A microwave oven has a power requirement of....

Question

A microwave oven has a power requirement of 1,251 W. A frozen dinner requires 5.4 min to heat on full power.

(a) How much electrical energy (in kWh) is used?
____ kWh

(b) If the cost of electricity is 11¢ per kWh, how much does it cost to heat the dinner? (Do not round your final answer. Fractional cent values are acceptable.)
____ ¢

Answer 1

Sorry, Michael J., $0.11 * 0.113 = 1.34 cents, not dollars.

 

Colsten, if you're trying to match the energy drawn with the heating delivered, you may want to read the appliance label (usually inside the door-close area). The actual klystron power (microwave) output is somewhat less than the total power usage of the oven as a whole, AND the energy delivered to the food is somewhat less than that. The reason for the latter is that not all the microwave energy is absorbed by the food -- some is reflected into a microwave trap, which is present in all consumer microwave ovens as a safety device to prevent the power bouncing back into the generating area and blowing the circuitry. If you get interested, you can download free circuitry diagrams from the manufacturer, and try to trace how that would occur!

Answer 2

Electrical energy has the units kWh. This tells you that you multiply the power and time together.

 

Energy = Power  x  time

 

Energy = ((1251 / 1000) kW * (5.4 / 60) h)

           = (1.251 kW) * (0.09 h)

           = 0.113 kWh

 

Next, is to find the cost.  The rate is $0.11 per kWh.

 

($0.11) * (0.113) = $0.0124

 

 

It costs $1.24