# capacitor and inductor opposition theory

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#### walters How does capacitors create opposition for voltage?

How does Inductors create opposition for current?

The phase displacement from a capacitor is from the charging and opposition?

The phase displasement from a inductor is from the charging and opposition?

Why does a Capacitor need to Charge? or have a charge time?

why does a inductor need to charge? or have a charge time?

why does a capacitor oppose voltage or current?

Why does a inductor oppose current or voltage?

#### thuvu

##### Member level 3 Better look on a basic electronics book.

It has to do with the equations of the two elements.
for capasitor V=I/sC
for inductor V=sLI

The first integrates and creates a smoother Voltage.

#### walters for capasitor V=I/sC
for inductor V=sLI

What does SC mean?

What does SLI mean?

#### v_c For a capacitor
$i_C = C \frac{d v_C}{dt}$
In the frequency domain the Laplace variable $s$ takes the place of $d/dt$, so you get
$I_c = s C V_c$
It is also general convention to use capital letter to denote frequency domain variables. You can do the same thing with an inductor.
$v_L = L \frac{d i_L}{dt}$
$V_L = s L I_L$
If you write each of these equations as $V=Z I$ where $Z$ is the impedance, you will see the the impedance of the inductor is $s L$ and the impedance of the capacitor is $1/(sC)$ with units in $\Omega$.

Does this help?

Best regards,
v_c

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#### walters Thanks for the help

Not really because its formulas and math i need the concept of why

How does capacitors create opposition for voltage?
How does Inductors create opposition for current?
The phase displacement from a capacitor is from the charging and opposition?
The phase displasement from a inductor is from the charging and opposition?
Why does a Capacitor need to Charge? or have a charge time?
why does a inductor need to charge? or have a charge time?
why does a capacitor oppose voltage or current?
Why does a inductor oppose current or voltage?

#### v_c walters said:
How does capacitors create opposition for voltage?
Capacitor creates opposition to voltage because the capacitor stores the energy in
the electric field created between the two plates. It takes time and force (voltage) to build up this electric field. You cannot change the capacitor voltage instantaneously because it would take an infiinte current to do that.

walters said:
How does Inductors create opposition for current?
This is very similar (and analogous) to the capacitor. The energy in the inductor is stored in a magnetic field and it takes energy to build this field. You cannot change the inductor current instantaneously.

walters said:
The phase displacement from a capacitor is from the charging and opposition?
Yes you can see that from i=C dv/dt -- taking a derivative is introducing a phase delay of 90 degrees. In a capacitor the current leads the voltage by 90 degrees.

walters said:
The phase displasement from a inductor is from the charging and opposition?
Yes it is from the delay in building the magnetic field. Again you can see this
from v=L di/dt -- the derivative introduces a phase delay of 90 degrees. In
an inductor the voltage leads the current by 90 degrees.

walters said:
Why does a Capacitor need to Charge? or have a charge time?
It needs to charge (and discharge) to build (and collapse) the electric field.
That is how it stores energy. Note that capacitors are storage elements and
don't dissipate average power. Real capacitors will have losses associated with
them -- but theeoretically they are lossless.

walters said:
why does a inductor need to charge? or have a charge time?
Inductor needs to build up (and collapse) the magnetic field to store energy.
Just llike the capacitor, it is a storage element and does not dissipate average power. Of course, real inductor will have some losses -- but in theory they are
lossless.

walters said:
why does a capacitor oppose voltage or current?

walters said:
Why does a inductor oppose current or voltage?

#### ferrite

##### Member level 1 the physical reason for the capacitor is that its voltage across it cannot be build instantly, it needs certain time to charge itself, thus it appears like it has opposition to the change of voltage

similar thing happens to inductor, since it cannot changes current through it abruptly due to the opposition of induced eletronic motive force, thus it shows that inductor opposites the chnage of current

walters said:
Thanks for the help

Not really because its formulas and math i need the concept of why

How does capacitors create opposition for voltage?
How does Inductors create opposition for current?
The phase displacement from a capacitor is from the charging and opposition?
The phase displasement from a inductor is from the charging and opposition?
Why does a Capacitor need to Charge? or have a charge time?
why does a inductor need to charge? or have a charge time?
why does a capacitor oppose voltage or current?
Why does a inductor oppose current or voltage?

#### walters Thanks v_c and ferrite alot for your information

delay in building the magnetic field
Delay in building the electric field

Why is there a Delay in building and storing energy in the magnetic field and electric field?

When the frequency changes the delay building and storing energy changes the magnetic field and electric field?

Does the Magnetic field and Electric Field create opposition for current and voltage and why does that please?

#### v_c Why is there an opposition to the increase of magnetic field. The answer is Lenz's Law. See here http://en.wikipedia.org/wiki/Lenz's_law

#### walters thanks for the help

Yes why is there an "opposition" to the increase of magnetic field from a inductor?

Why is there an "opposition" to the increase of electric static field from a capacitor?

#### _

##### Junior Member level 1 walters said:
Why does a inductor oppose current or voltage?

When a DC current runs through an inductor it builds up a magnetic field around it, when you increase the current you increase the magnetic field. While the current is going through the inductor it maintains or supports the magnetic field, as soon the the DC current stops the magnetic field collapse and converts into current and therefore generates high voltage over circuit resistance.

The "opposition" to current change comes from the compensation of magnetic field converting into current, when the input current decreases the magnetic field collapse in proportion to the drop in the input current and converts into current which slows down the drop in the current output.

#### walters Thanks for the information

But the magnetic field opposes the voltage and current in a inductor it produces opposition

The capacitors electric static field opposes the voltage and current it produces
opposition

I'm still confussed on how the magnetic field creates opposition

and how a capacitors electric static field creates opposition

#### _

##### Junior Member level 1 walters said:
Thanks for the information

But the magnetic field opposes the voltage and current in a inductor it produces opposition

From the following link it says

When current through an inductor is increased or decreased, the inductor "resists" the change by producing a voltage between its leads in opposing polarity to the change..................................................................
......................................................................
When the current through an inductor is increased, it drops a voltage opposing the direction of electron flow, acting as a power load. In this condition the inductor is said to be charging, because there is an increasing amount of energy being stored in its magnetic field.

www.opamp-electronics.com/tutorials/magnetic_fields_and_inductance_1_15_01.htm

#### jonw0224

##### Full Member level 4 walters said:
I'm still confussed on how the magnetic field creates opposition

Lenz's law states that a current will create a magnetic field (actually, it states much more, but the basic gist is that current is always accompanied by a magnetic field).

How does that create opposition?

Lets think about an ideal coil (no resistance). An ideal coil is basically a short. If I connect the coil to a voltage source the current will be infinite because there is no resistance. However, this is not true immediately because the coil has no magnetic field surrounding it to begin with. So, in passing the current through the coil I establish the magnetic field. A magnetic field can attract or repel objects which are also magnetic. So, therefore a magnetic field has the potential to do work and contains energy. To establish a magnetic field, I must put energy into it. To make a magnetic field stronger, I must put more energy into it, to make it weaker, I must take some away. The only method I have of adding or removing energy is adjusting my current through the coil.

Let's assume I connect this coil to an ideal voltage source. To begin with, I have no magnetic field. Since I have no magnetic field, I have no current BUT I am applying a voltage which is trying to force current into the coil. So, though I have zero current, the voltage source begins to push some current through the coil. This current begins to establish a magnetic field (remember that requires energy which is the same as voltage times current times time). So, my current increases gradually as I add energy to the magnetic field equal to the voltage times the current over time. The fact is, to establish the field, I must supply energy. Since I am supplying energy there is an opposition to current flow.

walters said:
and how a capacitors electric static field creates opposition

This is very similar, except instead of adding energy to a magnetic field I am adding energy to an electric field.

There is a more visible way to think of this one however. Electrons try to repel one another. But if I connect a capacitor to an ideal current source, I am supplying electrons to one plate (and removing them from the other plate). Since the electrons repel one another, pushing them close together requires work. So, I am storing work into the capacitor and this causes the opposition to current flow.

#### walters THanks alot guys for the help on this

Can i convert the Time constant into a Frequency?
Is a time constant a frequency

#### v_c Yes, the time constant is related to frequency. Let's say you have a RC lowpass filter. The time constant is $\tau = RC$ -- this is in seconds. The cutoff frequency of this RC filter is $f_c = \frac{1}{2 \pi \tau} = \frac{1}{2 \pi R C}$ -- this is in Hertz. Below the cutoff frequency the signal passes through and above the cutoff frequency the signal is attenuated (it is attenuated by 20dB for every decade above the cutoff). So if the cutoff frequency is 100Hz, at 1kHz the signal is attenuated (reduced) by 20dB (factor of 10). At 10kHz, the signal is attenuated by 40dB (factor of 100), etc.

Best regards,
v_c

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#### walters Thanks for the help V_C

Why is the Cutoff frequency the RC time?

Is the reciprocal of Time constant the frequency or cutoff frequency?

There is a "Time Element" for capacitors and inductors

Whats is the " time Element" for a capacitor?
Whats is the "time element" for a inductor?

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