# Energy stored in a capacitor graph

1. Jan 20, 2009

### nokia8650

What would a graph of Energy stored in a capacitor whilst charging and discharging a capacitor against time look like?

Would the graph of Energy stored vs. time whilst charging have the same shape as the Voltage/time graph?

For discharging, would it be exponential decay?

Thanks

2. Jan 20, 2009

### skeptic2

It seems to me you'll need a formula for energy versus voltage and capacitance. I wonder where you could find that.

3. Jan 20, 2009

### nokia8650

Thanks for the reply. E=0.5QV. This doesnt relate the energy to time, however.

Thanks

4. Jan 20, 2009

### skeptic2

For a charging capacitor the formula is V*(1-e^-(t/rc)) where V is the supply voltage used for charging, e is the number e, about 2.71, t is the charging time and rc is the charging resistance times the capacitance. RC is often called the time constant or tau.

For instance if R=1k ohm and C=1 uF, the time constant would be 1k * 1u or .001 seconds. This means that if you charged that capacitor for 1 millisecond it would be 1-e^-(.001/.001) or 1 - e^-1 or about 63.2% charged. Multiply that by the charging voltage to get the charged voltage.

5. Jan 20, 2009

### nokia8650

Thanks alot for the reply. So what would graphs of ENERGY vs. time look like? I am aware of the case for Voltage, charge and current.

Thanks