DiagramModerate1 mark
8(c)
The diagram shows a circuit with the key k open. Calculate:
the current drawn from the cell when the key k is open.
Topic: Current, resistance and circuits
Previous-year question practice database
Answer
Exam answer
Given: e.m.f. ε = 3.3 V, total resistance = 5.5 Ω (from part (i))
By Ohm's law applied to the whole circuit: I = ε ÷ (R + r) = 3.3 ÷ 5.5 = 0.6 A
Current drawn from the cell = 0.6 A
Explanation
Divide the e.m.f. by the TOTAL resistance, internal resistance included. Using the external 5 Ω alone gives 0.66 A, which is the standard error here.
The distinction matters: the e.m.f. drives current through the whole loop, and part of that loop is inside the cell. Use ε ÷ (R + r) whenever the question mentions an internal resistance at all.
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