Why is it so that current through external resistance is maximum when total internal resistance of the circuit is equal to external resistance?

Asked Shiny

Please refer to the following

- https://en.wikipedia.org/wiki/Maximum_power_transfer_theorem
- http://www.allaboutcircuits.com/textbook/direct-current/chpt-10/maximum-power-transfer-theorem/
- http://www.electrical4u.com/maximum-power-transfer-theorem/