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I am a little confused about the application of Faraday's law in a generator. If we imagine a magnet attached to a wheel (which spins when water falls on it) next to a coil, as the magnet spins around, current will be generated due to electromagnetic induction. However, when we measure the current, the needle on the galvanometer swings back and forth. Why doesn't the current cancel out when it goes from positive to negative on the galvanometer?

Jay
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  • What do you mean, "cancel out?" I have to assume that when you say, "coil," you must be talking about a closed loop because if the loop were not closed, then no current could flow. If you integrate the current over time, that should give you the total amount of charge that has moved past a given point, and if the integration is over exactly one turn of the wheel, that value should be zero. Is that what you mean by "cancel out?" – Solomon Slow Jul 27 '19 at 17:21
  • Maybe what you are asking is why doesn't the power cancel out? If you connect the output of an AC generator to a resistive load, then the net charge motion is zero because the current keeps changing direction; but the net power delivered to the resistor is positive because every time the current changes sign, the voltage also changes sign such that their product (power = current × voltage) always is positive. – Solomon Slow Jul 27 '19 at 17:25
  • To me, it seems like the current's value swings to say 2 amps, but then when the magnet turns around, you get -2 amps. So why isn't the net current 0 amps? – Jay Jul 27 '19 at 18:50
  • "Net current" isn't a phrase that I've heard before. What does it mean? There's instantaneous current, which continually changes sign in an AC circuit, and there's Average current which, when taken over any number of full cycles of the waveform, always is zero.There is also RMS Current, which electrical engineers use in power calculations. RMS always is positive by definition. But what is "net" current, and why are you asking? – Solomon Slow Jul 27 '19 at 22:14
  • I think the confusion stems from me trying to understand how the current is used. It's a little hard for me to think of how the current is used if it keeps switching directions. – Jay Jul 28 '19 at 02:08
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    All you have to do is accept that current can do useful work when flowing in either direction. If the load is a resistor, that happens automatically because the direction that does useful work is determined by the Voltage, and the current and the Voltage always switch direction together. If the load is a motor,... There's different kinds of motor. One kind uses current the same way a resistor uses it. Another kind of motor always rotates in sync with the AC frequency so that whichever way the current is flowing, the motor always is in position at that same instant to use it. – Solomon Slow Jul 29 '19 at 12:12
  • For electronic circuits, the AC power first is converted to DC using something called a rectifier. You can google that if you want to learn more. – Solomon Slow Jul 29 '19 at 12:13

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From what I understand maybe you mean that as the current keeps changing direction so why don't we say that the net current is 0 as there exist 2 equal and opposite currents.

Simply put it's because the currents are in opposite directions at different instances of time. At no point of time are there opposite currents simultaneously in the circuit. After half a rotation you get the change in direction.

I found this YouTube video particularly helpful myself while understanding this concept as it may get difficult to visualise.

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As the magnet turns it pushes electrons through the coil (and circuit since it must be a closed loop) first one direction, then back the other direction as the magnets poles passing the coil change. After one complete revolution the electrons are back approximately where they started (approximately, as no real generator is ideal), so you could say their movements cancelled each other out. But this does not mean no energy was transferred. Think of an incandescent light bulb in your circuit, as electrons are pushed one way through the filament, it starts to get hot, then polarity changes and the electrons are pushed back the other way, the filament gets even hotter. This cycle represents one revolution of the magnet. Eventually the filament will get hot enough to glow, even though the electrons are moving back and forth from their approximate starting point.

Adrian Howard
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  • The math also proves it: Assuming an AC generator connected to a resistive load, Every time the current changes sign, the Voltage also changes sign at the same exact instant. Since the power delivered to the resistor is the product of current times Voltage, the sign of the power never changes. – Solomon Slow Jul 27 '19 at 22:27