Question

a refrigerator has a power of 90w and is left on for 24 hours. A hair dryer has a power 900w and is on for 20 minutes. which statement correctly compares the energy used by these devices?

Answer 1

The refrigerator uses 6,696,000 J more than the hair dryer.

I just took the quiz and this was the correct answer.

Step-by-step explanation:

Other answer made me get it wrong.

Answer 2

Remember that the power P is the energy E per unit of time t:

`P= (E)/(t)`
from which

`E=Pt`
Let's find the energy used by the two devices and then let's compare them.

- refrigerator:
the total time the refrigerator was on is

`t= 24 h=86400 s`
Its power is 90 W, therefore the energy used is

`E=Pt=(90 W)(86400 s)=7.78 \cdot 10^6 J=7.78 MJ`

- hair dryer:
the total time the hair dryer was on is

`t= 20 min=1200 s`
its power is 900 W, so the energy used is

`E=Pt=(900 W)(1200 s)=1.08 \cdot 10^6 J =1.08 MJ`

So, comparing the two energies, we can say that the refrigerator has used more energy than the hair dryer.