Curry Soup and Physics

On Tuesday evenings I teach a class of university-level introductory physics in the evening from 6.30 pm to 9.30 pm. I have to eat something before I go to teach, and so last Tuesday I warmed up a bowl of Farmboy (a Canadian food retail chain) Coconut and Lentil Curry soup. I also tweeted about it, as a joke.

I microwaved it for a minute and then tested the temperature. The soup (which is quite thick) was warm around the edges, but cold in the centre of the bowl. As a physicist, this tells me that it has a high specific heat capacity (the energy needed to raise the temperature of a unit mass of soup by one degree), but has a low thermal conductivity — the hot portions of the soup don’t transfer their energy to the cold parts very quickly. That’s why you need to stir it. So I tweeted again about my findings while eating the soup.

And immediately got a reply from one of my Twitter followers in Germany. The reply set off my internal “This is a teachable moment about Physics in Everyday Life” alarm!

What can we learn about the simple act of microwaving a bowl of soup?

Here are my initial ideas, written as a stream of conciousness:

What is a Microwave?

Electromagnetic radiation — talk about EM Spectrum, visible, infrared, ultraviolet, x-ray, radiowaves, microwaves, X-rays, gamma rays…

Definition of wavelength, frequency and speed of light

History of Science — measuring the speed of light by various methods

Importance in Physics of the speed of light as a limiting speed for matter.

How does Microwave radiation interact with matter? And why does it heat the food up?

Frequency match of microwave radiation with vibration-rotation in water molecules, which in turn means discussing structure of atoms and chemical bonding

Resonance (match the natural frequency of an oscillating system to an external driving frequency) to produce large oscillations

Oscillating systems with natural frequencies — electrical, mechanical and molecular systems

Examples of resonance — High tidal swings in the Bay of Fundy; opera singers breaking wine glasses; tuning circuits in radios.

History of science — The cavity magnetron, the generator of microwaves) invented at Birmingham University at start of World War 2 (I used to work there, which adds a nice personal touch).

Applications of microwaves: radar, communications, cooking

Heat transfer in solids and semi-solids — the water molecules are excited by the microwaves and then collide with the other molecules in the food transferring energy, thereby raising the temperature.

Temperature scales and how they are defined.

Energy — what is it, and how do we use the concept in physics?

It would be easy to write an entire lecture or even two, based solely on the concepts needed to understand my observations that it takes some time to microwave my soup and that it needs stirring.

Everyone uses physics all the time, but few people realize that it’s possible to teach a lot of physics using everyday situations. And it doesn't have to be complicated, scary or include complicated equations.

P.S. The soup was delicious.