Voltage and Current

Electricity is invisible, which makes it hard to picture. So engineers, teachers and makers all use the same trick: imagine water flowing through pipes. It is not a perfect comparison, but it is close enough that it will get you through years of building.

Voltage is pressure

Voltage, measured in volts (V), is the push. It is the pressure behind the water in the pipe. A tall water tower gives high pressure; a small bucket on the floor gives almost none. In electronics, a 9V battery pushes harder than a 1.5V AA battery, and your Arduino's 5V pin pushes with a steady, moderate force.

Voltage is always measured between two points, which is why every circuit needs a ground connection. Saying "this wire is at 5 volts" really means "this wire is 5 volts higher than ground". Without ground, the number has no meaning — the same way "two metres higher" only makes sense if you know higher than what.

Current is flow

Current, measured in amps (A) or more often milliamps (mA), is how much electricity is actually moving. It is the amount of water rushing past a point every second. A tiny LED sips about 20 mA. A motor might gulp 500 mA or more. This is why motors need a driver board and LEDs do not.

Here is the crucial detail beginners often miss: voltage is supplied, while a load draws current. A power source provides a voltage, and the circuit's resistance helps determine the current. Never test any battery on your tongue or skin.

Resistance is a narrow pipe

Resistance, measured in ohms (Ω), is how much the pipe fights the flow. A narrow pipe lets less water through for the same pressure. That is exactly what a resistor does in a circuit, and it is why we put one in front of every LED — to keep the flow gentle enough that the LED glows instead of burning out.

Ohm's Law, the one bit of maths

These three quantities are tied together by a single, beautifully simple rule:

V = I × R — voltage equals current multiplied by resistance.

Rearranged, it tells you what resistor to use: R = V ÷ I. Suppose you have 5 volts, an LED that drops about 2 volts across itself, and you want 20 mA (0.02 A) of current. The resistor must handle 5 − 2 = 3 volts, so R = 3 ÷ 0.02 = 150 ohms. In practice everyone reaches for a 220Ω resistor because it is close, safe, and in every kit.

Staying safe

Most logic circuits here use low voltage, but batteries, short circuits, hot parts and moving mechanisms can still cause harm. Ask a trusted adult before powering a real build, always use a resistor with an LED, never connect 5V directly to ground, and unplug power before rewiring. Never experiment with wall-socket electricity.