In a potentiometer circuit, a cell of EMF E1 gives a balance length l1 = 60 cm. When replaced by another cell of EMF E2, the balance length is l2 = 80 cm. What is the ratio of their electromotive forces (E1 / E2)?
Answer and explanation
Correct answer: D. 3 / 4
.
- Step 1: In a potentiometer wire carrying a constant current, the potential difference across a length of wire is directly proportional to its length: , where is the potential gradient.
- Step 2: At the null point for each cell: and .
- Step 3: Taking the ratio: .
- Step 4: Substitute and :
Answer
3 / 4
Why not the others: '4 / 3' inverts the ratio; '9 / 16' squares the length ratio; '1 / 2' is mathematically incorrect.
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