# RC Circuits

This lecture is based on HRW, Sections 28.8
• When a capacitor in series with a resistor discharges, the charge on its plates decreases exponentially
```         q(t) = q0 * exp(-t/RC)
```
• The current created by the capacitor also decreases exponentially from its initial value
```         I(t) = I0 * exp(-t/RC)
```
• The product of resistance and capacitance, RC, is called the time constant of the circuit. The current and charge change by a factor of e (= 2.718) for every time constant which passes
• Charging a capacitor goes quickly at first, then slows down and approaches a limiting value
```         q(t) = q0 * [ 1 - exp(-t/RC) ]

= CV * [ 1 - exp(-t/RC) ]
```
where V is the voltage of the battery charging the capacitor
• The current flowing as a capacitor charges is large at first, but decreases with time
```         I(t) = I0    * exp(-t/RC)

= (V/R) * exp(-t/RC)
```

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