Ohm's Law

Imagine the stream of water that gushes out when you turn on a faucet. The harder the pressure pushes the water, the thicker and stronger the stream gets. But if the pipe is clogged with sand or is much too narrow, the water can only trickle through.

The same thing happens in an electric circuit. The force a battery uses to push electricity into the circuit is called voltage (V). The flow of electricity that this force actually drives along the wire is called current (I). Just as higher water pressure sends out more water, higher voltage makes more current flow through the wire.

On the other hand, the property that blocks and hinders the path of electricity is called electrical resistance (R). Like grains of sand in a pipe, it acts as an obstacle to the movement of electrons. In the end, the current in a circuit gets stronger as the "pushing force," voltage, gets bigger, and weaker as the "obstacle," resistance, gets bigger.

Ohm's law neatly turns the proportional relationship among voltage, current, and resistance into a formula. Voltage (V) equals current (I) times resistance (R), written V = I × R.

The best part of this formula is that if you know any two of the three, you can work out the third. For example, connect a light bulb with a resistance of 3 ohms (Ω) to a 9-volt (V) battery, and the calculation easily shows that a current of 3 amperes (A) will flow in the circuit.

In reverse, if you measure the voltage across a part and the current through it, you can work backward to find its resistance, meaning how much it blocks electricity. When you want to prevent fires caused by too much current in a wire, or choose the right parts so electronics don't break, this is the calculation that is the starting point of all electrical engineering.

Ohm's law is not an absolute law that holds perfectly for every material in the universe. In ordinary metal wires like copper or aluminum, doubling the voltage doubles the current exactly, a wonderfully tidy rule. Materials with this property are called "ohmic materials."

But semiconductors such as diodes and transistors, the core parts of smartphones and computers, behave very differently. When you apply only a little voltage, almost no current flows. Then, the moment the voltage passes a certain threshold, the current shoots up explosively. Things whose voltage and current don't trace a neat straight proportional line are called non-ohmic devices.

Even in a metal wire, things change once current flows and heat is produced. The atoms inside the wire absorb the heat and vibrate violently, blocking the electrons' path even more. That's why Ohm's law holds exactly only when the temperature stays constant.