4.38. Power http://www.ck12.org
FIGURE 4.73
Compared with a less powerful device, a more powerful device can either do more work in the same time or do the
same work in less time. For example, compared with a low-power microwave oven, a high-power microwave oven
can cook more food in the same time or the same amount of food in less time.
Calculating Power from Work and Time
Power can be calculated using the formula above if the amount of work and time are known. For example, assume
that a microwave oven does 24,000 joules of work in 30 seconds. Then the power of the microwave is:
Power=WorkTime=^2400030 sJ=800 J/s, or 800 W
Q: Another microwave oven does 5,000 joules of work in 5 seconds. What is its power?
A: The power of the other microwave oven is:
Power=^50005 sJ=1000 J/s, or 1000 W
Q: Which microwave oven will heat the same amount of food in less time?
A: The 1000-watt microwave oven has more power, so it will heat the same amount of food in less time.
Calculating Work from Power and Time
You can also calculate work if you know power and time by rewriting the power equation above as:
Work = Power×Time
For example, if you use a 1000-watt microwave oven for 20 seconds, how much work does it do? First express 1000
watts in J/s and then substitute this value for power the work equation: