A relay is required to operate 800 m from its power supply. The power supply has negligible
internal resistance. The relay requires 16.0 V and a current of 0.60 A to operate.
A cable connects the relay to the power supply and two of the wires in the cable are used to
supply power to the relay.
The resistance of each of these wires is 0.0050 Ω per metre.
What is the minimum output e.m.f. of the power supply?
A 16.6 V B 18.4 V C 20.8 V D 29.3 V

Question

jamesj · Answer

So calculate how much cable you need and the resistance of that wire.  From there, how much potential difference is lost over the wires.  Hence what potential difference you need at the source in order to have the desired V at the relay.

anonymous · Answer

The Total resistance of the relay and the wires comes out to be;
R=34.6ohms 
and the V will then be R=V/I

Got it thanks :)

anonymous · Answer

so the answer is C

jamesj · Answer

How did you find that value of R?

anonymous · Answer

Resistance of relay+resistance of wires
(16V/0.6A)+ (0.005ohmsx800mx2)

jamesj · Answer

Got it.  Good.

anonymous · Answer

yeah, thanks for guiding