JAMB - Physics (2014 - No. 31)
A circuit has a resistance of 200Ω. The resistance of the circuit can be reduced to 120Ω when
A 300Ω resistor is connected to it in parallel
An 80Ω resistor is conneceted to it in series
A 150Ω resistor is connected to it in parallel
A 240Ω resistor is connected to it in series
Explicació
A 300Ω resistor is connected to it in parallel
Let R\(_t\) = total resistance in parallel
\(\frac{1}{R_t}\) = \(\frac{1}{R_1}\) + \(\frac{1}{R_2}\) = \(\frac{1}{200}\) + \(\frac{1}{300}\)
R\(_t\) = \(\frac{200 \times 300}{200 + 300}\) = \(\frac{6000}{500}\) = 120 \(\Omega\)