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MCQ practice · SSC Physics Board questions · Chapter 11: Current Electricity

Sylhet Board 2023

Which of the following relations will be correct if two equal resistors are joined in series and in parallel?

Think it through first, then pick an option.

  1. a Rp = 4 Rs
  2. b Rs = 4 Rp
  3. c Rp = Rs/2
  4. d Rs = 2 Rp
Answer and explanation

Correct answer: b Rs = 4 Rp

Rs = 2R and Rp = R/2; so Rs : Rp = 2R : R/2 = 4 : 1, that is Rs = 4 Rp.

Why the other options are wrong

  • a Rp = 4 Rs: Rp = 4 Rs is upside down: for two equal resistors R, Rs = 2R and Rp = R/2, so Rs = 4 Rp.
  • c Rp = Rs/2: Rp = Rs/2 is not right: if Rs = 2R then Rs/2 = R, but Rp = R/2, which is Rs/4.
  • d Rs = 2 Rp: Rs = 2 Rp is not right: from Rs = 2R and Rp = R/2 we get Rs = 4 Rp.