A $1000\,watt$ microwave oven takes $90$ seconds to heat a bowl of soup. How many joules of energy does it use?

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Hint: Let us get some idea about the work, power and energy. The most commonly used words in Physics are work, energy, and power. Work and resources can be thought of as two sides of a single coin. Work is usually measured in terms of the force applied, while energy is measured in terms of other factors such as heat. Power is defined as the amount of work completed in a given amount of time.

Power is a measurement of how much work can be accomplished in a given period of time. The following equation can be used to describe power:
$\text{Power} = \dfrac{\text{Work}}{\text{Time}}$
Since work is expressed in joules (J) and time is expressed in seconds (s) in this equation, power is expressed in joules per second (J/s). This is the watt, which is also known as the SI unit for electricity (W). $1$ joule of work per second is equivalent to $1$ watt. Watts is a term you're probably already acquainted with. The watts of power that light bulbs and small appliances like microwave ovens provide are labelled.

Given: $\text{Power}\,(p) = 1000\,watt$, $\text{Time}\,(t) = 90\,\sec$
As we discussed the power formula above that:
$P = \dfrac{W}{t}$
Therefore by simplifying we will get:
$1000\,watt = \dfrac{W}{{60}}$
$\therefore W\, = 1000 \times 90\, = \,9000\,Joules$

So we can say that a microwave oven of $1000\,watt$ uses $9000\,J$ of energy in $90\,\sec$.

Note: To solve this problem there are some important points we should keep in our mind. Firstly $1$ joule of work per second is equivalent to $1$ watt, this is the most important part to approach the problem. Secondly the power formula, by using this we can get better clarification of the problem.