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Which cooks faster: Hot dogs in series or in parallel?
I did not come up with this idea. A lot of physics teachers I have spoken to have heard of it, but every year it is a lot of fun and definitely engages students. This is a great experiment to run as a demo, but precautions MUST be taken to ensure student safety (see my notes on safety below).
The Setup
First, create some "suicide cables." This means you need to find an old device and cut off its power cord. Separate each side of the power cord and strip the wires as shown in the picture. Then get some skewers and wrap each cable around the skewers to create electrodes. You will also need to devise a way to rig up the skewers so that they stand sturdily on their own. Get creative--clamps, duct tape--it's up to you! That's basically it, the only other thing you need to find are some hot dogs.
What to Expect
Assuming the potential difference across the series and parallel hot dogs is the same. We can combine Ohm's Law (V=IR) and the equation for power (P=IV) to eliminate current. We end up with P= (V^2) R. If you plug into a regular American wall socket, both of the parallel dogs have a potential difference of the full 110 Volts. The series dogs each only have 1/2 that potential difference (55 Volts). Since power depends on voltage squared, there should be four times as much power dissipated in each parallel dog compared to each series dog. This means the parallel dogs should cook approximately four times faster. (They don't cook exactly 4x faster because the hotter they get, the faster they lose heat to their surroundings.)
When performing this experiment, even if you take no measurements, the results are dramatic. The parallel dogs quickly start steaming and popping while the series dogs don't do so until much later. I have used infrared thermometers to monitor the temperatures of the dogs as they cook and this was a good way to quantify measurements, but is not really necessary because it's so easy to see the results.
Notes on Safety
As a general rule, I wouldn't run this demo if I felt I couldn't control my classroom. Remember, there are safety risks with high voltage so even though this demo is a lot of fun, I am careful not to be too blasé about the whole thing. I also ensure that I set up the demo in a location where it's easy for students to see, but not touch. Usually I lay down some tape on the floor to create a line that students aren't allowed to cross so that the hotdogs are out of their reach. I also check and double-check that wires are unplugged before touching the dogs.
I also demonstrate safe practices myself when the current is on. As a personal rule, I always keep one hand behind my back. This means that if I do happen to do something stupid and touch it (we all have those momentary lapses of reason) current isn't likely to flow through my torso. This is a good practices to explain and model for students as well.
Circuit Breakers and High Currents
Because I live in Korea and we use 220 Volts I chose to use a voltage converter (to 110V) to make it a bit more safe. You can see the converter in the picture of the setup. If I were in the US, I would plug my suicide cables right into a wall outlet. But, be aware! The dogs may have low enough resistance to draw sufficient current to trip your circuit breaker. I would recommend trying it out and seeing what happens. This is another reason I chose to use the voltage converter. The voltage converter will automatically limit the output current so that it won't trip the school's circuit breaker. This is a bit problematic, because it doesn't guarantee an exact 110 Volts. In fact, I tested the dogs with the a multimeter and the voltage across each dog was substantially less than 110 Volts, but not so much less that it affected this as a demonstration.