You are looking for a new electrical appliance that will last a ...

[FButt]

You are looking for a new electrical appliance that will last a long time. You are considering buying a high-efficiency model that uses an average of 60 W
 
  of power and
  costs 800 or a lower efficiency model that uses an average of 90 W and costs 600 . If the
  cost of electricity is 0.10/kWh and the appliances are operated continuously, how much
  money will you have saved by purchasing and operating the high efficiency model over the
  low efficiency model after 15 years?

Question 2

For a tidal energy device, bigger tidal range is better.
 
  What will be an ideal response?

[tuwy]

Answer to Question 1

The total cost of the more efficient refrigerator will be
800.00 + 60 W x 15 yr x 8760 h/yr x 0.10/kWh
= 800.00 + 788.4 kWh x 0.10/kWh = 878.84
The total cost of the lower-efficiency model is
600.00 + 90 W x 15 yr x 8760 h/yr x 0.10/kWh
= 600.00 + 1182.6 kWh x 0.10/kWh = 718.26
Far from saving money, it would cost you about 160 more for the more efficient model in
this comparison. This problem should impress on the student the need to study the energy
ratings and the additional costs involved.
Actually, the power given for the less-efficient model is probably too low (and some models
do even better than the quoted high-efficiency power consumption). If we used 120 watts
for the less-efficient refrigerator, though, we'd still save by buying the cheaper model
although only about 2 . Because a typical refrigerator lasts 19 yr, the more energy-efficient
model would save money over its entire lifetime.

Answer to Question 2

This is true because a larger tidal range means a greater gravitational potential
energy difference.

[FButt]

Gracias!

[mohan]

Excellent