Introduction: What Makes Circuits Work?
Every time you charge your phone, switch on a light, or use any electrical device, you're relying on the same three fundamental ideas: electric current, voltage (potential difference), and resistance. Together, these concepts , connected through Ohm's Law , explain how electrical energy moves through circuits and powers our world.
In this subtopic, we'll build up from the definition of current all the way to calculating power and analysing circuits with multiple resistors, including the behaviour of real batteries with internal resistance.
Think of an electric circuit as a water pipe system. Current is the flow of water, voltage is the pressure pushing the water along, and resistance is the narrowness of the pipe that restricts how much water can flow. A higher pressure (voltage) drives more flow (current), but a narrower pipe (higher resistance) limits it.
Note: This analogy is a useful starting point, but it has limits , it does not capture concepts like energy per unit charge (EMF) or the behaviour of semiconductor devices. Use it to build initial intuition, then rely on the formal definitions as you progress.
Electric Current: The Flow of Charge
Electric Current: The rate of flow of electric charge past a given point in a circuit. It is measured in amperes (A).
In a metal conductor, free electrons are the charge carriers. When no voltage is applied, these electrons move randomly in all directions , there is no net flow. When a potential difference is applied across the conductor, an electric field is established within it, exerting a force on the free electrons and pushing them in a consistent direction. This produces a net flow of charge , the electric current.
The mathematical definition of current is:
Where:
- = current (amperes, A)
- = charge transferred (coulombs, C)
- = time interval (seconds, s)
One ampere is defined as one coulomb of charge passing a point per second: .
Conventional current flows from the positive terminal to the negative terminal of a battery (i.e., in the opposite direction to electron flow). This historical convention is used in circuit analysis , don't let it trip you up!
Worked Example: A current of 2.0 A flows through a wire. How much charge passes through the wire in 10 s?
Answer: 20 coulombs of charge pass through the wire.
To measure current in a circuit, always connect an ammeter in series with the component. An ideal ammeter has zero resistance so it doesn't affect the circuit.