RRAAAnn
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#1
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#1
The power system of a spacecraft must provide a constant 28.0 V for the equipment on board, independent of the current supplied.
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

answer:
‘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!
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workhards
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#2
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hi, so first you're correct in saying you have to calculate the lost volts - but do that for A only. then find what the terminal p.d. of A would be (i.e. emf of A minus the lost volts you just calculated). Then we know that the terminal p.d. of A + the p.d. of B = 28V, so from there you should get the correct answer of 12V for B
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