Download Time Varying Voltages

Time Varying Voltages
Introduction
We have talked about the electromotive force (e.m.f.) created by chemical means in batteries.
Batteries produce an e.m.f. that does not change as time passes – at least it doesn’t change up
until such time as the battery wears out. There are other sources of e.m.f. that do change their
amplitude as time passes. One common source of time-varying e.m.f. is the mains electricity
supply.
In this lab experiment we will introduce a simple time-varying e.m.f. based on the idea of
switching the connections to a battery on and off repeatedly. We will progress to the
sinusoidal voltage waveform used for mains electricity.
Finally we will look at the oscilloscope as an instrument that can draw a picture of the way
that an e.m.f. varies relative to time – i.e. plot a graph of voltage versus time for a source of
e.m.f.
Equipment List:
220 Ohm resistor
Benchtop Power Supply Unit – PSU
Digital Multi Meter – DMM
Procedure:
1. Plug the PSU into the mains supply and connect the 220 Ohm resistor between the output
terminals of the PSU. Turn on the PSU and set the voltage to 10 volts. Turn the PSU off
again.
2. Turn the PSU back on and every five seconds for thirty seconds after turn-on, measure
the PSU output voltage using the DMM. Enter the voltage measurements in the table
provided on the lab record sheet.
3. After 30 seconds, turn the PSU off and repeat the measurement of PSU output voltage
every 5 seconds for thirty more seconds. Enter the voltage measurements in the table
provided on the lab record sheet.
4. Repeat steps 2 and 3 a second time and record the measurements.
5. Plot the measurements from the table on the graph paper supplied showing measured
voltage on the vertical axis and time elapsed on the horizontal axis. Why is this timevarying e.m.f. called a square-wave? What is the period of time, T, after which the
pattern starts to repeat? Calculate the number of repetitions of the basic shape that would
occur in one second – this is called the frequency.
6. Repeat steps 2-4 again but this time switch the PSU on for 10 milliseconds and then off
for 10 milliseconds with measurements of the PSU voltage taken every 2 milliseconds.
Enter the values in the table provided and plot as before. Calculate the period T and
frequency as before. Comment on any practical considerations associated with these
measurements.
7. In a separate experiment, a PSU was not simply switched on and off but instead an
operator adjusted the PSU voltage smoothly between the values specified at the 5 second
time intervals given in the table on the lab record sheet - a bit more tricky but not unlike
what you have just done. Plot the values from the table on the supplied graph paper –
remember that the operator adjusted the PSU voltage smoothly. Comment on the shape
of the waveform that you have just drawn. Calculate its period, frequency and the peak
amplitude in volts of the e.m.f.
Time (seconds)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
PSU voltage (volts)
Why is this time-varying e.m.f. called a square-wave?
What is the period of time, T, after which the pattern starts to repeat?
Calculate the number of repetitions of the basic shape that would occur in one second
– i.e. the frequency.
Time (milliseconds)
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
PSU voltage (volts)
What is the period of time, T, after which the pattern starts to repeat?
Calculate the number of repetitions of the basic shape that would occur in one second
– i.e. the frequency.
Comment on any practical considerations associated with these measurements.
T (mS)
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
EMF (V)
0.00
2.94
4.76
4.76
2.94
0.00
-2.94
-4.76
-4.76
-2.94
0.00
2.94
4.76
4.76
2.94
0.00
-2.94
-4.76
-4.76
-2.94
0.00
Comment on the shape of the waveform that you have just drawn.
Calculate its period, frequency and the peak amplitude in volts of the e.m.f.