Physics of a microwave - why does the turntable spin?

[tayles123]

Homework Statement

A microwave oven operates by injecting electromagnetic waves with a frequency of 2.45 GHz into the cavity formed by the conducting metal shields on each face of the oven box. Explain, with the aid of sketches and simple calculations, why it is generally necessary to rotate the food using a turntable during operation

The Attempt at a Solution

My guess is something to do with standing waves, and waves reflecting off of the metal shields causing interference and so different strengths of the EM waves will hit the food, so by rotating the turntable you get a more evenly cooked food.
However this is a 4 mark question, and I don't see why I need the frequency.
Help much appreciated!

 

[berkeman]

Homework Statement

A microwave oven operates by injecting electromagnetic waves with a frequency of 2.45 GHz into the cavity formed by the conducting metal shields on each face of the oven box. Explain, with the aid of sketches and simple calculations, why it is generally necessary to rotate the food using a turntable during operation

The Attempt at a Solution

My guess is something to do with standing waves, and waves reflecting off of the metal shields causing interference and so different strengths of the EM waves will hit the food, so by rotating the turntable you get a more evenly cooked food.
However this is a 4 mark question, and I don't see why I need the frequency.
Help much appreciated!

Use the frequency and the speed of light to calculate the distance between the peaks of the standing waves...

 

[tayles123]

Use the frequency and the speed of light to calculate the distance between the peaks of the standing waves...

Ah thanks I see that, but why would the distance between the peaks be relevant? Would it just be a tool in explaining why if the turntable was still, then not all of the food would get heated ~ evenly?

 

[berkeman]

Ah thanks I see that, but why would the distance between the peaks be relevant? Would it just be a tool in explaining why if the turntable was still, then not all of the food would get heated ~ evenly?

Yes, you are on the right track. What is the wavelength of 2.4GHz EM radiation?

 

[tayles123]

Yes, you are on the right track. What is the wavelength of 2.4GHz EM radiation?

0.125m so this is double the length of the standing wave?

 

[berkeman]

0.125m so this is double the length of the standing wave?

For the fundamental mode, yes.
http://media.opencurriculum.org/art.../standing-waves-and-musical-instruments/2.png

 

[haruspex]

0.125m so this is double the length of the standing wave?

What do you mean by "the length of the standing wave" that is different from the wavelength? Do you mean the distance between two antinodes?

 

[haruspex]

For the fundamental mode, yes.
http://media.opencurriculum.org/art.../standing-waves-and-musical-instruments/2.png

The frequency is given. I don't think the harmonics are supposed to be considered. If they are, you get into the question of power distribution across them.

 

[fresh_42]

I thought it's because of a better heat distribution. It might happen that you heat up only parts of your food, so spinning is at least an attempt to avoid this. Am I wrong assuming it's just that simple?

 

[haruspex]

I thought it's because of a better heat distribution. It might happen that you heat up only parts of your food, so spinning is at least an attempt to avoid this. Am I wrong assuming it's just that simple?

That's basically right, but only shows it could be a problem. You need to think about the wavelength to show there actually is a problem.