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A 35 kΩ is connected in series with a resistance of 40 kΩ. What resistance...

Physics
JAMB 2023

A 35 kΩ is connected in series with a resistance of 40 kΩ. What resistance R must be connected in parallel with the combination so that the equivalent resistance is equal to 25 kΩ?

  • A. 40 kΩ
  • B. 37.5 kΩ
  • C. 45.5 kΩ
  • D. 30 kΩ
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Correct Answer: Option B
Explanation

For the combination in series;

⇒R1 = 35kΩ + 40kΩ = 75kΩ

R is combined with 75kΩ in parallel to give 25kΩ

=  \(\frac{1}{R_eq}\) = \(\frac{1}{R}\) + \(\frac{1}{R}\) 

=   \(\frac{1}{25}\)     = \(\frac{1}{R}\) + \(\frac{1}{75}\) 

=  \(\frac{1}{25}\)     - \(\frac{1}{75}\) + \(\frac{1}{R}\) 

=   \(\frac{3-1}{75}\) = \(\frac{1}{R}\) 

=   \(\frac{2}{75}\)   = \(\frac{1}{R}\) 

=   \(\frac{75}{2}\)  = R

;      R = 37.5k Ω

 

 

 

 

 

 

There is an explanation video available below.


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Explanation Video

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