INTERNAL RESISTANCE

==Internal Resistance== All power supplies have some resistance between their terminals called internal resistance. This causes charges in the circuit to dissipate some electrical energy in the power supply itself. The power supply becomes warm when delivering a current. [image:http://i.imgur.com/njmlu8X.png] So ε = V~r~ + VR Since I is a constant in a series circuit and '''V=IR''' $$ε = I(R + r)$$$ V~R~ is known as the terminal voltage and represents the ‘useful’ voltage that the power supply can output to the circuit. $$V_R = ε - Ir$$ V~R~ is the ‘lost’ voltage If R is very small, it will end up sharing a good amount of the total EMF with the internal resistance. This will also lead to a higher current. The terminal voltage decreases. [image:http://i.imgur.com/r7QgWqJ.jpg] If R is large the terminal voltage will increase however the current will be smaller. It turns out that an EMF will deliver maximum power to the circuit when ''R is equal to r''. The potential difference across an EMF will drop as soon as current starts flowing as work is done against internal resistance.