Electronics > Beginners

Help me understand inductance.

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hamster_nz:

--- Quote from: bsfeechannel on October 17, 2018, 04:54:21 am ---
--- Quote from: hamster_nz on October 17, 2018, 01:55:09 am ---So this is the bit that troubles me. How would the "voltage reverse in order to try to keep the current flowing". In that statement is the cause and effect the wrong way round?

--- End quote ---

Voltages can induce currents as currents can induce voltages. They are the two sides of the same phenomenon.

Since inductors tend to maintain their currents, when you switch from a voltage source to a load, the voltage has to be inverted so as to maintain the current.

You find an analogous behavior with a capacitor. In order to maintain its voltage, it inverts its current when you switch from a source to a load.



--- End quote ---

Yes! This! It (kind of) makes perfect sense. I'm going to have to ponder/think/sleep on this one, but I can sort of feel the understanding starting to form....

hugo:
Just go to: http://www.falstad.com/circuit/

The "Circuits" menu contains a lot of sample circuits for you to try.  ;)

T3sl4co1l:
If you (think you?) understand capacitors:

Just swap V and I.

It's that simple. ;D

To really cook your brain -- and drive home your knowledge of circuits! -- demonstrate the equivalence of parallel-series circuits where V and I are swapped, L and C, and series and parallel.  The node voltage equations of one will be exactly the current mesh equations of the other (with V replaced with I, of course).

(Uh, you will have to be far enough into intro EE to know what those things are, and the basics of how to do them.  I don't know how "beginner" that is in this case...)

Tim

kosine:
If you read the following carefully (maybe twice!), the fundamentals should hopefully make more sense:

1. Electrons have momentum:
Think of inductance as length (L).
Say a short wire and a long wire both carry 1A; Both have the same current (I), BUT the longer wire contains more electrons in motion.
It therefore has more electrical momentum. (Compare this with pipes carrying water.)
Electrical momentum is termed "magnetic flux" and is given the Greek symbol phi.

Since longer wires contain more electrons, the total momentum (phi) increases with length.
But a greater current also means more electrons, so momentum (phi) also increases with current.
Taken together, momentum (flux, phi) is proportional to L, and also to I.
So phi = LI. (The units are chosen such that the proportionality constant is 1.)


2. Momentum accumulates over time:
Now, to get a current flowing at all, we need to apply an electromotive force. This is the voltage (V).
Increasing this applied force/voltage will increase the momentum, but so will applying it for a longer period of time (t) because momentum accumulates.

Therefore we can also deduce that phi is proportional to V, and also to t.
So phi = Vt = LI
This is the textbook inductor equation, and is analogous to classical momentum:
Momentum = Ft = dm/t (since F=ma and a=d/t^2. Think of "mass per second" as the current.)


Inductors (as a component) are essentially a very long wire wound into a coil to make them more compact. (The coiling also increases the effective length due to magnetic effects, but that gets a lot more complex to explain.) Inductors allow you to build up momentum in the circuit, thereby storing the electrical equivalent of kinetic energy. If you suddenly break a circuit with lots of momentum in it, all that energy has to go somewhere. It'll either be converted into potential energy (i.e., a voltage), which is how a SMPS works*, or you can safely dissipate the energy as heat by adding a diode. That allows current to circulate back through the inductor until all the stored energy is used up.

* If you break the circuit on the high voltage side of an inductor, all the electrons flow away from the break resulting in a negative voltage spike. Break the circuit on the low side, and all the electrons pile up and cause a high voltage spike. How large these voltages get depends on many things, including the length of the coil. At it's simplest, that's its inductance (L).

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