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Old 02-19-2009, 09:05 AM
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Aaron Aaron is offline
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Back EMF vs. Collapsing Magnetic Field Spike

Hi Everyone,

This does come up time to time and it isn't a matter of semantics. Back EMF simply is not the spike that you get from a coil when the field collapses.

You charge a coil - lenz's law describes the counter current or back emf that opposes the forward current and resists the forward current's ability to bring the coil's charge up...

Once the coil is charged and you disconnect power, the spike you get back is the "inductive spike" or "transient spike."

You can see Lenz's law here:
Faraday's Law
It is at the bottom.

Look at this nice simple answer:
WikiAnswers - What is the formula for transient spike computation in an inductive load

"E=I x R. The inductive spike occurs as the circuit is opened. The collapsing magnetic field causes the inductor to become the source of the circuit. For example consider a circuit consisting of a 10 volt battery, a 10 mh inductor, and a 10 ohm resistor all in series. With the switch closed, 1 amp will flow through the circuit (after 5 mS). The 5 mS is the time it takes the current to rise from 0 to 1 amp. This is given by the formual TC=L/R where TC is the time constant in seconds, L is the inductance in henries, and R is the resistance in ohms. It takes 5 time constants for the current to reach the maximum current which is determined by I=E/R (Ohm's Law). The delay is caused by the counter EMF generated in the coil as flux lines cut through adjacent turns of the inductor. After 5 time constants, the current is at 1 amp. When we open the switch, it will take 5 time constants for the current to drop to 0 amps. However, this will not be 5 mS because the resistance is now much larger do to the opening switch contacts. The voltage across the switch contacts will be whatever is necessary to maintain the current flow for the 5 time constants. After one time constant, the current will have dropped to 32% of the maximum current or in this case, 0.32 amps. If the resistance of the switch gap is 1 megohm, the the voltage will be 320,000 volts. More than enough to ionize the air and create a conductive path. If we assume an average resistance of 1 megohm, it will take 50 nS for the current to drop to 0. Of course during this time, the switch contact gets zapped. Placing a diode across the inductor such that the diode is reverse biased with the switch closed will give the current an alternate path as the polarity of the inductor reverses when the magnetic field collapses and the inductor becomes the source. This lowers the voltage from 1,000,000 volts to 0.7 volts. The downside is that the time it takes for the current decrease increases bo the ratio of 1,000,000/0.7. In a relay, this may cause the relay to "chatter" when opening. Adding a zener diode in series anode to anode with the spike suppressing diode will alleivate most chattering problems. A 34.3 volt zener will raise the voltage from 0,7v to 35v and shorten the time by a factor of 50 (35/.0.7). "

So you can see that it takes 5ms to charge the coil because the back emf opposes the forward current...that is the delay of charging the coil...the back emf.

You can see it takes 50ns to go back to 0. Why so fast? There is no more back emf opposing anything.

I don't agree that the calculation of the spike is as straight forward as this because other things come into play with sharp gradients.

But you can clearly see the back emf is NOT the spike that comes back. The spikes we are capturing and putting to use is the "inductive spike" or "transient spike" and I believe it does matter what it is called because there are very specific names for these very specific well-known events that have been established for a really long time.

People experimenting with the free energy stuff won't have much credibility in the general world of science calling the spike back emf.

They can believe what the want, that is fine but it is simply ample evidence for them to show that people in this "free energy" field don't even know what they're talking about and they would be correct. Let's not give them any ammunition. If they see that we do know the difference, it is just less resistance (back emf) that we have to work against in getting this stuff out there. It really is an inductive spike or transient spike and the back emf is already gone.
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Sincerely,
Aaron Murakami

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