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State Kirchhoff’s first law.
A cell with internal resistance 
is connected to two resistors of resistances 
and 
as shown below.
The potential differences (p.d.s) across and are 
and 
respectively. The terminal p.d. across the cell is 
. The current in the circuit is 
. Use Kirchhoff’s laws to show that the total resistance 
of the external circuit is given by
The electromotive force (e.m.f.) of the cell in the figure above is 
. The values of and are 
and 
respectively. The terminal p.d. of the cell is 
. Calculate the internal resistance of the cell.
A resistor of resistance 
is added to the circuit above so that the circuit is as shown.
State and explain the effect, if any, of this change on:
the current in the cell
the terminal p.d. of the cell.
.
