# Homework Help: How much energy does a bicycle diname produces?

1. Sep 16, 2006

### Veselin

So my question is actually really simple.I want to know approximately how much energy does a normal dynamo produce when a bicycle wheel is spinning (at let's say 10 km/h).Please answer me with a number not with a formula because i'm really bad at physics but i just want to test a theory of mine.

P.S: ah.I almost forgot. I want to know what friction does the dynamo make with the wheel.And for my comfort if you could tell me will this friction significantly slow the normal 10km/h and how much exactly ?

Thank you!

2. Sep 16, 2006

### Andrew Mason

The force required to power the generator depends on the load. Lets assume it is 12 volts and draws one amp. This is 12 watts. If the generator is about 80% efficient, this means it is drawing 15 watts of power from your pedalling.

You would have to figure out how much power a human can generate but I expect it is significantly greater than that - probably 10 times that. The amount you would slow down would be the proportion that 15 watts is of the total power the person can output.

AM

3. Sep 17, 2006

### Veselin

Well actually i'm wondering if the following is possible. If you put enough dynamos on a bycicle wheel could they produce enough energy to keep the wheel spinning?A kind of a perpetuum mobile maybe?Why shoudn't it be possible when you can put many dynamos?But will the friction be so huge that they couldn't power the engine that has to spin the wheel?

4. Sep 17, 2006

### Andrew Mason

You will always have to input more energy than the machines produce. Machines are never perfect so it takes more energy to run them than they can produce. But even with a perfect machine, you could not get MORE energy out than you put in. This is the law of conservation of energy (which is also the first law of thermodynamics).

Furthermore, the human being is a thermodynamic engine. It produces kinetic energy (of the bicycle) by converting heat energy into mechanical work. The second law of thermodynamics says that you cannot convert all of the energy in heat into useable work.

So, in order to produce a perpetual motion machine you will first have to find some way to prove that the laws of physics are wrong: specifically, the first and second laws of thermodynamics.

AM

5. Jan 13, 2010