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# Magnetic Flux watch

1. A question I've just come across, when the magnetic flux through a coil decreases, does the voltage induced become negative, or is it still positive?
2. If it was positive when the flux increased, then it will be negative when the flux decreases.
3. Yes it was positive when the flux increased, therefore the latter is also true. Thanks.
4. (Original post by Stonebridge)
If it was positive when the flux increased, then it will be negative when the flux decreases.
Ooh, sorry to butt in here, but I haven't done this and it seems like it's the kind of thing I need to know for my exam on Thursday. Could you possibly explain why this is, if you have the time and inclination, please? Thankyou!
5. I only have a few moments to answer this just now. It's to do with Lenz's Law and Faraday's Law. The voltage (emf) that gets induced in the coil, is always in a direction such that it tries to oppose the change that produced it. (Lenz's Law) So if increasing the magnetic field (the change you made) caused the coil voltage to go positive, then decreasing the field must cause the opposite.
I'm sure someone will be able to jump in and write a longer and better answer than I can at this moment.
I will have a look in again tomorrow and add more if no one else has.
6. (Original post by Stonebridge)
I only have a few moments to answer this just now. It's to do with Lenz's Law and Faraday's Law. The voltage (emf) that gets induced in the coil, is always in a direction such that it tries to oppose the change that produced it. (Lenz's Law) So if increasing the magnetic field (the change you made) caused the coil voltage to go positive, then decreasing the field must cause the opposite.
I'm sure someone will be able to jump in and write a longer and better answer than I can at this moment.
I will have a look in again tomorrow and add more if no one else has.
Ok thanks, that's great! I wasn't expecting a massively long reply, don't worry - I'm just having a bit of a last minute exam panic, about physics in particular .

So if I to understand it correctly: By Faraday's law, induced emf depends on the rate of change of flux, and by Lenz's law, the direction of induced emf is such as to oppose the change that caused it. Therefore changing the magnetic field initially will make the emf flow in one direction, and reversing the change will reverse the direction of emf? If that's right, then it makes sense. Yay!
7. You got it!

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