Capacitors

I know this might come across as very silly.. But im confused so. I need to ask, sorry.

"When a charged capacitor is connected across a resistor, the p.d. drives a current through the circuit. This current flows in the opposite direction from the charging current."

So.. If the electrons flow from the negative plate to the positive plate, driving a current through the circuit.. Wouldnt the initially positive plate then become negative, and the negative plate become positive, yet again, producing a PD across the plates, and now driving a current in the opposite direction?

Please clarify in simple terms.. Im really confused


Posted from TSR Mobile

Think about the atoms making up the plates of the capacitor.

Protons have a +ve charge and electrons have a -ve charge. Most importantly, the force exerted by both types of charge has the same magnitude but differ in sign only.

Atoms are made up of a nucleus (+ve charged protons and no charge neutrons) with -ve charge electrons orbiting the nucleus.

The forces in the atom all like to reach equilibrium. In most cases, there will; be exactly the same number of protons as there are electrons. So the charge force exerted by that atom on other charged particles will be zero because the +ve charge force cancels out with the -ve charge force. (think of it like a tug of war where both teams are equally matched. The flag stays in one place)

But if there are more electrons than protons in any given atom, then the combined -ve force of the electrons will exert a greater influence than the combined +ve protons. The atom has gained a net -ve charge. (one team member leaves his team and joins the other team. The flag now gets pulled to the side with the bigger team)

If on the other hand the atom loses electrons, the +ve charge protons outnumber the electrons and the combined +ve force of the protons will exert a greater influence in the opposite direction than the combined -ve electrons. The atom has gained a net +ve charge. (the other team is now bigger so the flag moves the other way)

Going back to your question, in order for a current to flow through the circuit, there needs to be a charge imbalance between the ends of the circuit.

That charge imbalance comes from the excess of electrons on the plates of the capacitor and the exactly opposite +ve charge on the other plate. (one plate has more electrons than protons and the other plate has more protons than electrons)

Because protons are fixed in relation to other atoms in the materials atomic structure, only electrons can migrate through the circuit, drawn to the net +ve charge on the other plate.

As electrons leave the -ve plate, that plate becomes less -ve charged and heads towards neutral (balanced) charge. As electrons arrive at the +ve charge plate, that plate must now become less +vely charged and it too heads towards neutral charge. When the forces between the plates equalise, no more electrons can flow and bith plates will once again have a neutral charge.

In terms of the tug of war, it's like one team gained two new members form outside (charged capacitor). This team were winning, but the judge told one of them to go over to the other side and when he did so, the tug of war once again became equally matched and no forces win out.

That makes sense but when the capacitor was getting charged, where were the protons then? As you said, protons are FIXED, so shouldnt they have stayed there whilst electrons accumulated on that plate, leading to no charged plate in the first place?

Posted from TSR Mobile

The protons were still fixed. BOTH plates have the same number of protons for all intents.

The atoms of the capacitor plate are conductors. Which means that not all of the spaces in their outer shells are filled with electrons - just enough to balance out against the electric force exerted by the protons.

This is how conductors work. Electrons can migrate form one atom to the next because there are free spaces available for them.

Take two neutral charge conductor atoms. Each has a neutral charge because the number of protons balances the number of electrons. But each atom still has spare capacity (get it) to take new electrons.

So an electron leaves one atom (which makes that atom net +ve charge because there are now more protons than electrons) and that electron joins the other atom which now becomes net -ve charge because there are now more electrons than protons.

Critically, the number of protons on the two plates remain UNCHANGED. It's the loss or gain of electrons that makes each plate either +ve or -ve charge respectively.

The donor atom has gained a new space for more electrons (vacated space means it's now net +ve charge) and the neutral receptor atom has gained and electron meaning it lost a space for more electrons (now net -ve charge).

If you think about it from a power supply perspective, a neutral capacitor atom can still gain electrons because there are spaces still available for them to do so in the outer valence (conduction) shell.

Makes sense for now! Can you clear up one more thing for me? I used to think current is the rate if flow of electrons.. But it isnt. Conventional current is such an abstract concept for me.. What exactly is moving in conventional current?

Posted from TSR Mobile

Yup. Current is most definitely the rate of flow of electrons (charge).

In reality they flow from the -ve to the +ve supply terminals.

However, conventional current flow is defined as from +ve to -ve which is exactly the opposite of what is physically happening.

This is all the result of historical context. When Benjamin Franklin set out his ideas for the flow of charge in the 18th century, he had no idea about the concepts of atomic structure or electrons and protons.

But nonetheless he defined the direction for the displacement of charge from the materials he experimented with. i.e. from wool to wax when they are rubbed together. That is why electrons are labelled as -ve charge because he assumed charge moved in the opposite direction to the way it actually moves.

That arbitrary labelling system stuck and by the time it was discovered he was wrong very many years later, it was so widely used that no effort was made to change it.

So we still use his convention today. It makes no difference whatsoever to the reality of current flow because an electron would still behave the same way even if it were called nice-charge jtbteddytrons!

But current is the flow from the positive to the negative terminal, which is the opposite to electron flow.. So how can current be the rate of flow of electrons?

Sorry, Im still confused. So realistivally, theres nothing flowing from the positive terminal to the negative one? It was just an assumption?


Posted from TSR Mobile

Yes, it was just an assumption that turned out to be opposite to reality when electrons were discovered. But that convention stuck and is still used today.

When dealing with circuits and practical applications of electricity we always use this so called 'conventional current' description. In other words the Franklin version.

It makes absolutely no difference to calculations as we can see from all of the electrical and electronic devices in use today. It's just a convention, the same as if we called debt positive and credit negative. As long as the convention is used throughout all calculations, the answer will still be the same.

When dealing with individual charges at the atomic level such as with particle physics, the correct vector magnitude and direction of electrons and forces must be used.

Don't worry, it confuses the hell out of a lot of people. Other examples of arbitrary naming conventions: magnetic north and south poles, east and west, left and right, up and down etc. etc.