Question

36 cells, each of emf 4V are connected in series and kept in a box. The combination shows an emf of 88V on the outside. Calculate the number of cells reversed.

• A. 2
• B. 5
• C. 10
• D. 7

Hint:

In a series combination, reversing a cell reduces the total emf by twice the emf of one cell.

Answer 1

The Correct Option is D

Step 1: Understand the setup.
The total emf of 36 cells, each with an emf of 4V, when all are connected in series, is: \[ 36 \times 4V = 144V \] However, the outside emf is given as 88V, indicating some cells are reversed, contributing a negative emf.
Step 2: Calculate the number of reversed cells.
Let \( x \) be the number of reversed cells.
The emf contributed by the reversed cells is \( -4x \).
Thus, the total emf is: \[ 144 - 4x = 88 \] Solving for \( x \): \[ 4x = 144 - 88 = 56 \quad \Rightarrow \quad x = \frac{56}{4} = 7 \]