# Dielectric Constant of Food in microwave

I made the experiment up myself so my errors may be a result of this, but I heated up marshmallows, eggs and cheese in the microwave and measured the distance between the hotspots in order to get the wavelengths. For the marshmallows I got 0.13cm, for the eggs I got 0.16cm and for the cheese I got 0.10cm.
However when I plugged these into the equation to get dielectric constant:
c=(λ)(f)√ε

I got marshmallows=0.97, eggs=2.2, and cheese=0.87
and I read somewhere that dielectric constants can't be less than 1.
We don't cover dielectric constant in my physics class, and my teacher doesn't know too much about it in relation to food
I don't know if it was a flaw in my experiment, if the heat of the microwave or fact that it was food instead of a gas, or maybe that dielectric constants in food follow different guidelines,
but if anyone has any idea about this and could just explain where I went wrong I would appreciate it so much!

## Answers and Replies

CWatters
Homework Helper
Gold Member
I think you mean 13cm not 0.13cm etc.

I'm not sure how you got your values, but using the standard microwave oven frequency f = 2.45 GHz, along with your formula, I get the relative dielectric constants to be:

Marshmallow: 0.89; Egg: 0.59; Cheese: 1.5

I would expect these values to be much higher (as in, something on the order of 10 or 100) if they are indeed the dielectric constants of these materials.

I could be wrong, but I suspect the problem is that the distance between hotspots is not actually the wavelength of light within the food. Rather, it has more to do with the air in the microwave chamber and the geometry of the chamber. So basically, instead of measuring the wavelength of light inside the food you're heating, you're getting a very rough measurement of the wavelength of light inside the chamber (i.e. the wavelength of light in air).

That's just my guess though, can anyone confirm/deny?

CWatters
Homework Helper
Gold Member
Google found this article which suggests there is a lot more to it. It questions the whole issue of measuring the speed of light or wavelength using a microwave oven..

http://www.wensh.net/archive.php/topic/1527.html

VI. Conclusions

E&M field in microwave oven is analyzed. It is not a simple plain wave or a 1D standing wave. Thus the method of measuring speed of light by measuring 6cm apart hot spots does not make sense. Instead, the method by measuring “components” of wavelength is more reasonable. However, it is valid only in the ideal resonant cavity approximation. In a real microwave oven, all sorts of perturbation can fail this method.