Capacitors and their function in Alternating Circuits

[Fishn sparky]

Good afternoon all,
Regarding capacitors in alternating circuits.....

What is your best approach at describing their function in an alternating circuit. More in particular, how do they provide energy to the circuit. I'm aware that energy is stored in the electrostatic field between the two plates and when the applied voltage changes magnitude and the capacitor discharges to maintain a voltage and current (charge) during the applied voltage direction change. Thus "smoothing out" the change and providing constant voltage. Is this a correct assessment?

In regards to volt amps reactive in a capacitor, capacitors are a source of VARS. They don't use energy but provide energy through the electrostatic field. While inductors are a consumer, capacitors are a source. Inductors consume VARS due to the concept that they use energy to create a magnetic field which stores energy. Capacitors do not use energy but create a field via laws of physics and attraction and repulsion of electrons.

Are these correct statements? Any help would be appreciated if I am off the mark.

Thanks gents!

 

[charlie b]

You have some ofthis right, but you have some misconceptions as well. Capacitors do not provide energy to thecircuit; they are not an energy source. Both capacitors and inductors share the characteristic that they drawenergy from the circuit during one half of the cycle, and return it to thecircuit on the other half of the cycle. They are just opposite in their timing. Also, the one stores energy in the form of an electrical field, and theother in the form of a magnetic field. Finally, although a capacitor will tend to smooth out a waveform, theywill not provide constant voltage. They’rejust not that good.

As you described, acapacitor will tend to oppose a change in the voltage that exists between itsplates. It does that by creating acurrent that is oriented in a direction that is in opposition to thechange. On the other hand, an inductorwill tend to oppose a change in the current flowing through its coil. It does that by creating a voltage that isoriented in a direction that is in opposition to the change. In both cases, the result is that the voltageacross the component and the current flowing through the component will hittheir respective peak values (and zero crossing values, and negative peakvalues) a quarter of a cycle offset in time from each other. For an inductor, the voltage will hit itspeak, and 90 degrees later the current will hit its peak. For a capacitor, the current will hit itspeak, and 90 degrees later the voltage will hit its peak.

Since the real-worldcircuits we deal with also have resistive elements in them, the peak values ofvoltage and current will occur closer to each other than 90 degrees. For most of our real-world circuits, theimpact of inductors outweighs that of capacitance. So the voltage will lead the current (as seenby the source) by a value between 0 and 90 degrees. The number of degrees that separate the twois commonly expressed as the power factor of the circuit. To be precise, power factor is the cosine ofthat angle.

So, does this helpor more greatly confuse the situation for you?

 

[Ingenieur]

They both store/absorb and release/supply var
by convention for math sign consistency the C is +var supply, and L is -var, absorb

in an LC ckt the energy/var oscillates between them

 

[gar]

171020-0556 EDT

v*a is not energy. Rather it is instantaneous power.

v*a*dt is incremental energy over the incremental time period dt. The energy over some specific time period is the integral (summation) of all the small incremental elements of energy over said specific time period.

When power is applied to a resistor that input power is dissipated as heat. The amount of electrical energy input to the resistor over some time period is all output as heat energy over that same time period.

Ideal capacitors and inductors do not dissipate energy. They store or give up energy over time as determined by where they are in a circuit. You can not instantaneously change the voltage of a capacitor, nor the current of an inductor.

If I supply current to a capacitor the voltage across the capacitor will gradually change. If I apply voltage to an inductor its current will gradually change.

Think about these concepts for a while, and study what happens when you introduce a series resistor to a capacitor or inductor and with a switch apply a fixed voltage.

.

 

[Sahib]

Fishn sparky:

Are you asking about the functions of a capacitor or the nature of the capacitor?

 

[Ingenieur]

171020-0556 EDT

v*a is not energy. Rather it is instantaneous power

.

I think we all know that
P = work/t where work ~ energy = P t

are you saying no energy is transferred in an LC ckt

 

[ggunn]

Good afternoon all,
Regarding capacitors in alternating circuits.....

What is your best approach at describing their function in an alternating circuit. More in particular, how do they provide energy to the circuit.

They don't. They sink energy during part of a cycle and source most of it back (nothing is 100% efficient) during another part of a cycle.

 

[gar]

171029-1005 EDT

Ingenieur:

are you saying no energy is transferred in an LC ckt

No. I made no statement about an LC circuit.

The use of "P" would usually mean average power over some defined time period. Whereas "p" is likely to mean instantaneous power. "t" usually means some instant in time, and "T" some time duration.

So energy, E, over some time period T would be:
T = "average power over T" * T = P*T.

I would describe an amount of energy, E, dissipated in a resistor, or transferred to something as
E = the integral from t1 to t2 of v*i dt where "v" and "i" are the instantaneous values of voltage and current to the something.

.

 

[Jim1959]

A simplistic understanding I remember from years ago, is that a capacitor resists a change in voltage, an inductor resists a change in current.

 

[Sahib]

171029-1005 EDT

Ingenieur:

No. I made no statement about an LC circuit.

The use of "P" would usually mean average power over some defined time period. Whereas "p" is likely to mean instantaneous power. "t" usually means some instant in time, and "T" some time duration.

So energy, E, over some time period T would be:
T = "average power over T" * T = P*T.

I would describe an amount of energy, E, dissipated in a resistor, or transferred to something as
E = the integral from t1 to t2 of v*i dt where "v" and "i" are the instantaneous values of voltage and current to the something.

.

However, electric field in capacitor and magnetic field in inductor store energy and not power.

 

[Ingenieur]

171029-1005 EDT

Ingenieur:

No. I made no statement about an LC circuit.

The use of "P" would usually mean average power over some defined time period. Whereas "p" is likely to mean instantaneous power. "t" usually means some instant in time, and "T" some time duration.

So energy, E, over some time period T would be:
T = "average power over T" * T = P*T.

I would describe an amount of energy, E, dissipated in a resistor, or transferred to something as
E = the integral from t1 to t2 of v*i dt where "v" and "i" are the instantaneous values of voltage and current to the something.

.

Again
we know that
the OP is asking a very basic question in a simple form
he is not interested in how much you or I know
he wants a conceptual understanding/explanation
otherwise he would crack a calculus level physics or EE power text

 

[Russs57]

I think the poster needs to define the circuits the capacitors will be used in.

In normal building electrical wiring I’d say caps are mainly used for power factor correction or to create a virtual phase.

In electronics they are mostly used for coupling, decoupling, and timing.

 

[Besoeker]

Was my power factor correction comment removed?

 

[Besoeker]

I think the poster needs to define the circuits the capacitors will be used in.

In normal building electrical wiring I’d say caps are mainly used for power factor correction or to create a virtual phase.

In electronics they are mostly used for coupling, decoupling, and timing.

And in variable frequency drives as the DC link bucket capacitor.

 

[electrofelon]

Let's not forget the Flux capacitor, which makes time travel possible

 

[gar]

171020-1112 EDT

Ingenieur:

I agree that "Fishn sparky" wants a simple answer. But he has been somewhat misdirected by some of the simplified information he has previously received.

Prior to about 1890 AC circuit analysis was done using calculus equations. Then the concept evolved that steady state analysis could be performed with algebraic equations. This was extended to the concept of phasors as a graphical approach.

Extending DC algebraic circuit analysis to AC circuits required a basic assumption to be made that one was dealing with steady state conditions and linear components.

But the basic characteristics of devices remains. Ideal resistors instantaneously dissipate energy, while capacitors and inductors instantaneously stored or export energy and dissipate no energy.

Capacitors and inductors are opposites of a sort, and on a steady state basis can compensate one another to an extent. There may be side effects in this compensation.

So what does "Fishn sparky" need to know to clarify his thinking? I don't know.

One item that might be important is that the instantaneous sum of two sine waves of exactly the same frequency and phase related is just another sine wave of that frequency and some possibly new phase relationship.

If one sine wave lags a reference by 90 degrees and another leads by 90 degrees, then these two are 180 degrees different. Thus, the sum is the instantaneous difference. If the magnitudes are equal, then the difference is zero, and perfect compensation has occurred.

If one has an ideal parallel circuit of a resistor, capacitor, and inductor, then the input current is the sum of the three individual currents. This circuit can look as resistive with or without a reactive component. The kind of reactive component is a function the frequency and the relationship of the inductor and capacitor at that frequency.

.

 

[gar]

171020-1226 EDT

If we have an ideal capacitor and inductor connected in parallel, also means they are in series, and we inject some energy into this circuit, then that energy will bounce back and forth between the capacitor and inductor in a sine wave form at the resonant frequency.

.

 

[Sahib]

171020-1226 EDT

If we have an ideal capacitor and inductor connected in parallel, also means they are in series

.

If an ideal capacitor and inductor are connected in series, it could result in dead short circuit.

 

[jumper]

Was my power factor correction comment removed?

No posts in this thread have been removed. One was deleted, but it was not yours or any others involved in this discussion.

 

[Besoeker]

No posts in this thread have been removed. One was deleted, but it was not yours or any others involved in this discussion.

Probably me with finger trouble then........

And thank for responding!