Simple circuits

Current needs a path

An electric circuit is a complete path through which electric charge can flow. A simple circuit might include a battery, wires, a switch, and a light bulb. When the path is complete, charges move through the circuit and the bulb lights. When the path is broken, current stops.

Electric current is the rate at which charge flows. It is measured in amperes, or amps. The symbol for current is usually $I$. A larger current means more charge passes a point each second.

Voltage as energy per charge

Voltage is often described as electrical pressure, but a more physical description is energy per unit charge. A battery provides a voltage by using chemical energy to move charges into a higher-energy arrangement. When charges move through a bulb or resistor, electrical energy is transformed into light, thermal energy, or other forms.

Voltage is measured in volts. A higher voltage source can provide more energy per unit charge. This can produce a larger current if the circuit's resistance stays the same.

Resistance

Resistance describes how strongly a material or device opposes electric current. A thin wire has more resistance than a thick wire of the same material and length. A long wire has more resistance than a short one. Materials also differ: copper has low resistance, while rubber has extremely high resistance.

Ohm's law relates voltage, current, and resistance:

$V = IR$

Here $V$ is voltage, $I$ is current, and $R$ is resistance. If resistance is constant, increasing voltage increases current. Increasing resistance decreases current for the same voltage.

Energy in circuit elements

A light bulb glows because current passes through a filament or electronic component that transforms electrical energy into light and thermal energy. A resistor warms because moving charges interact with atoms in the material, transferring energy to microscopic motion.

Electric power is the rate of energy transfer:

$P = IV$

A device with greater power transforms energy more quickly. Power is measured in watts.

Series circuits

In a series circuit, components are connected one after another along a single path. The same current flows through each component. If one bulb burns out or one switch opens, the path is broken and all current stops.

Series circuits are simple, but adding more bulbs increases total resistance and usually makes each bulb dimmer if the same battery is used.

Parallel circuits

In a parallel circuit, components are connected on separate branches. Each branch has its own path. If one bulb goes out, current can still flow through the other branches. Household wiring is mostly parallel because devices need to operate independently.

In parallel, each branch receives the same voltage from the source, though the total current supplied by the battery or outlet depends on how many branches are connected and their resistances.

The big idea

A circuit requires a complete conducting path. Current is flowing charge, voltage is energy per charge, and resistance opposes flow. Ohm's law connects these quantities. Series circuits provide one path; parallel circuits provide multiple paths. Together, these ideas explain flashlights, appliances, electronics, and electrical safety.