# I Capacitor - why does current flow

1. Jun 24, 2017

### Janez

I have question, why can condesator get charged, when connected to voltage source, if for current to flow, there most be closed wire, but here it is not, but there still is current? Thanx for answers.

2. Jun 24, 2017

### Buzz Bloom

Hi Janez:

A current flows while electrons accumulate on one side of the capacitor, and are depleted from the other side. When the capacitor has reached it's maximum state of charge, then no more current flows.

Regards,
Buzz

3. Jun 24, 2017

### phinds

the current does not flow through the capacitor, it moves in the rest of the circuit to stabilize the charge on the cap.

4. Jun 24, 2017

### Janez

Why do we need capacitator then? If the wires are metalic and we have voltage source, we just disconnect wires at some point and wires should get charge on them?

5. Jun 24, 2017

### FactChecker

The plates in the capacitor allow room for the charges to accumulate that a simple wire does not. As long as the charge can continue to flow into the capacitor plate, it seems just like they are going through on a normal wire. Once the capacitor plate is filled up, the charge stops flowing. If the charge changes direction, the accumulated charge flows back out and once again appears as though it is going through a normal wire. So the capacitor plates provide a storage area for charges to come and go. If the charge changes direction often (high frequency), a capacitor acts very much like a simple wire. If the charge doesn't change direction often (low frequency), the capacitor acts more like a disconnected wire (higher resistance).

So a capacitor will block a constant voltage that never oscillates (after the charge has built up). It will also react differently toward different frequencies -- low resistance to high frequencies and high resistance to low frequencies. Those are very good uses for capacitors.

Last edited: Jun 24, 2017
6. Jun 24, 2017

### sophiecentaur

That is too simplistic. If the Capacitor is in a 'black box', you can measure a current flowing into the + terminal and out of the - terminal. If you looked at just the instantaneous current (a single quick reading of IN and OUT)) you would not know what component was in the box and you could even conclude that it was a resistor (of an appropriate value). HOWEVER, over time, the current flowing would be changing (and the PD across it). The slower the changes, the greater the Capcitance.
We 'need' Capacitors because we can use this time dependence of Current and PD to cause phase changes, frequency selectivity and timing delays in a circuit which you just can't do with resistors alone.

7. Jun 24, 2017

### SlowThinker

Disconnected wires are a capacitor but with very low capacity. If we need more capacity, and we often do, we need to increase the area of the electrodes, bring them close together, and replace air with a better material. All of these increase the capacity.

8. Jun 24, 2017

### phinds

If and when you ever study electric circuits you will quickly see why we need caps.

9. Jun 25, 2017