Understanding Charge and Current in A-Level Physics

Charge and Current Basics

Charged particles are everywhere around us, and they're simpler than you might think. Protons carry positive charge whilst electrons carry negative charge - like opposite ends of a magnet, opposite charges attract and like charges repel each other.

The key thing to remember is that charge is measured in coulombs (C). A single electron or proton has a tiny charge of ±1.6 × 10⁻¹⁹ C, so you need about 6.25 × 10¹⁸ electrons to make up just one coulomb!

Free charges are particles that can move freely through a material. In metals like copper wire, these are called delocalised electrons - they're not stuck to any particular atom and can drift through the metal structure. This is what makes metals such good conductors compared to insulators, which have very few free charges.

Quick Tip: Remember that ions in solutions (like copper sulphate) are also free charges - they're not just limited to electrons in metals!

Movement and Drift of Charges

Even when a wire isn't connected to anything, electrons are still zipping about at incredible speeds due to thermal velocity - we're talking hundreds of thousands of metres per second! However, their average velocity is zero because they're moving randomly in all directions, so there's no net movement.

Everything changes when you connect a battery. The electrons still have that random thermal motion, but now they also drift slowly towards the positive terminal. This drift velocity is much slower than thermal velocity, but it's consistent and directional.

Current (I) is simply the amount of charge flowing past a point each second, measured in amperes (A). One ampere equals one coulomb per second, which you can calculate using: I = ΔQ/Δt (current = charge moved ÷ time taken).

The battery works by pulling electrons towards its positive terminal and pushing them away from the negative terminal. When the battery dies, it's because the chemical reactions inside have been used up - the electrons themselves don't disappear.

Remember: An ammeter measures current and must be connected in series with the component you're measuring!

Conventional Current

Here's something that might seem backwards at first - conventional current flows in the opposite direction to electron flow! This historical convention assumes current flows from positive to negative terminals, even though we now know electrons (which are negative) actually flow from negative to positive.

Think of conventional current as showing the direction positive charges would move if they were the charge carriers. It's like a agreed-upon standard that makes circuit diagrams and electrical calculations consistent worldwide.

Key Point: Conventional current direction always matches how positive charges would drift, not how electrons actually move!