A cell with electromotive force (e.m.f.) $E$ and internal resistance $r$ is linked to an external resistor of resistance $R$, as illustrated. What is the power dissipated in the external resistor?
- A$\frac{E^2(R+r)}{R^2}$
- B$\frac{E^2R}{(R+r)^2}$
- C$\frac{E^2(R+r)}{r^2}$
- D$\frac{E^2 r}{(R+r)^2}$