Discussion in 'Homework Help' started by quarkeboy, Aug 2, 2012.

  1. quarkeboy

    Thread Starter New Member

    Aug 2, 2012
    hi, it is written in my book capacitors absorb energy when voltage is positive or increasing and it also absorb energy when voltage is negative or decreasing.i am confused that the capacitor should release energy when voltage is negative or decreasing.plz tell me whether i am correct or not if not then why i am not correct.
  2. #12


    Nov 30, 2010
    Capacitors don't absorb energy (except for a very tiny bit, and that is considered to be a defect due to the type of insulator being used). A capacitor just holds a conductive plate close to another conductive plate and when current enters one plate, the other plate becomes more or less attractive to electrons. The capacitor is a very "don't care" kind of device. It just stands there and electrons flow to or from of the plates according to the voltage you apply.
  3. mlog


    Feb 11, 2012
    The energy (U) stored in a capacitor is U=\frac{CV^{2}}{2}. The voltage can be negative or positive. If the voltage is initially positive and heads negative towards V=0, then the stored energy is decreasing. On the other hand, if the voltage continues negative past V=0, then the stored energy will increase again but with the opposite polarity.

    Think of it as a mass on spring at rest. For all practical purposes (neglecting gravity), the spring has stored 0 energy. If the mass and spring are compressed and held, then they have stored energy. If the mass and spring are slowly allowed to return to the resting point but are then pulled in tension, the mass and spring again have stored energy but with the opposite polarity.

    So you see, "increasing or decreasing" can refer to voltage, distance, energy, force, etc. Decreasing voltage might mean decreasing energy, but it could also mean increasing energy in the opposite direction. Same goes for a mass/spring. Increasing distance in one direction might mean increasing force and increasing store energy, but if you reverse direction you can move past the "at rest" or neutral condition and increase force and energy in a way that seems like it's in a negative direction.

    I hope this helps and doesn't add to the confusion.
  4. #12


    Nov 30, 2010
    A very different point of view but perfectly valid. Capacitors can store energy, but they don't absorb it.
    steveb likes this.
  5. hexram

    New Member

    Feb 29, 2012
    May I be allowed to comment on the answers provided until now? The concept of energy is not an easy and straightforward one for sure, but you can be on the safe side most of the time if you relate energy to work. Recalling the physics definition of work you can say it is force times distance; please don't crucify me regarding the enormous amount of details I have obviated so far. This may be interpreted, in the case of capacitors, as that they can exhibit the capacity -pun certainly intended- to exert work on electrical charges, that is, to push them and make them move. Resistors, on the other hand, do not exhibit this capacity; they just oppose, restrict, impede the movement of the electrical charges and thus there is the need for an external energy source to keep them moving. In fact, a resistor and a capacitor together in an electrical circuit behave in a similar fashion: If there is an external energy source then the capacitor stores some of this energy; when there is no external energy source then the capacitor supplies the stored energy. In both cases, the resistor transforms the energy into heat, which is a less desirable form of energy.
    We say that capacitors store energy in an electric field. This is an abstract concept that takes into account the fact that there is the possibility to observe a force in some region even though there is nothing moving; think of a river with nothing floating on its surface: you know you will see movement if you drop something that floats on it, even though you don't see no movement right now. If you now replace electric with magnetic, then you can explain the behavior of another electric component called an inductor. Together, capacitors and inductors tend to work in tandem: when one of them is storing energy, the other is releasing its stored energy. One of the most common applications of this behavior is in the ignition system of combustion motors.
    In a nutshell, energy is not absorbed; it is transformed. Capacitors and inductors can store tiny amounts of energy for equally tiny amounts of time. Resistors cannot store energy, they can only transform it into heat. Real capacitors and inductors heat themselves while at work, because every substance exhibits some amount of resistance; thus, energy storage in capacitors and inductors is not perfect: stored energy is transformed into heat by the resistance in real capacitors and inductors.
  6. WBahn


    Mar 31, 2012
    The problem I have with your description doesn't have anything to do with "absorb", it has to do with the word "or". Here is how I would state it:

    A capacitor absorbs energy (which is stored in it's internal electric field) when the voltage is positive AND increasing, and it also absorbs energy when the voltage is negative AND decreasing. In other words, a capacitor is absobing energy whenever the absolute value of the voltage across it is increasing.

    To be clear, we need to agree that of the voltage is -10V, that it increases if it becomes -9V and decreases if it becomes -11V. We tend to naturally think in absolute terms and that a voltage or current are decreasing if the magnitude is getting smaller and increasing if the magnitude is getting larger. In electronics, we have to be careful because usually we need to think in terms of algebraic terms in which increasing means moving to the right (towards positive infinity) on the number line and decreasing means moving to the left (towards negative infinity).

    The key things to note are:

    1) If the voltage across the capacitor is not changing, then there is no absorbing (or relinquishing) going on -- the energy stored remains the same.

    2) Knowing only that the voltage on the capacitor is positive or negative is insufficient to determine whether energy is being absorbed, staying constant, or relinquished.

    3) Knowing only that the voltage on the capacitor is increasing or decreasing is insufficient to determine whether energy is being absorbed, staying constant, or relinquished.

    As you can see, people have different meanings for different terms and part of the joy of communicating with other human beings is trying to be close enough to being on the same page to actually communicate something coherently. It's not always easy.

    For my part, I am willing to live with the use of 'absorb' energy, when talking about R,L,C components, to mean that electrical energy has flown into the part and did not come back out as electrical energy (at least not yet). So, as others have said, the energy has been transformed into something else. In this context, I don't have a problem using 'absorb' and 'transform' as synonyms (though someone could probably construct an example in which I would agree they really aren't). The difference between a reactive component (the L and C) and a resistive component (the R) is that energy that is absorbed by a reactive component is stored in a form that can readily (and will eventually) be transformed back into electrical energy and delivered back to the circuit, while the energy that is absorbed by a resistive element is dissipated, usually as heat but at least as some form that will likely not be transformed back.
  7. MrChips


    Oct 2, 2009
    The quick explanation is found in the formula.

    Since the energy is a function of V^{2} the sign of V does not have any effect.
  8. Rbeckett


    Sep 3, 2010
    Heres a very simple analogy of what a cap does. Imagine if you will a water system with a surge tank. The surge tank helps keep pressure and volume steady when the pump is cycling on and off and the spigot is being opened or closed. A cap is essentialy a surge tank in an electronic system. They put AC to ground while smoothing the peaks and valleys caused by ripple in the incoming and out going current. Hope this helps you understand how they are emplyed in a very basic sort of way. Sometimes a different point of view makes a huge difference in understanding how a component works in the bigger picture of a circuit.
  9. WBahn


    Mar 31, 2012
    While a surge tank (a.k.a., an accumulator) is often a useful analogy for a cap, it doesn't capture the point the OP is trying to understand in this case because the accumulator doesn't get at the notion of stored energy very easily and not at all in an analogous way. What does it mean to have "negative" water in the surge tank and to have the same energy stored whether you have positive or negative water. Without capturing the square-law nature of the energy-storage mechanism, the analogy doesn't work for this purpose.
  10. WBahn


    Mar 31, 2012
    It doesn't have any effect on the energy stored at a given voltage, but it does have an effect on the relationship between the sign of the voltage, which direction the voltage is changing, and whether additional energy is being stored or not. The key there is that we are looking at the change in energy storage as it related to a change in voltage, namely

    <br />
\frac{dU/dV}\;=\;CV<br />
\;<br />
dU\;=\;CV\cdot dV<br />

    Hence, for the energy change to be positive (more energy being stored), the sign of V and the change in V must be the same, either both positive or both negative.