# Homework Help: DC and capacitive reactance

1. Nov 5, 2008

### TuAst

1. The problem statement, all variables and given/known data
Hi

I hope I write in the right thread, if not, my apologies to the moderator. I am not a physisist, but I am very interested in physics and I am doing medical physics right now, and wondered if you guys could help me with something (probably easy for you, but I don't seem to find the answer anywhere);

What is the "relationship" between capacitive reactance and a DC circuit? I mean, is there any at all in a DC circuit. I would really appreciate a "for the idiots" explanation on this if someone has the time.

Thank you very much in advance!

2. Relevant equations

3. The attempt at a solution

2. Nov 5, 2008

### lvlastermind

Hi tuast

When a capacitor is connected to a dc source (such as a battery) the capacitor plates quickly acquire equal and opposite charges but no steady current flows in the circuit. A capacitor prevents the flow of a dc current. But if a capacitor is connected to an alternating source of voltage, an alternating current will flow continuously. This can happen because when the ac voltage is first turned on, charge begins to flow and one plate acquires a negative charge and the other a positive charge. But when the voltage reverses itself, the charges flow in the opposite direction. Thus, for an alternating applied voltage, an ac current is present in the circuit continuously (Physics for Scientists and Engineers with Modern Physics). I think you might be a little confused about capacitive reactance. Capacitive reactance is the capacitors opposition to a change in ac voltage. In a purely capacitive circuit, capacitive reactance is the imiting factor in current flow.

In conclusion, capacitive reactance is a measurement for ac voltage and not dc. If you are interested in physics i would suggest reading this book.

Physics for Scientists and Engineers with Modern Physics