We commonly use meters to make measurements in electric circuits but meters are relatively simple devices and if the property we are measuring changes over a short period of time then a meter will not show this clearly (if at all). But a different instrument, the oscilloscope, has the power to reveal such changes in full detail.

An earlier post (DC and AC Electricity Pt1) described the basic differences between DC and AC electricity in terms of the movement of electrons: in DC electricity, the electrons move forwards in a constant direction whereas in AC electricity the electrons move backwards and forwards (oscillate) at a constant rate.

The movement of electrons is driven by the supplied energy, which is measured by potential difference (voltage). We can therefore visualise the movement of electrons through components in DC and AC circuits by plotting a graph that shows how the potential difference (voltage) varies with time. An oscilloscope does this automatically.

Oscilloscopes work by passing a dot of light across a screen, and when the dot moves quickly, it draws a line. If the potential difference changes as the dot crosses the screen, the dot will be deflected upwards by a positive voltage and downwards by a negative voltage. Oscilloscopes have a grid on their screen and an adjustable scale, marked in volts-per-division (square) so they can be used to measure both the magnitude and the sign (direction) of the potential difference.

In the case of DC electricity, where electrons are always moving forwards, the voltage will have a fixed value and the graph of DC electricity is therefore a horizontal straight line. If the voltage is zero, the line will cross the middle of the screen, as shown in the picture below.

Zero voltage is indicated by a flat line that crosses the middle of the oscilloscope screen.

If there is a positive voltage, the line will be in the top half of the screen but if the voltage is negative then the line will be in the bottom half of the screen. The greater the voltage (either positive or negative) the further the horizontal line will be away from the middle of the screen (the zero line). The two pictures below illustrate this difference. The scale was set to 2 V/div for both pictures so the two voltages shown are 4 V and -6 V for DC electricity.

The screen on the left shows a positive DC voltage; the screen on the right is a negative DC voltage.

In the case of AC electricity, where electrons move both forwards and backwards, the voltage varies between positive and negative values and the graph of AC electricity looks like a wave. The line will be in the top half of the screen when the voltage is positive and in the bottom half of the screen when the voltage is negative. The voltage changes smoothly between positive and negative limits with a constant rate of oscillation, resulting in a familiar wave pattern, as shown below.

AC electricity is always represented by a wave that crosses above and below the zero line (mid-screen).

In the same way that the vertical scale on an oscilloscope screen measures voltage, the horizontal scale measures time. So by counting the number of squares the AC signal takes to complete a full wave, and knowing the time taken for the beam to cross each square, we can find the period of the wave. This in turn can be converted into the frequency of the AC electricity.

The AC picture was captured with a time scale of 5 ms (0.005 s) per division. There are four divisions (squares) between two adjacent peaks of the wave giving a total time of 0.02 s. To find the frequency of a wave, we take the reciprocal of its period, which is done using the equation;

frequency (Hz) = 1 / period (s)

In this case, the frequency ( given by 1 / 0.02 ) is therefore equal to 50 Hz, meaning that there are 50 waves created each second.

We can also look at the vertical scale to find the amplitude of the wave. The voltage scale is set to 0.5 V/div and the wave goes two squares above the zero line so its peak voltage is 1 V.

Importantly, a wave that is entirely in the top half of the screen is a varying DC signal, not an AC signal. This is because the values change but they are always positive, so there is no change in direction for the electricity – and that means it cannot be alternating current (AC). Varying DC signals are normally called ripples. An example is shown in the picture below.

This wave is a DC ripple, not AC electricity, because the wave remains entirely above the zero line.

To test your ability to interpret signals on an oscilloscope screen, take the test by clicking here.

If you want to experiment with a free online oscilloscope simulator (the one that was used to create the displays shown here) then head over to https://physics-zone.com/virtual-oscilloscope/

Published by physbang

Writer and photographer with experience in teaching (mostly secondary physics), computer programming and research metallurgy. Previously the lead for both e-safety and e-learning for all schools in Jersey. Author of multiple books on photography and articles on other subjects ranging from unreliable online information to customising multiple-choice question papers.

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