In the latest episode of the MythBusters, Adam and Jamie wanted to see if you could make a rocket fueled by gummy bears (they also tried poop). You can get a lot of energy from sugar (in the gummy bears) so why not use this energy to lift a rocket?
But how do you measure the energy you can get from different materials? The answer is a bomb calorimeter.
Where does the energy come from?
It’s tempting to say that there is energy stored in the sugar. That’s not quite true. Actually, you get energy out of sugar by breaking up the molecules of sugar (which requires energy input) and forming new bonds to make stuff like carbon dioxide (which gives more energy than you put in). If you want more info on energy and chemical bonds, take a look at this older post on energy and chemistry.
The key point is that we are not just breaking up sugar to get energy. We have to create a new molecule to get any energy out of the sugar. For the case of gummy bears, we make a reaction between the sugar and oxygen and one of the products is carbon dioxide. But how do you measure the change in energy?
The basic idea of a calorimeter is to measure the amount of energy you get when you burn some substance. Here is the old device that chemistry students use in lab.
Yes, that just looks like a box with stuff coming out of it. Let me draw a simplified diagram of the inside.
Here’s how it works.
- Put a sample in a bomb (it’s called a bomb, but it’s just metal container that doesn’t explode).
- Add oxygen to container and seal it up.
- Put the bomb inside the water (at a known temperature) inside of the insulator.
- Use an electric current to ignite the sample. It burns in the oxygen (there has to be oxygen in there for it to work).
- When the sample burns, it heats up the container and the water. By knowing the specific heat capacity of water and the bomb, you can determine the total energy output.
- Done. That’s it.
This gives the total change in energy for the substance as it burns. You can then convert this to calories—if that is your favorite unit for energy. For me, I like the Joule.
If you do this for poop or gummy bears, it should give an upper limit for the total amount of energy you could get from a poop-based rocket. If you want to find the edible food calories for gummy bears, this might not give the best answer. There is a big difference between a bomb calorimeter and a human’s digestive system. Humans don’t normally completely consume all of the energy in food. Some gets “exported” before being completely used up. This is one of the reasons that people complain about calorie counts on food labels.