Preview
IB DP Physics — Unit B: The Particulate Nature of Matter | SL & HL | B.5 Current and Circuits
📋 What You Will Learn
- Observe how current and voltage behave differently in series and parallel circuits using real-world examples
- Define resistance, electromotive force (emf), and internal resistance with precise IB notation
- Derive the rules for total resistance in series and parallel combinations from first principles
- Apply Kirchhoff's current and voltage laws to analyse multi-loop circuits
- Calculate electrical power dissipated in resistors and delivered by sources using multiple equivalent expressions
- Analyse V–I graphs and circuit diagrams to determine unknown quantities and evaluate circuit behaviour
🌍 Real-World Context Every home is wired so that lights, appliances, and sockets can be switched on and off independently — turning off the kitchen light does not extinguish every bulb in the house. Yet a string of old-fashioned Christmas tree lights, wired in a different configuration, fails completely when a single bulb blows. These two behaviours arise from a fundamental difference in how components are connected: in series or in parallel. Understanding this distinction is not merely academic — it determines how power grids are designed, how fuses protect circuits, and why your phone charger gets warm while charging.
🔍 Guiding Question How does the arrangement of resistors in a circuit determine the distribution of current, voltage, and power — and what role does the internal resistance of a source play in limiting the energy delivered to an external load?
Physical Setup: What Is a Circuit?
An electric circuit is a closed conducting path through which charge can flow continuously. At its simplest, a circuit consists of a source of electromotive force (a battery or power supply), connecting wires, and one or more components such as resistors, bulbs, or motors. The source drives charge around the loop; the components resist that flow and convert electrical energy into other forms.
The two fundamental ways to connect components are in series — one after another along a single path — and in parallel — side by side along separate branches that share the same two endpoints. These two arrangements produce strikingly different distributions of current and voltage, which we will now explore systematically.
Examining the series diagram, notice that there is only one path for current — every charge carrier must pass through every resistor in turn. In the parallel diagram, the current has a choice of routes; charge can travel through any branch independently. This single structural difference is the origin of all the rules we will derive in this topic.
Core Concepts and Definitions
Before we can analyse circuits quantitatively, we need to establish precise definitions for the key quantities. These definitions are not arbitrary — each one captures a measurable physical reality that can be probed with instruments and tested in the laboratory.
📖 Key Definition — Electric Current Electric current \( I \) is the rate of flow of electric charge past a cross-section of a conductor. It is defined as \( I = \frac{\Delta Q}{\Delta t} \), where \( \Delta Q \) is the charge flowing in time \( \Delta t \). The SI unit of current is the ampere (A), where 1 A = 1 C s⁻¹.
📖 Key Definition — Potential Difference (Voltage) The potential difference \( V \) between two points