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So I was visiting this insctructable safety light for bikes which takes advantage of magnetic induction to get current and power some 2V red LEDs. How does it work if no magnetic field gets trough the coil? Does it work even if the magnets get near it?

I was going to make my own, would you give me any suggestion, for instance the use of a capacitor?

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PS: I haven't covered this argument at school yet, so I read my textbook to get a (probably leaking) basic knowledge of this topic, any correction is welcome

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According to Faraday's Law of magnetic induction, a changing magnetic flux $\phi_B$ will produce an electromotive force (emf $\epsilon$) in an adjacent coil loop.

$$\epsilon =-\frac{d \phi_B}{dt}$$

So as the permanent magnets move past the coil, the magnetic flux through the coil changes, resulting in an electromotive force which can be used to power a light source such as an LED.

Its the changing magnetic flux through the coil which induces a voltage. The magnet itself does not need to go through the coil.

As for building one yourself, it looks simple enough. The main thing is to make sure the coil has adequate number of windings to produce the required voltage. More windings will result in higher voltage for a given flux rate.

Also, the wire in the coil must be able to carry the load current. LED lights are usually quite low current so this is unlikely to be an issue.

The two magnets will cause a voltage in the coil to pulse as the wheel goes around. If you add a capacitor in the circuit, you may be able to make the circuit sustain a light rather than flashing, but a flashing LED is more likely to draw attention and is therefore usually preferred.

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  • $\begingroup$ usually 'not' preferred right ? $\endgroup$
    – Gowtham
    Commented Dec 29, 2014 at 12:49
  • $\begingroup$ flashing light is preferred on a bike because it draws more attention (which is what you want at night... so the car drivers will notice and hopefully less likely to collide with you) $\endgroup$
    – theo
    Commented Dec 29, 2014 at 12:59
  • $\begingroup$ but why are the lights in bikes/cars in the world not flashing (i mean the rearlights). Is the reason that it maybe distracting ? $\endgroup$
    – Gowtham
    Commented Dec 29, 2014 at 13:02
  • $\begingroup$ bicycles.stackexchange.com/questions/2294/… $\endgroup$
    – theo
    Commented Dec 29, 2014 at 13:20
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    $\begingroup$ @maxpesa The total emf $epsilon$ across $N$ coils, each of which is the boundary of a surface $S$ which intersects a changing magnetic flux $\Delta \Phi_B$ in a given time $\Delta t$ is given by $\epsilon=-N\frac{d \Phi_B}{dt}$. The magnetic field (or magnetic flux density) is given by $B=\mu_0 H$, where $H$ is magnetic field intensity, which in turn is realted to the current $I$ (or current density $J$) in the wire by $\oint H.dL=I=\int J.dS$. $\endgroup$
    – theo
    Commented Dec 29, 2014 at 22:22

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