The physics of cooking

[tielfan]

I need some help with the physics of cooking, to be used when people start freaking out over the alleged dangers of microwave ovens.

 

1. It seems to me that hot food must be radiating into the infrared no matter how it was heated. In the case of microwave heating, a lower frequency is used to stimulate a frequency in the food that is higher than the frequency used to induce it. Correct?

 

2. A stove burner transmits heat by conduction and an oven does it by convection. But how is the heat generated in the first place? Does an electric oven use electricity to generate infrared in the heating element, whose energy is then transmitted by conduction or convection? Or is something else going on? The heating element turns red so it seems that radiation is being generated in the visible spectrum.

 

3. Does a gas stove generate infrared at all, or is the heat purely the result of the chemical reaction that occurs when we burn the gas? Do the flames look blue because they're generating blue photons, or because they're reflecting the blue photons that were already bouncing around the room?

 

4. Is cooking over a fire different from cooking with gas, in terms of chemical heat versus electromagnetic radiation? I assume that a different chemical reaction is involved, since we're burning different materials.

 

Thank you for your help.

Edited March 7, 2017 by tielfan

[swansont]

I need some help with the physics of cooking, to be used when people start freaking out over the alleged dangers of microwave ovens.

 

1. It seems to me that hot food must be radiating into the infrared no matter how it was heated. In the case of microwave heating, a lower frequency is used to stimulate a frequency in the food that is higher than the frequency used to induce it. Correct?

 

2. A stove burner transmits heat by conduction and an oven does it by convection. But how is the heat generated in the first place? Does an electric oven use electricity to generate infrared in the heating element, whose energy is then transmitted by conduction or convection? Or is something else going on? The heating element turns red so it seems that radiation is being generated in the visible spectrum.

 

3. Does a gas stove generate infrared at all, or is the heat purely the result of the chemical reaction that occurs when we burn the gas? Do the flames look blue because they're generating blue photons, or because they're reflecting the blue photons that were already bouncing around the room?

 

4. Is cooking over a fire different from cooking with gas, in terms of chemical heat versus electromagnetic radiation? I assume that a different chemical reaction is involved, since we're burning different materials.

 

Thank you for your help.

 

 

 

1. The spectrum for something in the range of ~400-500 K will peak in the infrared (500K peaks at 5.8 microns). As things get even hotter that spectrum starts including the visible, starting with red (hence the term "red hot")

 

2. Joule heating, i.e. resistance from current flow (see comment above about red hot).

 

3. The flame color is dominated by from electrons dropping down into lower states (the combustion reaction excites and ionizes electrons), so the color is not indicative of the temperature. It's the color of the photons being emitted by the de-excitation.

 

4. Temperature is primarily what matters here. That's the main effect. If the pan gets to a certain temperature, the food doesn't care how that happened. There are even inductive heating units, that transfer the energy via a changing magnetic field and the joule heating is from eddy currents in the pan itself.

[MigL]

Further, temperature is a measure of inter and intramolecular vibrations.

A microwave generates radiation at a frequency specific to the intramolecular bonds of water, causing them to vibrate more energetically.

In effect it heats the moisture in your food, boiling it from the inside.

That's why you can't 'char' your food in a microwave, no matter how hot you get it.

[tielfan]

Thank you, that's very helpful!