# Voltage & Capacitance: LanguageNerd's Questions

• LanguageNerd
In summary: Displacement current is just a fancy word for current that flows because the electric field has changed.
LanguageNerd
As everybody already knows, voltage is defined as the work done/energy per unit charge. I see voltage as being the idea of giving the charge carriers a 'want' to move: when a closed circuit is attached to a battery, the voltage makes the charges want to move, and they can. Yet, in an open circuit they want to move but can't. Eventually if you were to increase the voltage between the gap in the circuit to the breakdown voltage of the air, you'd get a spark and a quick burst of current - is this all correct so far?

However, I've stumbled across something when revising capacitance that's confused me:

We know that current is the flow of charge, and when a circuit is broken, the current(flow of charge) stops. In a capacitor, there are two metal plates, separated by an insulating material, thus meaning the circuit is broken, therefore no flow, therefore no movement of charge carriers; no dispute.

The book then goes on to say that the electrons spread out across the plate attached to the negative terminal of a battery. Now I thought there would need to be a current for the electrons to move and "spread out" along the surface. Evidently, there is a voltage, therefore a 'want' to move, but as I understand, because the circuit is broken, they shouldn't be able to do so?

Thanks for any help given in advance,
LanguageNerd

There is very rapid current flow for a very short amount of time. During this time, negative charges build up on one plate of the capacitor, and deplete from the other plate. This goes on until the negative charge on the negative plate and the positive charge on the positive plate just balance the voltage across the capacitor. Then the flow of current stops. This all takes place very rapidly, and involves very little flow of charge.

1 person
Chestermiller said:
There is very rapid current flow for a very short amount of time.

How, exactly, can this current flow when the circuit is not connected? Throughout all my studies, they've drawn the conclusion that when the circuit breaks, or in this case never was connected, the current immediately stops , or never flows to begin with.

Your studies have always considered the 'steady state' after things have settled out. When you 'charge something up', current flows for a short time.

To elaborate on what SC said, this isn't quite like two wires with a big open space between them. The capacitor plates are very close together, and the short burst of current that occurs allows charge to build up on the negative plate and get depleted from the positive plate. They call it a capacitor because it has the capacity to build up charge.

That's a point well worth making.
The capacitance between two random wires near each other will be a few picoFarads, whereas the capacitance of a 'big' capacitor, available these days, can be 1 Farad (10^12 times bigger!)

Keep in mind that there is something called displacement current which propagates across the gap of a capacitor as it charges and discharges. This confusion about capacitors is very common based on many questions on these boards. It also confused Maxwell who discovered the displacement current in an effort to explain it.

The displacement current occurs when the electric field changes in time. It has the same units as current but does not require mobile charges to propagate.

## 1. What is voltage?

Voltage is a measure of the electrical potential difference between two points in an electrical circuit. It is often referred to as "electrical pressure" and is measured in volts (V).

## 2. How is voltage related to capacitance?

Voltage and capacitance are closely related, as capacitance is a measure of the ability of a capacitor to store electrical charge at a given voltage. The higher the voltage, the more charge a capacitor can store.

## 3. What is capacitance?

Capacitance is a measure of the ability of a capacitor to store electrical charge. It is determined by the physical characteristics of the capacitor, such as the distance between the plates and the type of material used.

## 4. How do voltage and capacitance affect each other in a circuit?

In a circuit, voltage and capacitance have an inverse relationship. This means that as voltage increases, capacitance decreases, and vice versa. This relationship is important in controlling the flow of electricity in a circuit.

## 5. What are some practical applications of voltage and capacitance?

Voltage and capacitance are important in many electronic devices, such as computers, cell phones, and televisions. They are also used in power grids to regulate the flow of electricity. Capacitors are also used in electronic circuits to filter out unwanted signals and to store energy for short periods of time.

Replies
7
Views
475
Replies
7
Views
2K
Replies
1
Views
1K
Replies
9
Views
1K
Replies
21
Views
2K
Replies
18
Views
2K
Replies
16
Views
2K
Replies
4
Views
2K
Replies
10
Views
2K
Replies
24
Views
3K