Short Circuit?
This circuit, consisting of four resistors, is connected to a voltage source. The value of the resistance marked is unknown.
When we accidentally drop a screwdriver across the wires, thus connecting points A and B directly, there is no noticeable change in the current that flows through the circuit.
Calculate the resistance , in ohms.
The answer is 25.
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If the short between A and B has no effect, there must be no current flowing through it. This means that A and B have the same potential.
The ratio at which the voltage is distributed in a series branch of the circuit is equal to the ratios of the resistances. It follows that the top and bottom branch of the circuit have equal ratios for the resistances: 1 2 : 1 0 = 3 0 : x . The solution is x = 1 2 1 0 ⋅ 3 0 = 2 5 Ω .