Internal resistance

• Every component has internal resistance.
• E is the EMF of the cell/battery.
• V is the potential difference across the terminals.
• r is the internal resistance of the cell/battery.

Equation

E=Ir+V = Ir + IR = I(r+R)

Example calculations

If a battery has an internal resistance of 3\Omega and an EMF of 9V, what is the current flowing and terminal p.d. when it’s connected to a load of 12\Omega?

• r=3
• R=(12+3)=15
• E=9
• I=\frac9{15}
• V=0.6*12=7.2

• E=3
• V=2.8
• R=8
• I=2.8\div8=0.35
• $