Inductance is the phenomenon where a changing magnetic field causes (induces) an electric current
to flow in a wire. Self inductance occurs when a changing magnetic field caused by a current in turn
causes another current to flow in the opposite direction and so partly cancels out the original current.
Currents add algebraically, so the total current would be the original minus the self induced current.
We dont actually measure these two currents individually however, we only measure the total current.
So we have induction, which is a change in magnetic field producing a current in a wire, and self induction
where a change in current produces a change in magnetic field which in turn produces another current
in the opposite direction and so cancels out some of the original current which reduces the total
current flow at that point in time. An inductor exhibits this behavior so we measure the inductance
and this is what is called the self inductance. It's in units of Henries.
There are a couple of equations if you would like to try to work out some examples.
The first and simplest one for inductance is:
V=L*di/dt
where
V is the voltage across the coil,
di is the change in current through the coil,
dt is the change in time between points we are looking at, and
L is the self inductance we talked about above.
The above is sometimes estimated by calling di and dt "delta i" and "delta t" where these two are relatively
small changes in the variables i and t.
For example, we might look at this equation at two points in time at t=10.0us and t=10.1us, so dt=0.1us here,
and measure current levels i=1.000 amps and i=1.001 amps, where di would be 0.001 amps here.
Knowing the self inductance L for that coil, we could then calculate V.
So, the changing magnetic field produces a current in the opposite direction which reduces the current
in the original direction.
It doesnt matter if there are turns or just straight wire because a straight wire has a surrounding
changing magnetic field too.
If the magnetic field stop changing, so does the reverse current effect.
That's why we have to use a changing quantity to measure the inductance of a physical inductor or wire.
The higher the self inductance, the more the changing field reduces the original current.
We only measure the net effect when we measure the current so we see it as a reduction of current
as compared to a wire that had no inductance at all.
Inductance is when a magnetic field 'induces' a current in a wire.
Self inductance is when the very device that produced the magnetic field induces
another current within itself that reduces the original current.
(electro-tech-online.com/general-electronics-chat/118906)
Calculating straight wire inductance.....
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