Direct Current (DC) and Alternating Current (AC)

Electric current is the flow of electric charge. We measure current in amperes (A). The size of a current depends on how much charge passes each second: $I = \dfrac{Q}{t}$. Conventional current is defined as flowing from positive to negative. In metals, electrons actually move from negative to positive.

Direct Current (DC)

DC flows in one direction only. A battery or cell gives DC because its positive and negative terminals stay fixed. On a current–time graph, ideal DC is a flat line above zero (constant value). DC has a fixed polarity: devices that care about polarity (like LEDs) must be connected the right way around.

Alternating Current (AC)

AC repeatedly changes direction. The current grows to a peak in one direction, falls to zero, then reverses to a peak in the opposite direction. On a graph, simple AC looks like a smooth wave (sine wave) crossing zero regularly.

• Frequency is how many complete waves happen each second (unit: hertz, Hz). $f$.
• Period is the time for one complete cycle (seconds). $T$.
• They are linked: $$f = \frac{1}{T}$$
• Amplitude/peak shows how large the current or voltage swings to its maximum.

Mains electricity is AC (typically 50 Hz in many countries, 60 Hz in some). A simple generator produces AC.

Comparing DC and AC

• Direction: DC one way; AC reverses.
• Sources: DC from batteries or DC power supplies; AC from mains or generators.
• Graph: DC is a flat line; AC is a wave crossing zero.

Measuring Current (Ammeter)

• Connect an ammeter in series so the whole current flows through it.
• Select the correct type: DC setting (straight line symbol) for DC; AC setting (tilde ~) for AC.
• Start on a higher range to avoid damaging the meter, then reduce for a clearer reading.
• Analogue meters use a needle; digital meters show numbers.