An LED requires a steady current of 20 mA and its forward voltage drop at this current is 2.0 V. The LED is to be operated from a 9.0 V battery of negligible internal resistance using a series resistor.
(i) Calculate the value of the series resistor needed to obtain the required current.
(ii) Determine the power dissipated in the series resistor.
(iii) The LED converts 25 % of the electrical power it receives into visible light. Calculate the percentage of the total power supplied by the battery that is converted into visible light.