sort of.  Any two pieces of metal have some capacitance between them, but of course it only matters when there is an ac voltage between them to drive the flow of the displacement current.

The thing to realise is that capacitance falls with distance and rises with area facing the other 'plate' and in cables is tens to hundreds of pF (picofarads - 1E-12 Farad) per metre - large capacitors use a swiss-roll construction of rolls of foil and thin plastic to get two conductors very close with a large area facing each other - by being a hundred times closer and areas of many square metres, capacitors a million times larger (in the microfarads to tens of microfarads range) are then quite possible and handy sized,

If you  are in the habit of using the 'juice' mental model of electricity as being fluid like in pipes in place of wires, then the analogue of a capacitor would a rubbery membrane stretched across the area of the pipe - allowing changes in pressure to be transmitted but not a DC flow. Inductance however is more like some coupling between a propeller in the moving liquid and a heavy flywheel - where it is the moving of the charges that is important, rather than the pressure.

Happy to discuss further and fill in any areas you feel are patchy for you.

Mike

So is it the change in current caused by the AC wave form which allows the current flow through the capacitor due to the constant charge and discharge they might not be the right words but I think it shows my point. 

What do you mean by the area facing the other plate is that referring to the size of the opposite plate or for example a cable? 

So having an inductor in a circuit is basically like having a large form of resistance does this work in a similar principle as a capacitor where it reacts to the change in alternating current. 

Thank you for taking the time to reply it's definitely made things that bit clearer

Sam 

So is it the change in current caused by the AC wave form which allows the current flow through the capacitor due to the constant charge and discharge they might not be the right words but I think it shows my point. 

Getting there I think. The rubber membrane analogy is a good one I think. As the pressure (voltage) on one side increases the rubber membrane stretches a little and pushes a bit more fluid out of the other side - until the membrane tension and the pressure balance out. If the pressure increases further the membrane stretches a little more and a bit more fluid flows in on one side and the other fluid is pushed out from the other side. Or if the pressure drops the membrane relaxes and a little fluid flows backwards on both sides. So it's the change in voltage (pressure) that drives the current flow. Bigger capacitors allow more current to flow for the same change in pressure.

  - Andy.

Ah okay so as the pressure (voltage) builds in one side of the capacitor it begins to push from one side of the capacitor into the other. Then once the capacitor becomes fully charged as more pressure try to enter the capacitor it pushes some out of the other side basically replacing the existing charge. 

Would the same thing occur on a DC circuit or does the capacitor just build up until it's full and then discharge once the DC voltage is removed? 

Thank you for your patience 

Sam 

well the inductance is a resistnace that rises with frequency - hence my 'juice' analogue of a propeller with a heavy flywheel - once moving it keeps moving, DC is no issue, but to get current  moving initially or to change direction takes time and voltage drop. The reverse of the capacitor case where fast things are easy, and slow ones take more voltage.

In a transformer the propellers in two streams of 'juice' are geared to a common drive shaft....

Pursuing the capacitor analogy further - the catastrophic breakdown of a capacitor is rather like the rupture of the rubber membrane - then there is a DC path and current (juice) flows right through.

Mike.,

Once the capacitor has charged to match the supply voltage, (Q= C/V where Q relates to the number of electrons displaced relative to their at-rest position, C is the capacitance and V is the voltage) current stops *. As the voltage falls, current flows out and the displaced charges relax back to their original places (well more or less, I am ignoring losses)

Mike.

actually and amp second = A coulomb of electrons is a lot, about 1E19 of them - that is a 1 and 19 zeros - they are really small.....

Sam Ayre said:

occurs between an energized conductor and a non energized conductor due to the difference in potential difference

Just one thing that I don't think has been picked up in the excellent replies - better to think of "at least one energized conductor". You've picked up the important point that you'll notice the effect if there's a potential difference. This may be between, say, one conductor at 230V and another at 0V (one energized, one non-energized using your wording), or one at 230V and one at 110V in phase so 120V potential difference, or both at 110V but in opposite phases so 220V potential difference between them.

It's all about the potential difference.

Sam Ayre said:

Would the same thing occur on a DC circuit or does the capacitor just build up until it's full and then discharge once the DC voltage is removed? 

Basically yes, the rate of charge falls exponentially over time. So initially (when the voltage is first applied) there's a high charge and it drops away until it becomes immeasurably small. And the same with the discharge.

This is important as it explains the behaviour of capacitive coupling for ac signals. For faster ac signal changes (higher frequency) the charging / discharging spends a higher proportion of its time at that initial high charge point, it can't do much charging before it has to start discharging. This is why as the frequency goes up you see more average current being drawn to carry out this charging / discharging. We tend to think of this as ac current "flowing through" the capacitor (and I find that a perfectly workable way of thinking of it), but it isn't really.

Cheers, Andy