Question 8
The charge that an
electric battery can deliver is specified in ampere-hours.
For example, a battery
of capacity 40 ampere-hours could supply, when fully charged, 0.2 A for 200
hours.
What is the maximum
energy that a fully charged 12 V, 40 ampere-hour battery could supply?
A 1.7 kJ B 29
kJ C 1.7
MJ D 29
MJ
Reference: Past Exam Paper – June 2015 Paper 11 Q35
Solution:
Answer: C.
Power = VI
Power = Energy / time
Combining the 2 equations,
Energy / t = VI
Energy = VIt
The battery is described as ’12 V, 40 ampere-hour’.
This means that
e.m.f. V = 12 V
From the example give, 40 ampere-hours means that a
current of 0.2 A can be supplied for 200 hours.
Notice that, when we take the product (0.2 A × 200 h) gives the number of
ampere-hours (= 40).
But this is NOT in SI unit as hour is not an SI unit.
We know that 1 hour = 3600 s
So, 40 ampere-hours = 40 × 3600
ampere-seconds
‘Ampere-hours’ or ‘ampere-seconds’ actually represents
the quantity ‘current × time’ (= It).
Energy = VIt
Energy = 12 × 40 × 3600
Energy = 1 728 000 J = 1.7 MJ
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