Required Practical 9

Discharging - Equipment

Electrolytic capacitor  Resistor Battery Voltmeter Switch

Discharging - Method

Set up the apparatus as shown in the diagram  Set the switch to the A position to allow the capacitor to fully charge Move the switch tp the B position and start the stopwatch. Observe and record the voltage reading V at time t = 0 and at 5s intervals as the capacitor discharges until about 120s have passed Repeat the experiment twice more and obtain the average V at each t

Required+Practical+9(1) (binary/octet-stream)

Discharging - Graphs and calculations

Calculate the natural logarithm of V at each t and tabulate this. Plot a graph of ln(V) against t and draw a line of best fit. This should yield a straight line graph with negative gradient, showing that the decay of voltage across the capacitor is exponential. The gradient will equal -1/RC V=V(0)e ^(-t)/(RC)  ⇒ ln(V) = (-1/RC)t +ln(V(0)) If C is known, you can find the time constant using RC and also using the graph and check if they are the same. If C is not known, you can find it using the graph

Discharging - Safety

Ensure the capacitor is connected with the correct polarity and that its voltage rating exceeds the voltage of the battery used to prevent it from exploding and releasing harmful chemicals

Discharging - Improvements and notes

You can also plot a graph of V against t which will give an exponential decay curve. The time constant can be found from this by finding t when the voltage is approximately 37% of the original voltage (the voltage of the battery)

Charging - Equipment

Electrolytic capacitor  Resistor Battery Voltmeter Switch

Charging - Method

Set up the apparatus as shown in the diagram Close the switch and observe and record the voltage reading V at time = 0 and at 5s intervals as the capacitor charges until about 120s Repeat the experiment twice more and obtain the average V for each t

Required+Practical+9(2) (binary/octet-stream)

Charging - Graphs and calculations

Plot a graph of V against t. This graph will show an exponential growth curve  V=V(0)(1-e^(-t)/(RC))

Charging - Safety

Ensure the capacitor is connected with the correct polarity and that its voltage rating exceeds the voltage of the battery used to prevent it from exploding and releasing harmful chemicals

Charging - Improvements and notes

The time constant can be found from the exponential growth curve by finding t when the voltage is approximately 63% of the maximum voltage (the voltage of the battery)