# RC Circuit

1. Feb 19, 2005

### 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?

2. Feb 19, 2005

### Staff: Mentor

3. Feb 19, 2005

### ehild

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

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