Preview
IB DP Physics — Unit B: The Particulate Nature of Matter | SL & HL | B.5 Current and Circuits
📋 What You Will Learn
- Observe and describe the flow of charge in a conductor using a microscopic model of electron drift
- Define electric current, potential difference, and resistance with precise IB-aligned definitions
- Derive Ohm's Law and the relationship between resistance, resistivity, length, and cross-sectional area
- Apply \( V = IR \) and \( R = \frac{\rho L}{A} \) to solve figure-based and graph-based problems
- Analyse current–voltage (\( I \)–\( V \)) characteristic graphs for ohmic and non-ohmic conductors
- Evaluate common misconceptions about current, voltage, and resistance in circuit contexts
🌍 Real-World Context Every time you charge your phone, stream music through a speaker, or switch on a light, billions of tiny electrons are being pushed through metal wires in a coordinated flow. The brightness of a bulb, the heat produced by a toaster, and the speed at which a circuit responds all depend on three fundamental quantities: how much charge flows per second, how much energy each unit of charge carries, and how much the material resists that flow. Understanding these three quantities — current, potential difference, and resistance — is the foundation of all electrical circuit analysis.
🔍 Guiding Question How do the microscopic properties of a conductor — the density of free electrons and the structure of the material — determine the macroscopic electrical behaviour we observe in circuits?
Physical Setup: Charge Flow in a Conductor
Imagine a metal wire connected between the two terminals of a battery. Inside the metal, there are vast numbers of electrons that are not bound to any particular atom — these are called free electrons or conduction electrons. Before the battery is connected, these electrons move randomly in all directions, with no net flow in any one direction. The moment the battery is connected, it creates an electric field inside the wire, and this field exerts a force on every free electron, nudging them all in the same direction.
The result is a slow, net drift of electrons along the wire — superimposed on top of their rapid random thermal motion. This net drift is what we call electric current. It is important to appreciate that the electrons do not travel quickly from one end of the wire to the other; instead, they drift very slowly (typically a fraction of a millimetre per second), but because there are so many of them, the total charge flowing per second can be enormous.
Notice in the diagram that conventional current is defined as flowing from the positive terminal to the negative terminal — opposite to the actual direction of electron flow. This historical convention was established before the discovery of the electron and is retained throughout physics and engineering. When we say "current flows through a resistor from left to right," we mean positive charge would flow in that direction, even though it is actually electrons moving right to left.
Core Concepts and Definitions
To quantify the flow of charge, we need to count how much charge passes a given point in the wire per unit time. Think of it like measuring the flow rate of water in a pipe — instead of litres per second, we measure coulombs per second. The more