A.C/D.C

As Mr Clydesdale says “A very good band”

The date is really 2020, but I need this post under the main Electricity notes section.

Mr Sharkey demanded I take screenshots of the traces for his OneNote ClassNotebook as he was made to leave against his will! So he asked, and I did!

Here are the A.C /D.C traces

from Nat 5C 2020

We’ve plugged in a 1.5 V cell to the picoscope, put a voltmeter in parallel and noted the reading on the voltmeter and the looked at the value on the picoscope.

The picoscope was picking up some of the electrical signals from the computers and power around the room.

Notice on these images the reading on the picoscope and the voltmeter are the same. The cell is a source of D.C, direct current. In direct current the current /charges only flows in one direction. The free electrons in the ciruit are always drifting around the circuit in one direction.

When the polarity is reversed (swapping the positive and negative connections to the cell) the trace moves below the zero line showing that the current is now in the opposite direction. The voltmeter reads -1.5 V (the negative indicating that the current is in the opposite direction).

When we connect up to the A.C supply of the usual school power supplies we can see that the trace indicates the current flows in both directions. We can tell this as the trace of the voltage goes above and below the 0 V line on the picoscope. The trace shows a wave indicating the voltage and hence current is changing direction and magnitude many times per second. In the case of the mains voltage the frequency of the supply is 50 Hz.

Notice that the reading on the voltmeter reads 6.69 V. The power supply is set to 6V, but the peak of the trace is greater than this, about 9.5 V. The peak voltage of an A.C. trace is always greater than the quoted voltage of the supply. This is because we want to be able to compare A.C and D.C traces and so the quoted value is 1.414 times smaller than the peak voltage, try this.

When the polarity is reversed it makes no difference to the trace.

Another power supply in the Department is the 5.0 V regulated power supply. We can see this is a D.C trace and that the value of the voltage and hence the current is steady.

We can see when the polarity is changed (the connections to the power supply are swapped over) We can see the the trace of the voltage goes below the zero line, indicating the current is moving in the opposite direction. The voltmeter reading is the same as the value on the picoscope.

However, when we connect the picoscope to the usual Lockmaster power supply on the D.C. setting we get a rather unusual trace. The trace is D.C, remember direct current tells us that the current remains in one direction. However, the voltage and hence current isn’t constant. This is an unsmoothed D.C trace, and is common in cheaper power supplies. The trace never goes below the zero value on the screen.

Reversing the polarity shows us that the voltage is opposite, we get a negative value on the power supply but the trace never goes above the line. The current remains in one direction.

So in summary

In DIRECT CURRENT the current always moves in one direction.

In ALTERNATING CURRENT the current changes direction, usually many times per second. The current also usually changes magnitude (size).

With cells or regulated power supplies the D.C trace gives a constant value. In an unregulated trace the current also changes magnitude, but never direction.

Signature
December 2020

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