Datalogging in Science

Measure frequency of alternating current

Overview

In this demonstration a transformer is used to step down the alternating supply to a voltage of about 4 V (The frequency remains unchanged). This alternating voltage is connected to a resistor. A voltage sensor and current sensor are included in the circuit and are connected to a datalogger recording in fast mode. The current and voltage values are plotted against time by the Graph program. The software is also used to measure the time interval between peak values and so to calculate the frequency of the a.c.

Apparatus

Transformer capable of delivering a low a.c. voltage (e.g. 4 V).
ResistorVoltage sensor (+/– 10V),
Current Sensor (+/- 10A),
Datalogger capable of recording in Fast mode.

Procedure
1. Assemble the apparatus as shown

2. Connect the voltage sensor to socket 1 and the current sensor to socket 2 of the datalogger.

3. Open the Graph program
and set it to record current and voltage in Fast mode.

4. Switch on the current and set it to a low value.

5. Start logging the data and after a short time when it has been
retrieved from the logger, observe the graph of current against
time on the computer screen and observe also voltage plotted
against time.

6. Analyze the graph using the various software options available.

The graph may look something like the one shown below:

Results

The results should be saved on the computer before analyzing.

Questions
1. What is the frequency of the a.c. current?
2. What is the frequency of the a.c. voltage?

3. How does the peak voltage compare to what the transformer voltage setting suggests was supplied to the resistor?


Further tasks

Repeat the experiment for different voltage settings of the transformer.
Questions: What remains the same? What is different?

Background information on Alternating Current (a.c.)

In this context, alternating means frequent change of direction. Alternating current is very effective at liberating heat from a wire. It is more cost effective to meet our heating needs through supplying alternating current to our homes than direct current. IA.C. can be generated by the relative motion of magnets and coils of wire. Alternating voltage used to drive the alternating current can be easily stepped up or stepped down using transformers. Alternating current can be converted to direct current using rectifiers, but the output may not be perfectly smooth.

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.