Direct and alternating current
Figure 13.24(a) shows a circuit with a cell, a resistor and a centre-zero galvanometer connected in series. The resistor stops a large current flowing through the galvanometer.
Figure to be added
The galvanometer shows a steady deflection, so a steady current flows through it. Plotting the current against time gives a straight line, as in figure 13.24(b).
| t (cycles) | 0 | 2 |
|---|---|---|
| V | 0.7 | 0.7 |
Figure 13.24(b) — direct current: the current (or potential difference) stays the same against time.
Now connect the AC dynamo discussed earlier in series with a resistor and the centre-zero galvanometer, as in figure 13.25(a), and turn the dynamo's armature by hand. The galvanometer pointer swings back and forth between + (positive) and − (negative). Plotting the current (or potential difference) against time gives a curve like figure 13.25(b).
Figure to be added
| t (cycles) | 0.25 | 0.75 | 1.25 | 1.75 |
|---|---|---|---|---|
| V | 1 | -1 | 1 | -1 |
Figure 13.25(b) — alternating current: the current (or potential difference) swings positive and negative in turn.
Direct current (DC)
The direction of the current does not change with time.
From cells, batteries and solar cells.
Alternating current (AC)
The direction of the current changes with time.
From AC dynamos and the mains supply.
Varying is not the same as alternating
A current can rise and fall and still be direct, as long as it never changes direction. The test is the sign: a current is alternating only if it goes both positive and negative.
Why the galvanometer tells them apart
A centre-zero galvanometer has its zero in the middle of the scale, so it can swing either way: one side for current entering one terminal, the other side for current entering the other. With a cell, the pointer moves to one side and stays there. With the dynamo turned slowly by hand, it swings to one side, back through zero and over to the other, following the turning coil of the last pages. Turned quickly, the swings come too fast for the pointer to follow — which is why the mains, reversing 100 times a second at 50 Hz, would just leave the pointer trembling at zero.
Direct one way, alternating both ways
A current whose direction does not change with time is direct; one whose direction changes with time is alternating.