Title:                     Study of capacitors in charging            

       And discharging processes                                  

Date of Experiment:            2007/03/21    

Objective:

• To study the factors affecting the charging and discharging processes of capacitors
• To observe the charging and discharging currents

Apparatus:

Result:

1. Study of capacitor in charging

Firstly, a 470μF capacitor was connected to a 1.5V battery with 1000Ω resistor and a centre-zero galvanometer as shown below. Then the reading of the galvanometer was observed when circuit was completed.

When the circuit was completed, the reading of the galvanometer increased from 0mA to 1.14mA instantaneously at once. Then the reading decreased to 0mA gradually.

2. Study of capacitor in discharging

Both sides of the 1.5V cell was connected it a wire shown by the dash line in figure above. Then the reading of the galvanometer was observed when circuit was completed.

When the wire was connected,the reading of the galvanometer decreased from 0mA to –1.14mA instantaneously at once. Then the reading increased to 0mA gradually. The case was similar to Part A of the experiment but with opposite direction.

C. Investigation of factors affecting charging and discharging

Part A and B were repeated with the other capacitors (100μF, 220μ

For 1000Ω resistor:                         For 350kΩ resistor:                  

The patterns of the two current pulses were similar. During charging, the voltage increased to a maximum at once, and then slowly decreased down to zero. During discharging, the pattern was similar to that

1. The apparatus given was not accurate or precise enough. (e.g. galvanometer) They may also affect the experimental data. Using better quality of apparatus instead can minimize the inaccuracy.

2. There were wrong assumptions. The wires and the galvanometer had small resistance which were neglected. The 1000Ω resister had resistance <>1000Ω.They affected the experimental result actually. To reduce errors, the resistances should be considered also.

3. The dry cells were not exactly 1.5V due to long time using. So, before the experiment, the voltages of dry cells should be checked by CRO first, or using new dry cells

4. The scales on the galvanometers were observed by the human naked eyes. The records were inaccurate. Using a hand lens when reading, Reading the scale when the pointer is directly on top of its image in the mirror behind was important to lower the error. Apart from these, the scale was read by several people repeatedly could help the accuracy too.

Conclusion:

In this experiment, the charging and discharging currents were found to be directly proportional to the capacitance of capacitors and the voltage across the capacitors, but inversely proportional to the resistance of the circuit.