IB Physics-B.5-Electricity-Paper2-Qc#3-Potential divider

The diagram shows a potential divider circuit used to measure the emf E of a cell X. Both cells have negligible internal resistance.
a. State what is meant by the emf of a cell.
b. AB is a wire of uniform cross-section and length 1.0 m. The resistance of wire AB is 80 Ω. When the length of AC is 0.35 m the current in cell X is zero.
Show that the resistance of the wire AC is 28 Ω.
c. Determine E.
d. Cell X is replaced by a second cell of identical emf E but with internal
resistance 2.0 Ω. Comment on the length of AC for which the current in the second
cell is zero.
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