An electrical circuit is a connected system of electrical components and conducting paths that provides a route through which electric charge can move and electrical energy can be transferred. A basic circuit may contain a source such as a battery, conducting wires, a load such as a lamp or resistor, and a switch. For sustained current through a simple DC circuit, there must generally be a continuous conducting path between the source terminals and the load.
Opening a switch interrupts the path and prevents ordinary continuous current from flowing through the intended circuit. A circuit is more than a collection of components because the connectivity between those components determines how voltage and current are distributed. Circuit analysis uses mathematical models to represent these relationships and predict behaviour. Circuits can be simple or extremely complex, ranging from a battery connected to a lamp to integrated circuits containing billions of semiconductor devices.
The fundamental elements of a circuit perform different functions. A source supplies electrical energy by establishing an electromotive force or potential difference. Loads absorb electrical power and convert it into other forms, such as heat in a resistor, light in a lamp or mechanical motion in a motor. Conductors provide pathways for charge movement and electrical interconnection. Control components such as switches, relays and transistors determine when and how current flows.
Passive components include resistors, capacitors and inductors, while active components include devices such as transistors, operational amplifiers and integrated circuits that can control or amplify electrical signals when appropriately powered. Sensors can convert physical quantities into electrical signals, while actuators can convert electrical energy into mechanical or other physical effects. A practical circuit may combine all these functions into a coordinated system.
Circuits can be described using series, parallel and more complex network arrangements. In a series circuit, components are connected along a single path, so the same current flows through ideal series elements while the supply voltage is divided among them according to their characteristics. In a parallel circuit, branches share common nodes and therefore have the same voltage across ideal parallel branches, while current divides between the branches.
Real circuits often contain combinations of series and parallel sections, forming networks that require systematic analysis. Kirchhoff's current law states that the algebraic sum of currents entering and leaving a node is zero, reflecting conservation of electric charge in the lumped circuit model. Kirchhoff's voltage law states that the algebraic sum of voltage changes around a closed loop is zero, reflecting conservation of energy under the relevant circuit assumptions.
Circuit behaviour is determined by the relationships between voltage, current, resistance, capacitance, inductance and other component properties. Ohm's law describes resistive elements that behave approximately linearly under specified conditions. Capacitors store energy in electric fields and have current relationships that depend on changing voltage. Inductors store energy in magnetic fields and oppose changes in current through their induced voltage. Together, resistors, capacitors and inductors can form filters, oscillators, timing networks and transient response systems. Semiconductor devices introduce nonlinear switching, amplification and control behaviour. Digital circuits use voltage levels to represent logical states, while analogue circuits process continuously varying electrical signals. Modern circuits can therefore represent information, perform calculations, regulate power, communicate data and control physical systems.
Circuits can operate using direct current or alternating current. In a DC circuit, voltage polarity and current direction are generally fixed or vary relatively slowly according to the system. Batteries and many electronic power supplies provide DC. AC circuits involve quantities that periodically vary with time and may include phase differences between voltage and current. Mains electrical systems are examples of AC networks. Frequency becomes an important variable in AC circuits because capacitors and inductors have frequency dependent impedances.
A circuit that behaves one way at low frequency can behave very differently at high frequency. Transmission lines and microwave circuits require further consideration of distributed electrical effects because the physical dimensions of the system become significant compared with the wavelength of the signal. Circuit theory therefore includes both low frequency lumped models and more advanced distributed electromagnetic models.
A complete practical circuit includes physical interconnections as well as electrical components. Wires have resistance and inductance, connectors have contact resistance, batteries have internal impedance, and components have tolerances and parasitic properties. At high frequencies, even a short piece of conductor can behave as a significant inductive or transmission line element. Printed circuit board layout can influence signal integrity, electromagnetic interference, grounding and power distribution.
Circuit designers therefore consider not only schematic connectivity but also physical placement, trace geometry, return paths, shielding, thermal conditions and electromagnetic compatibility. Ground is often used as a reference potential in circuit diagrams, but it does not automatically mean that every ground point is physically identical or at exactly the same potential. Good circuit design requires understanding both ideal circuit models and the limitations introduced by real physical systems.
Circuit protection is necessary because abnormal conditions can produce excessive current, excessive voltage or unwanted energy transfer. Fuses and circuit breakers can interrupt current when it exceeds specified limits. Current limiting circuits can protect sensitive electronics, while surge protection can reduce damage from transient overvoltages. Earthing and bonding arrangements provide defined paths for fault currents and help maintain safe accessible voltages in appropriate electrical installations.
A short circuit can create a very low resistance path and potentially produce extremely high current. An open circuit creates a break that prevents ordinary continuous current through that path. Fault finding therefore often involves determining whether a circuit is open, shorted, incorrectly connected or operating outside its expected electrical parameters.
Circuits are fundamental to nearly every modern electrical and electronic technology. Domestic lighting, computers, telephones, industrial machinery, vehicles, medical equipment, telecommunications systems, power grids and spacecraft all depend on circuits. Integrated circuits miniaturise complex networks of semiconductor devices onto chips, allowing processors, memory, sensors and communication systems to perform sophisticated functions. Circuit analysis provides a framework for predicting voltages, currents, power, signal behaviour and transient response before hardware is built.
Simulation tools can model circuits numerically, while oscilloscopes, multimeters, logic analysers and other instruments allow physical circuits to be tested. The concept of a circuit therefore links fundamental electromagnetism with practical engineering by providing a structured representation of how electrical energy, charge and information move through interconnected components.
