Datalogging in Science

Charging and discharging of a Capacitor

Overview
In this demonstration the difference in time taken to charge and discharge capacitors, is explored. The charge that a capacitor can store and then deliver when discharged is also measured. An LED is included with the resistor so as to give a visual indication of charging and discharging.

Theory
The charge stored on a capacitor is given by the following equation:

Q = C V

Q = the charge in Coulombs.
C = the capacitance in farads.
V = the voltage.
When a capacitor is charged a current flows.
The relationship between charge and a steady current is:

Q = i t

i = the current flowing in amps.
If the current is variable, as in this experiment, then the charge flow is the AREA under the current time graph.

Apparatus

1. Voltage Sensor .....( 0 - 10V)
2. Current Sensor ......(100mA)
3. Resistors ...............( 330 Ohm )
4. Capacitors........(470 micro Farad)
5. Low voltage DC supply. Dry cell 9V.
6. Electric circuit kit or breadboard
7. Wires and crocodile clips.
If not using the recommended values, consult the table below for suitable alternatives.

Procedure
1. Assemble the circuit as shown but do not connect the power.
If using electrolytic capacitors ensure the correct polarity is observed.
Connect the Voltage Sensor and Current Sensor to the datalogger.

2. Note down the values of the resistors used and the
value of the capacitor.

3. Launch the Graph software.
Put the switch into the discharge position "B" and connect the power supply.

4. Use Test mode to check that the capacitor is fully discharged, the Voltage and Current Sensors should read zero. It may be necessary to short the terminals of the capacitor using a piece of wire.

5. Begin datalogging.

6. Immediately logging starts put the switch to the "A" position.

7. When the graph lines have levelled out and the values are almost constant put the switch into the "B" position. There may be time to do a second complete charge/discharge cycle.

8. If a complete charge/discharge cycle was not obtained on the graph, repeat the recording

9. When finished, discharge the capacitor by putting the switch to the discharge position.

Results

The graphs obtained are of Current v Time and Voltage v Time.

The analysis features of the datalogging software may be used to determine
1. The time interval of charging and of discharging.

2. The total charge stored, by finding the area under the discharge section of the Current v Time graph.

3. The peak voltage, by identifying the highest point on the Voltage v Time graph.
Footnote

Choice of method

1. A capacitor will fully charge in approximately 6 times the time constant.

2. Whereas with real time recording, students can see the curves developing as the logging takes place, fast logging will give more reliable results. However fast logging is unseen and the result not visible until the data is downloaded to the computer.

3. A 30-second interval gives enough time to collect two full charge discharge cycles with a time constant of 1 second.

Background information on Capacitors
Capacitors are devices that can store electric charge; they do not produce the charge. Capacitors were originally made from two metal plates separated by an insulating material called a dielectric. They are now made from a wide range of materials. There are many different types of capacitor available and they come in a variety of shapes and sizes. The unit of capacitance is the farad and values will range from Pico farads (10-12) to farads. Capacitors are used in: timing circuits, energy storage, frequency filters and smoothing voltages.

Data Harvest users

1. Connect the sensor to the datalogger
and the logger to the computer
2. Then click the Set Up icon
to the right of this message.
3. When the software opens,
click the Play button.