Question

What steady current can be drawn from an accumulator of capacity $80{\text{Ah}}$ ?
(A) $4{\text{A}}$ for ${\text{20}}$ hrs
(B) $40{\text{A}}$ for ${\text{6}}$ hrs
(C) $20{\text{A}}$ for ${\text{5}}$ hrs
(D) $2{\text{A}}$ for ${\text{100}}$ hrs

Answer 1

To solve this question, we have to find the quantity whose value has been given in the question. For this we have to consider the unit of the quantity given. Then we have to check each of the options which match with the given value, which will be the correct option.

Complete step by step answer
The capacity of the accumulator is given in the question as $80{\text{Ah}}$. Its unit is equal to the multiplication of the two units, namely, ampere and hour. We know that the ampere is the unit of current, while the hour is the unit of time. Also, we know that multiplying current with the time gives us the value of the charge. So multiplying the units of the current and the time will give us the corresponding unit of the charge. Therefore we have been given the charge capacity of the accumulator, which is equal to $80{\text{A}}$ for two hours.
In the first option, we have a current of $4{\text{A}}$ for ${\text{20}}$ hrs. So the charge is
$Q = 4 \times 20{\text{Ah}}$
$\Rightarrow Q = 80{\text{Ah}}$
This matches with the value given in the question. So the option A is correct.
In the second option, we have a current of $40{\text{A}}$ for ${\text{6}}$ hrs. So the charge is
$Q = 40 \times 6{\text{Ah}}$
$\Rightarrow Q = 240{\text{Ah}}$
This is more than the charge capacity given in the question. So option B is incorrect.
In the third option, we have a current of $20{\text{A}}$ for ${\text{5}}$ hrs. So the charge is
$Q = 20 \times 5{\text{Ah}}$
$\Rightarrow Q = 100{\text{Ah}}$
This charge is more than the charge capacity given in the question. So the option C is also incorrect.
Lastly, in the fourth option, we have a current of $2{\text{A}}$ for ${\text{100}}$ hrs. So the charge is’
$Q = 2 \times 100{\text{Ah}}$
$\Rightarrow Q = 200{\text{Ah}}$
This charge is also more than the charge capacity given in the question. So the option D is also incorrect.

Hence, the only correct answer is option A.

Note
An accumulator is a device which stores energy and releases it when needed. They are used for reducing the power requirement of a hydraulic system.