Power, Cost, Efficiency

Efficiency is always _____________

• less than 1%

• more than 100

• more than 50%

• less than 100%

A washing machine uses 250J of electrical energy to produce 125J of useful energy. How efficient is it?

• 100%

• 5%

• 50%

• 55%

A 30 W device was used for 16.667 seconds. How much energy was transfered? round the value.

How much time is needed to produce 720 Joules of work if 90 watts of power is used? 

• 0.125 s

• 8 s

• 64800 s

• 80 s

A clock has a power rating of 4 watts. The cost of electricity is $0.11 per kWh. What is the cost to use the clock for a year?

• $385.44

• $38.54

• $3.85

• $0.39

Laptop A uses 255 kWh, Laptop B uses 154 kWh and Laptop C uses 234 kWh. Which is the most efficient?

• Laptop A

• Laptop B

• Laptop C

• There is not enough information to tell.

You want to look your best for science class, so you decide to blow dry your hair everyday of the semester. The 1.1 kW hair dryer is used for 15 minutes each day. The cost of electricity is $0.09/kWh. What would be the cost to use the dryer for the 97 days of the semester?

• $2.40

• $24.03

• $144.04

• $8.64