# Question 41090

Feb 24, 2017

Time to charge the battery is 15.4 hours.

#### Explanation:

$2.0 M J$ is a unit of energy which is defined as power multiplied by time.

$E = P \times t$

We know that: $1 M J$ of energy = $277.8 W h$

So $2.0 M J = 2.0 \times 277.8 = 555.6 W h$

Note the is decimal is rounded off here to match the single decimal place in all the given values.

The battery charger uses $40.0 W$ of power, but it only supplies 90%# or $\frac{9}{10}$ of that power to the battery.

Then: $P = 40.0 \times \frac{9}{10} = 36.0 W$

Substituting these values into the equation;

$E = P \times t$

$555.6 W h = 36.0 W t$

$t = \frac{555.6}{36.0} = 15.4 h$