electric circuit

Q: the resistance of AB is 0, then why there has no short circuit occur??
A: Who says that the resistance of AB is zero. There is no indication in the question that this is so.

Q: why there has internal resistance of 20ohm??
A: From the given information, the box probably consists of a voltage source because there is a voltage measured in fig.1) and some resistances, as there is a drop in measured voltage in fig.2

The solutuon to the problem is:

Assume the voltage source is of emf E, and all the other resistances are represented by an equivalent resistance r.

Since no current is drawn from the source in fig.1, the measured voltage is the emf of the source, i.e. E = 60 v

In fig.2, since voltage emasured across 100-ohm resistor is 50 v,
hence, 50 = 60 x (100/(r+100))
solve for r = 20 ohms

If the 100-ohm resistor is connected in series with the voltmeter, the voltmeter measured the emf of the source, i.e. its reading is 60 v

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