internal resistance of a supply

Last lecture I finished D.C circuit. This lecture I discuss an important thing that is internal resistance of a supply.

figure 1

All supplies must have some internal resistance, however small. This is shown as a series resistor external to the supply. figure(1) shows a cell of e.m.f  E volts and internal resistance r.when the cell is delivering no current ,the potential difference across the terminals will be equal to e.m.f. E of the cell as shown in figure (2)

figure 2

When some load resistance R is connected across the terminals of the cell, then current I start flowing in the circuit. This current causes a voltage drop across internal resistance r of the cell so that terminal voltage v available will be less then E. the relationship between E and V can be established by following equation.

I=E/(R+r)

IR=E-Ir

But IR-V , the terminal voltage of the cell.

V=E-Ir

for enlarge this picture click on the image

Next lecture I will discuss about network analysis