Units (Electricity)
5 marks · NormalA student uses a 12 V, 18 W lamp. During an experiment, a total charge of 90 C passes through it.
Calculate the current in the lamp.
Calculate the time during which this charge flowed.
State the units for electric charge and current, and explain their relationship.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) 1.5 A
(b) 60 s
(c) Electric charge is measured in coulombs (C), current in amperes (A). 1 ampere means 1 coulomb of charge flows each second.
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correct formula I = P / V. (follow-through)
A1 (a) · 1 mark · Correct answer (1.5 A) with unit.
M2 (b) · 1 mark · Rearranges Q = I x t to t = Q / I. (follow-through)
A2 (b) · 1 mark · Correct answer (60 s), follow-through if value of I is wrong from (a). (follow-through)
B1 (c) · 1 mark · Correct unit names and describes their relationship.
Worked solution
a) I = P / V = 18 W / 12 V = 1.5 A
b) Q = I x t So t = Q / I = 90 C / 1.5 A = 60 s
c) Charge is measured in coulombs (C); current in amperes (A). 1 ampere equals 1 coulomb flowing per second: I = Q / t.