# Could a Bug Circus Charge Your Phone?

By Rhett Allain

Hat tip to John Burk (@occam98) for sharing this video (an ad for Snapdragon):

Well? Could this work? First, I am going to assume this is fake. Although I don’t know a whole bunch about bugs, I assume you can’t train a praying mantis to ride a bike.

Fine, it is most likely fake. But is it possible anyway? Ok, let me try to estimate this possibility. First, let me point out that some of these circus acts probably aren’t for the generation of power. The merry-go-round for instance. It seems like those bugs are just having a good time and not working hard like the scorpion and beetles. Maybe they are on their union break.

Let me at least get an estimate for the power produced from the tarantula on the inclined treadmill.

Here are some starting assumptions:

- The treadmill is inclined at an angle of about 30 degreea.
- The spider is about 5 cm long and as a mass of 50 grams (Wikipedia page on tarantulas).
- The tarantula is moving with a speed of about 1 cm/second (relative to the treadmill).
- This treadmill-generator is 70 percent efficient. This is probably way too high for something like this, but I am trying to give the bug-charger the best chance of working.

One final assumption: The work done by the spider (and the power) are the same for this treadmill as it would be for the spider walking up the incline. So, let me calculate the power the spider would need to walk up that incline at the same speed. Here is a force diagram:

If the spider is moving at a constant speed, these forces have to all add up to the zero vector (no net force means no change in velocity). What is the *F*_{f} force? This is friction. I have drawn the forces ON the spider, not the force the spider pushes on the track. However, since forces are an interaction between two objects, the force the spider pushes on the track is the same magnitude as the friction force.

I can find the value of this frictional force (and the force the spider pushes) by looking at the forces along the plane. The plane-component of the weight and the frictional force have to have the same magnitude:

Now, what about the work? The work done by the spider will then be:

Where *x* is some distance up the incline. To determine the power, I need to know the time it takes the spider to go this distance. If the spider moves with a speed *v*, then *t* = *x* / *v*. This means the power is:

Oh, let me call the efficiency *e* then the electrical power output from the device would be:

Using the estimated values above, this gives a power production of 0.0017 Watts. WOOO HOO! That is some serious power. Instead of looking at all the bug devices, let me just say that there are about 10 of these spider-treadmills that all produce the same power. This would put the circus at 0.017 Watts.

## Charging a phone

I have looked at phone charging before (charging a phone by typing). From that, I used a 1420 mAh battery at 3.7 volts. This gives a total amount of stored energy at 1.89 x 10^{4} Joules.

Using my bug-circus, how long would it take to charge this phone?

That is just over 1 MILLION seconds or almost 13 days. Yes, thirteen days seems like a long time. However, you can at least enjoy the greatest bug show on Earth while you wait.

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