# What is RC (physics)

Can someone help please? I'm revising physics, and have realised I have no idea what RC is. I know it's a product and is called the time constant, and has the units of seconds. But what is it?

Ed
it is the time for q (the charge) to fall by a factor of 1/e of its initial value.

love Katy ***
Thats what I like, a simple response I understand. I'm sure I did know that, just couldn't find it in my books!

Cheers Katy.
CR is the time constant of a capacitor - the time taken for the current, charge or PD across the capacitor to fall to 1/e of its current value. A circuit with a large time constant discharges slowly. You use it in an equation like:

Q = (Q0)e^(-t/CR)

Where Q is the charge you are trying to find at time t, and Q0 is the charge at t=0.
When there is a capacitor discharging in a circuit RC is the product of the circuits total resistance and the capacintence of the capacitor. Now RC is the time taken for the current in the circuit to reduce to 1/e of its starting value. Now thats about 36.8% of its original value.

Its kind of like a half life but for capactiors, and instead of being half, its 36.8%.

Since it is the time taken for the current to reduce to 1/e of its original value, and current is directly proportional to voltage of the capacitor, it is also the time taken for the voltage to fall to 1/2 of its orginal value.

Hope this helps. Anymore questions just ask.

Richard
Hi Richard,

"it is also the time taken for the voltage to fall to 1/2 of its orginal value."

Why is that so?

Jacob
Isn't it the time taken for the current to fall to 1/e rather than 1/2 of it's original value?
Regarding the above post: Damn, I meant voltage! Sorry, my mind is wandering.
AFAIK, it can be voltage (pd), charge or current. The formula

x = x0e^(-t/CR)

is generic to all of those quantities (just replace x with what you need).
Sorry everyone.

When I said 1/2 I meant 1/e.

Richard