Inductive Kickback

[citfta]

Synchro will you please put together the simple circuit Tinman has provided?  I have reattached it here.

Now short out the cap to make sure it is fully discharged.  Then momentary tap the switch closed and then leave it open.  Now check the voltage on the cap.  If you are correct about the current reversing then the top of the cap should be positive and the bottom negative.  However you will see the bottom of the cap is positive and the top of the cap is negative.  This is because the coil has become a source for the short time it takes the magnetic field to collapse.  Please answer my question about which way the current flows inside of a battery.

Carroll

[tinman]

@Tinman,

"It is also opposite to that which created it. This is where you are lost. Power was supplied to the inductor to create the magnetic field.Then the magnetic field returned the energy which created it in the first place".

What is this some kind of attempt at burlesque?

The collapsing magnetic field is what inverts the voltage across that inductor. Although the magnetic field is collapsing around that inductor,and this inverts the voltage,the magnetic polarity remains the same,and this maintains the current flow direction,so as it is the same as it was before the power source was interrupted. In order for the current flow to change direction,the magnetic field polarity would also have to invert-which it dose not.

[MileHigh]

I think there is a lot of confusion around the "reversing voltage" and the explanation for that is quite simple.

When you energize the inductor you have an external voltage source across the terminals of the inductor.  So an external power supply defines the voltage across the inductor.

Then when the switch opens, the external voltage source is gone - it has no influence at all on the voltage across the inductor.

When the switch opens the inductor now acts as a power source.  The inductor as a power source outputs current.   The inductor works to keep the current flowing in the same direction.  The load creates the counter-EMF under the imposed current flow from the inductor.

So, in the truest sense, the inductor is just a hapless rag doll along for the ride when it comes to the voltage across its terminals.   In the first case, the external power supply is imposing its voltage across the terminals of the inductor.  In the second case, the load itself is imposing its voltage across the terminals of the inductor as the inductor discharges a current flow into the load.

In both cases, the inductor is a "follower" when it comes to the voltage across its terminals - like a hapless rag doll.

[MileHigh]

An entire cottage industry is built around the "secrets" of the inductor.

Here is a fun thought demonstration:

In one hand you have a one-farad capacitor with one volt across it discharging across a one-ohm resistor.

In the other hand you have a one-henry inductor with one amp traveling through it discharging through a one-ohm resistor.

If you put a scope across the resistor in both cases you would see an identical exponentially decaying voltage waveform.  Since it is a one-ohm resistor, you would also be looking at an identical decaying current waveform.

For the capacitor circuit, as you decrease the value of the load resistor, it starts to discharge faster and faster with higher and higher currents through the load resistor.

For the inductor circuit, as you increase the value of the load resistor, it starts to discharge faster and faster with higher and higher voltages across the load resistor.

If you can wrap your head around this then you will have an understanding about how capacitors and inductors work as energy storing devices and how they discharge their stored energy into a load.

[Jeg]

Hi Tinman

There is really no sense for voltage to change polarity while the field is collapsing. Voltage and current have a strict relation between the two. Current always flow from higher voltage potential to a lower one and never the opposite even momentarily. As current keeps going the same direction after collapsing, the same is with voltage. Not only it doesn't alter its polarity, but also becomes magnitudes higher also momentarily like current does.