How exactly does a capacitor discharge?

[davidbenari]

I've been asked to do an electric bug swatter in class, and several questions have come up to my mind.

Whenever two rackets (each connected to a different terminal on the capacitor) touched, a spark emerged. This spark is supposedly ionised gas, which makes sense. But why do people say that when this spark emerges, the capacitor has discharged?

I mean, the capacitor is charged with electrons, and I don't see how it can be discharged other than by losing those electrons. When the air ionises, does it also suck in the electrons in the wire?

If you touch this spark, do you absorb the charge in the capacitor?

Does it make sense/can one calculate the current of this spark?How exactly would a capacitor discharge here?

Thanks.

 

[SredniVashtar]

Ok, I'm still a bit groggy, but I should get this one right...

The capacitor is charged by displacing electrons from one armature to the other (via the external circuit). It does not store charge, it stores the electric field that is a result of charge displacement. It then discharges by having those electrons go back in their place (actually, it's not necessary for a particular electron to actually go from one armature to the other, what it counts is that an electron (any electron) takes that place).
When the air ionizes and the spark happens, charge is transferred from one armature to the other, via an external circuit that include air itself.

 

[Drakkith]

I mean, the capacitor is charged with electrons, and I don't see how it can be discharged other than by losing those electrons. When the air ionises, does it also suck in the electrons in the wire?

When a capacitor charges, one plate loses electrons while the other gains electrons. The total number of electrons the capacitor has stays the same, some are just on the other plate now. When you discharge the capacitor, the electrons from the negatively charged plate flow into the circuit, while electrons flow from the circuit onto the positively charged plate. This continues until there is no longer a charge imbalance between the two plates and it becomes neutral.

 

[CWatters]

This spark is supposedly ionised gas, which makes sense. But why do people say that when this spark emerges, the capacitor has discharged?

Air is normally a poor conductor of electricity so the air gap between the bars of the bug zapper appear like an open circuit and the capacitor doesn't discharge. However if you apply a high enough voltage across an air gap the electric field (volts per meter) will ionise the air. Ionised air (or rather the molecules that make up air) does conduct electricity. As a result the current increases dramatically (causing the spark) and that's why the capacitor discharges. The larger the gap the higher the voltage or electric field required to cause a spark.

In the case of a bug zapper the voltage between the bars is roughly constant and not enough to cause a spark. However when the bug (mostly water) goes between the conductors it either a) bridges the conductors causing a current to flow through it's body or b) it doesn't quite bridge the gap. In this later case it reduces the size of the air gap. That causes the electric field (volts per meter) to go up to the point where the air becomes ionised and a spark occurs.

 

[CWatters]

It might be constructive to look how a neon or relaxation oscillator works.

http://www.allaboutcircuits.com/vol_3/chpt_7/2.html