How efficient is a microwave oven? Does the efficiency decline as the oven ages?
Our 15-year old kitchen microwave appears to heat things slower than it didwhen new, so I did an experiment to see how long it would take to heat 2 cups of water from near freezing to boiling. It took about six minutes, but, by my estimate, should have been accomplished in under one minute: (2 cups)*(2.366e-4 m^3/cup)*(1000 kg/m^3)*(1000 g/kg)* (4.186 J/g-degC)*(100 degC)*(1 sec/5400 J)*(1/0.64)W sec. The microwave is either 6500W or 5400W, depending on the voltage supplied, so I assumed the lower. 4.186 is the specific heat of water. 0.64 is Wikipedia's estimate of a typical oven efficiency (no citation given). The time would have to be increased to account for the heating of the Pyrex glass container, but glass has a specific heat about 20% that of water, so little additional energy is needed to heat the glass. Am I missing something?
Our 15-year old kitchen microwave appears to heat things slower than it did