Electric Charge and Current Basics

Think of electric charge like water in a pipe - it needs to flow through the right materials to be useful. Charge is measured in coulombs (C), and incredibly, you'd need about 6.3 × 10¹⁸ electrons just to make one coulomb of charge!

Not all materials let charge flow through them. Conductors (like metals) are brilliant at letting charge move freely, whilst insulators (like plastic) block it completely. This is why your phone charger has plastic coating around metal wires.

Current tells us how fast charge is flowing - it's like measuring how quickly water rushes through a pipe. The formula is dead simple: I = ΔQ/Δt (current equals change in charge divided by time). Current is measured in amperes (A).

Quick Check: If 10 coulombs of charge flow past a point in 2 seconds, the current is 5 amperes!

How Electrons Create Current

Here's where it gets fascinating - metals contain free delocalised electrons that can move around easily thanks to metallic bonding. When you connect a battery, these electrons drift from negative to positive, creating the current we use.

The drift velocity equation I = nAve shows how current depends on several factors. Here, n is charge carrier density, A is the wire's cross-sectional area, v is drift velocity, and e is electron charge.

Surprisingly, electrons actually move quite slowly through wires - their drift velocity is typically just millimetres per second! Yet electricity seems instant because the effect travels through the wire at nearly light speed, like dominoes falling.

The derivation is straightforward: current equals the number of electrons passing through per second, multiplied by each electron's charge. This gives us the incredibly useful relationship v = I/(nAe) for calculating drift velocity.

Real World: In a typical household wire carrying 1 amp, electrons drift at about 0.1 mm per second - slower than a snail!