Solving an RC Circuit with 68.0% Charge in 0.980s

[Kawrae]

A capacitor in an RC circuit is charged to 68.0% of its maximum value in 0.980 s. What is the time constant of the circuit?

>> I know the time constant is equal to Q/(Q/t) and that t = 0.980 s. I also know that Q = CE and E is a constant. But they don't tell me the capacitance to use... Bah I'm really stuck and confused :( Does anyone know how to solve this?

 

[Astronuc]

Capactive (and inductive) time constants - http://hyperphysics.phy-astr.gsu.edu/hbase/electric/filter.html#c2

The capacitive time constant $\tau$ is just the product RC, where R is the Resistance and C is the Capacitance.

Don't forget V = Q / C.

 

[ehild]

A capacitor in an RC circuit is charged to 68.0% of its maximum value in 0.980 s. What is the time constant of the circuit?

>> I know the time constant is equal to Q/(Q/t) and that t = 0.980 s. I also know that Q = CE and E is a constant. But they don't tell me the capacitance to use... Bah I'm really stuck and confused :( Does anyone know how to solve this?

The time dependence of voltage across a capacitor when it is charged from a source of emf E is

$$U=E(1-\exp(-t/\tau))$$ ,

$$\tau$$ being the time constant and the maximum voltage is E across the capacitor, so

$$U/U_{max}=1-\exp(-t/\tau)$$

Just plug in the values and solve for $$\tau$$.

ehild