuit is providing an electo-motive force (emf) and any device taking it out has a potential difference (pd) across it.

Both emf and pd are measured in volts, V, as they describe how much energy is put in or taken out per coulomb of charge passing through that section of the circuit.

The best way to think of them is:

Emf - is the amount of energy of any form that is changed into electrical energy per coulomb of charge.

pd - is the amount of electrical energy that is changed into other forms of energy per coulomb of charge.

Sources of emf:

Cell, battery (a combination of cells), solar cell, generator, dynamo, thermocouple.

Internal Resistance

Cells and batteries are not perfect (what is - apart from the moment your last exam finishes, of course?). Use them for a while and you will notice they get hot.

Where is the heat energy coming from?

It's from the current moving through the inside of the cell. The resistance inside the cell turns some of the electrical energy it produced to heat energy as the electrons move through it.

It is easy to explain if you imagine that each cell is perfect except that for some bizarre reason (probably part of a plot to take over the world, masterminded by Dr Evil) the manufacturers put a resistor in series with the cell inside the casing.

Therefore, inside the cell, energy is put into the circuit by the cell (the emf) but some of this energy is taken out of the circuit by the internal resistor (a pd).

So the pd available to the rest of the circuit (the external circuit, as some questions may refer to it) is the emf minus the pd lost inside the cell:

V = E - Ir

Where:

V = pd across the external circuit (V)

E = emf of the cell (V)

I = current through the cell (A)

r = value of the internal resistance (Ω)

(Ir = the p.d. across the internal resistor)

Note: V is sometimes called the terminal pd as it is the pd across the terminals of the cell