Basic Inductor Question

[greg123]

Are inductors the same as caps in that they store current...where as caps store voltage?

[Donald S. Lambert]

Hi,
I am not a guru and I can't explain it furthur but if a capacitor it charged and removed from a circuit it still holds the charge. But if you take an inductor out of the circuit there is no voltage or current in it. <p>When I took a course in 1943 in radio repair they referred to the capacitors as condensors. And I don't know why they were called that. Greybie

[bodgy]

Perhaps a left over from the Leyden Jar days when it was thought that the current was condensed into the Jar?<p>bodgy

[Chris Smith]

Electricity is made up from current, Voltage is a measurment of pressure of that Current.

[Ron H]

Newbie, you are right on. If you charge a perfect capacitor and open circuit it, the voltage will remain the same forever. If you charge a perfect inductor with a current and then short circuit it, the current will flow forever. Remember, the time constant of an RL circuit is L/R. If R is zero, the time constant is infinite, which means that any current that is flowing in the inductor will not decay. Unfortunately, inductors are generally much farther from ideal than are capacitors. I would assume that superconducting inductors would have very high Q, but you won't find one in your average transmitter. <p>Ron

[unknown_entity]

inductors store voltage in a magnetic field and as soon as the applied voltage is removed that field collapses rapidly creating a high voltage spike between the two ends of the coil.<p>Caps are kind of like a battery in that the voltage will remain there after power is removed And they can be charged an discharged instantly.

[jgpjr]

Ahem, Chris. <p>Electricity is energy and the term voltage means the energy that makes current (the motion of charges) flow.

[MrAl]

Hello,<p>To add to the other posts a little...<p>
Both the capacitor and the inductor are passive circuit elements.
Even though they can both store energy, they are still considered
passive because they can not deliver an unlimited amount of energy.<p>Although a rigorous definition of duality is a little more
involved, the capacitor and the inductor are sometimes called 'Duals'
because the capacitor acts on voltage the way the inductor acts
on current. The capacitor stores energy in the form of a voltage,
while the inductor stores energy in the form of a current.
Another way of looking at it is that a circuit that includes
capacitors can be replaced with a circuit that uses inductors
if all the circuit elements are replaced by their duals, which
means all the caps would be replaced by all inductors.
The opposite is true also, in that a circuit that includes
inductors can be replaced by a circuit that uses all caps,
with all the inductors being replaced by all caps.
Of course in real life this is almost never possible, because
there are other characteristics about each type of element
that gives it special properties that are hard to obtain
in the other.<p>There are important dc properties of caps and inductors, most
noteably that you cant change the voltage across a capacitor in a
zero amount of time, and you cant change the current through
an inductor in zero time.<p>There are also important ac characteristics to look at.
The capacitors' reactance decreases with increasing frequency,
while the inductor (the dual of the capacitor) is just the
opposite: it's reactance increases with increasing frequency.<p>The dc and ac characteristics of the two circuit elements
make them useful in a wide variety of electrical circuits,
and understanding both of these characteristics takes the
mystery out of circuit analysis.<p>Good luck with your circuits,
Al

[unknown_entity]

What about power factor? Nobody mentioned that yet, well sort of. <p>*In a pure capacitive circuit current leads voltage by 90deg.<p>*In a pure inductive circuit current lags voltage by 90deg.<p>*In a pure resistive circuit current and voltage are in phase.<p>
That's why on large inductive loads you can put an appropriate sized cap across the line to bring the power factor closer to unity. <p>Does anyone know a formula to determine the right sized cap for a given load?<p>[ September 10, 2002: Message edited by: unknown_entity ]<p>[ September 10, 2002: Message edited by: unknown_entity ]</p>

[MrAl]

Hello,<p>One way to do it would be to resolve your
load into an equivalent inductor in
parallel (or in series) with a resistor.
Then, combine that with a parallel
capacitor. Solve the resulting network equation
for cos(angle(i)).
Then, try different value caps, or enter the
network equation into a calculator equation
solver that works with complex variables
and solve for C after setting cos(angle(i))
equal to maybe 0.95 or something close to 1.
I tried this with a TI85 and it took about
10 seconds to come up with an answer.<p>Good luck with your circuits,
Al

[Ron H]

I found an excellent discussion on power factor correction at http://www.lmphotonics.com/pwrfact.htm<p>Ron

[Chris Smith]

To Jay<p>"Electricity is made up from current, Voltage is a measurment of pressure of that Current".<p>There is nothing inaccurate here. <p>Voltage is pressure, currrent is the actual electrons,... and low or high pressure [voltage], is not an actual object like the electron, but rather the pressure or force trying to move electrons. Voltage doesnt move, current or electrons do.

[Dean Huster]

Ron, apparently your power factor correction site self-destructed after one read. Recheck the URL?<p>Dean

[Ron H]

Dean, I just now clicked on the URL I posted. Works for me.<p>Ron

[jeremyrfoster]

Best analogy I heard was this...<p>An inductor is like a propellor in a tube of water... it resists a change in current. If the water is not flowing it makes it hard to get going, and if the water is flowing it makes it hard to stop.<p>A capacitor is like a rubber membrane in a tube of water. No current actually passes through a capacitor. If you add water pressure to one side it will store that pressure.<p>Because they are affected by CHANGE, inductors and capacitors are primarily used in AC circuits.<p>Jeremy