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Does My Bike Induce Electricity?
An anonymous reader asks: "I have a steel frame road bicycle and recently began riding on a path that parallels high-tension power lines for several miles. My question is: Does my bike induce an electric current by passing through the electric fields from the power lines? I normally ride the section at about 18 miles per hour, estimate the distance to the lines at about 75-100 feet, and think they're 200KV lines."
Answer: Yes. Is it enough to matter? No.
alright, I'm a long way from being an EE, but I remember my high-school physics pretty well.
a magnetic field will form in concentric rings around any conductor that's carrying current, and the strength of that magnetic field will be proportional to the amount of current flowing through the conductor, and will drop off sharply with distance.
now, currents are only induced by a changing magnetic field, and the current will be proportional to the change in magnetic field strength. so if the power lines were carrying DC, and you were riding parallel to the lines, no current would be induced in you because the strength of the magentic field where you were riding would stay about the same -- but the power lines are probably carrying AC, so even if you stay still, the magnetic field will be reversing polarity every 1/60th of a second, and a current will be induced in you.
however, there are a few mitigating factors.
one is that you're probably several meters away from the lines, and magnetic fields don't couple well through air. transformers often have an iron core to guide the magnetic field between the two coils, because otherwise not much power gets transferred between them.
another factor is that the magnetic field around a single conductor is not very strong, unless it's carrying really a lot of current (like wire-melting amounts). again, transformers and electromagnets use coils with hundreds or thousands of turns to get a decent magnetic field going.
the last factor is that the lines are carrying high voltages. the comment in your submission seems to indicate that you think this will create a stronger magnetic field, but actually the opposite is true. for our purposes, electric power has two aspects, voltage and current; the voltage determines the strength of the electric field, and the current determines the strength of the magnetic field. for the same amount of power, if you increase the voltage, you reduce the current. the power company doesn't want current being induced in random objects, because that power gets lost as heat and costs them money (actually, they probably care more about the fact that large currents create heat in the wire itself -- but it works out to the same meaning for you). they run long lines at very high voltages as part of a strategy to minimize this power loss and save money.
The reason power is transferred via high volatage AC power, and not any kind of DC power is simple:
The lower the current you send down a wire, the less energy lost due to resistance. (P=IE, E=IR, so P=R*I^2). This is a major concern over long wires, where the overall resistance can be quite high. With AC power, you can transform from high current / low volatage, to low current / high voltage, and back quite easily. So you can put your powerplant far away from the consumer. However with DC, you can't do this. DC can't be transformed (at least, not easily?), and so you have to run it down the powerlines at the same voltage/current as the consumer will use, which is quite high. Hence, you loose a lot of energy on powerlines very quickly, and thus your powerplant must be close (within a couple miles actually) of the customers.
I should also mention (although I think I see posts saying this later on) that, indeed, the bike is a one loop conductor, and that the powerlines (via the right hand rule) have a varying magnetic field (because the current is varying), but the induced emf is very small. This is because the current in the wires is small, and there aren't very many of them, and you are 50+ ft away, and your bike is only one loop. I'd call the induced emf negligable personally.
-Jeff
Physicist, Programmer
Yes, you do in fact get an induced current. However, as demonstrated vividly by a question on my final when I was taking first semester E&M, you need somewhere on the order of 200,000 loops of wire to get enough induced current to be worthwhile from 20,000V power lines. (The exact question was, is it feasible to steal power by placing a coil between two 20,000V power transmission lines.) I'm pretty sure you don't care about the negligible effects (unless you're paranoid, or your bike is composed of 200,000 loops of wire).
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