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This can be achieved using two separate power supplies A and B connected in the series as shown in the diagram below.
Power supply A has an e.m.f. of 20.0 V and an internal resistance of 0.40 Ω.
Power supply B provides a variable voltage and has negligible internal resistance. Its voltage
varies automatically to maintain the constant 28.0 V.
When the current is 10.0 A,
(i) show that the voltage output of B is 12.0 V
‘Lostvolts’V=Ir=10.0×0.40=4.0V (1) VB =28.0–VA =28.0–(20.0+4.0)=12.0 (1)
I'm quite confused here. The question says the emf is 20V. Is the emf not lost volts+ terminal potential difference ? I'm confused as to why they calculate lost volts and add it to the emf??? HELPPP!