Answer to Question 3

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Short out the 80 ohm resistor:

Notice how the current flows:

We can work out the current from A to B:

I1 = 15 V 40 W = 0.375 A

I2 = 30 V 60 W = 0.500 A

Total current = 0.875 A

 

Now remove the cells and short out the gaps.  These cells have negligible internal resistance, so we don't have to worry about it.

These resistors are in parallel

RT-1 = 40-1 + 60-1 = 0.0417 W-1

RT = 24 W

Here is the Norton Equivalent:

We haven't finished yet.  We now put back the 80 ohm resistor:

Work out the parallel resistance:

RT-1 = 24-1 + 80-1 = 0.05417 W-1

RT = 18.46 W

Now work out the voltage:

V = 0.875 A 18.46 W = 16.154 V = 16 V (2 s.f.)

Now we can work out the current in the 80 ohm resistor:

I = 16.154 V 80 W = 0.202 A = 0.20 A (2 s.f.)