# Simultaneous Charging and Discharging Capacitor

sorry if I sound little noobish. Though I have a fairly good understanding of physics, I sometimes don't understand the electrical aspects.

Say there is a capacitor. This capacitor is expected to act as a storage buffer. By extension, the capacitor will have a "charge" interface and a "discharge" interface. There may or may not be an electrical circuit between these interfaces and the capacitor. The expected behavior of the system is, electrical energy may be input to the system via the charge interface, which will charge the capacitor, and energy may be simultaneously drawn also, via the discharge interface, which will draw energy from the capacitor, and this process can happen so long as the energy stored within the capacitor is within its maximum and zero. The actual path of the energy may be from the interface, through the circuit, to the capacitor, and back through the circuit to the other interface; or part of the energy may be routed from one interface to the other by the circuit, and the net energy difference between the two interfaces be actually sent to, or drawn from the capacitor.

Though I may think that this is possible, I'm not aware if any such system exists currently. The home inverter seems to be doing quite the same thing, but both the cycles don't happen at the same time though.

Edit: Yeah, a diagram will help me also to explain better what I have in mind.

The two interfaces are part of the circuit which shields the capacitor. This circuit may work in two possible ways, which I've mentioned as flow 1 and flow 2. In Flow 1, "all" of the energy which flows into/out of the system, does so through the capacitor. In Flow 2, the circuit redirects part of the energy flow in one interface to the other interface, and only the net difference between the energy flows is actually transmitted to/from the capacitor.

Hope this makes it more clear. Now, let me restate my question. Is such a system possible, importantly, such a circuit possible. Are any systems available today, which do exactly the same thing. And your own views on this is really welcome.

• i think a drawing of what you're actually asking is the way to go for stuff like this. but, it sounds like at least you're misunderstanding how a capacitor works. there are no separate charge or discharge "ports", you can think of the capacitor just like a re-chargeable battery in this context. you hook it up to a power line and it will absorb energy if the power voltage is high and release energy if the power voltage is low. very simplified... so, the process you describe is the standard process for all "storage buffers" (or UPS, uninterruptible power supplies). – BjornW Mar 25 '14 at 10:24
• It's not entirely clear what you're suggesting (draw it!), but sounds like you could build a basic AC to DC converter with it. – Kvothe Mar 25 '14 at 10:43
• Have edited the question and added a diagram to explain what I was asking. – Ashley Mar 25 '14 at 12:17
• You may be trying to ask about the behavior of a capacitor in parallel with some other circuit elements (though that is not what you have draw---you drew it in parallel with a bare wire). That is an easy thing to analyze when the other elements are also capacitors, but the difficulty grows from there. Have you encountered Kirchoff's Rules yet? – dmckee Mar 25 '14 at 13:19
• You have got the concept of a capacitor horribly wrong. A capacitor in the very basic sense is two conducting plates placed parallel to and not touching each other. When you apply a potential difference across these plates, charge accumulates on them and the capacitor gets charged. – udiboy1209 Mar 25 '14 at 14:30

If at time $t_0$, the voltage across an unconnected capacitor is $V_0$, then the capacitor will charge if an externally applied voltage $V_B > V_0$ in a circuit or will discharge if $V_B < V_0$. One can't do both at the same time.

As an analogy, one can't simultaneously raise and lower the quantity of water in a tank, even though one might simultaneously add water to and drain water from a tank.

• Thanks Edward, for your simple explanation. I guess you've gotten it dead right with your water tank analogy. Yes, water cant be raised or lowered at the same time. But just like how you can have two separate pipes for filling and draining the water, I wanted to know if there was an electrical system equivalent of it, wherein you would input electrical energy into the system, and simultaneously be able to extract electrical energy from it, such that the net difference between energy input and energy drawn will result in that much amount of energy addition or removal from the system. – Ashley Mar 25 '14 at 15:23
• Agreed that a capacitor has only a simple interface where it can either be charged or discharged. But surely the circuit that sits atop it will encapsulate this functionality of simultaneous charge-discharge right? In a sense, the capacitor will simply be the water tank here. – Ashley Mar 25 '14 at 15:24
• Energy and voltage are two different things in the same way that potential energy and the elevation of a mass are two different things. Imagine using a rope and pulley to hoist a weight. You can't simultaneously raise AND lower the weight in just the way you can't charge AND discharge a capacitor. You can apply energy to hoist a weight and then get some of the energy back as the weight lowers (as in a grandfather clock) but not both at the same time. – Edward Mar 25 '14 at 17:15

This brings back old memories, I was kinda obsessed in making a system where we charge several capacitors and continuously discharge them making spectacular sparks. Apparently it is not possible.

The only way we can charge and discharge is one by one. This technique is widely used in camera flashes where a large capacitor(in capacity, not in size) is charged and then shorted to make a burst/flash of charge.

• As soon as the capacitor charges it gets out of the circuit!
• Before it is charged, it simply behaves as a separate device/element(like a resistor).
• If you want to discharge the capacitor you need it at a potential higher than any other source like the battery. (Separately/Previously charged at higher potential)
• As soon as capacitors potential drops below that of the source, it starts charging again and does not participate in discharge again.

However in the case of AC source,

Note while the potential is dropping in case of AC source, the capacitor does discharge! But that only helps in maintaining a more stable/constant potential at the discharge place

Since you seem to be asking for a capacitor circuit which itself charges while itself discharging just as well, even this case is not useful.

• It's misguided to say that the capacitor acts as a resistor when it is charging. – eqb May 25 '14 at 20:50
• how about now @eqb? – Rijul Gupta May 25 '14 at 20:53