The Basics of Electric Circuits

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The definitions of current, voltage, and resistance
Current is the amount of electric charge flowing along a wire, measured in amperes (A); voltage is the force pushing that charge along (the potential difference), measured in volts (V); and resistance is the degree to which something impedes the flow of current, measured in ohms (Ω). It's easier to understand if you compare voltage to a difference in water height, current to the amount of water flowing, and resistance to how narrow a pipe is.
Ohm's law (V=IR)
Ohm's law is an equation expressing the relationship between voltage (V), current (I), and resistance (R), expressed as V=IR (voltage = current × resistance). Using this equation, you can find the current when you know the voltage and resistance (I=V/R), or conversely calculate the resistance you need — this is the basic calculation used to design and analyze circuits.
The difference between series and parallel circuits
A series circuit has only one path for current to flow, so the same current flows through every component, but the total resistance grows as the sum of each individual resistance, and if one component breaks, the entire circuit stops working. In a parallel circuit, current splits and flows through multiple branches, the same voltage is applied across each component, and if one component fails, current can keep flowing through the other paths — this is why most household outlets are wired in parallel.
Why a closed circuit is necessary
For electricity to flow, there must be an unbroken loop — a closed circuit — starting from the power source's (+) terminal (such as a battery), passing through the wires and electrical components, and returning to the (−) terminal. If a switch is turned off or a wire is broken, the circuit becomes an open circuit, current cannot flow, and a light bulb, for example, turns off.
The roles of fuses and circuit breakers
A fuse is a safety device that prevents fires by cutting off a circuit when excessive current beyond the rated amount flows through it — its thin internal metal wire melts and breaks from the heat on its own. A circuit breaker, by contrast, detects overcurrent and automatically flips an internal switch to cut the circuit; unlike a fuse, it can be reused simply by flipping it back on by hand once the underlying issue has been resolved.
Calculating power (P=VI)
The amount of electrical energy an electric device uses per unit of time is called power, measured in watts (W), and power (P) is calculated as the product of voltage (V) and current (I), or P=VI. For example, a microwave drawing 1A of current from a 220V outlet uses 220W of power, and the larger this value, the greater the power consumption that affects your electricity bill.

Why is it good to understand electric circuits?

From a smartphone charger to the outlets in your home, every electronic device around us operates based on an electric circuit. Understanding the basic relationship between current, voltage, and resistance, and how circuits are configured, lets you understand in principle why a light bulb won't turn on, or why it's dangerous to plug too many devices into a power strip.

Electrical energy also ultimately follows the law of conservation of energy

The power consumed in an electric circuit is a process of electrical energy converting into other forms, such as heat or light. If you're curious about the principle behind how energy is conserved as it changes form, see our guide to understanding the law of conservation of energy.

Frequently Asked Questions

Is household electrical wiring series or parallel?

Mostly parallel. Wiring in parallel means the same voltage (220V) is applied to every outlet and light fixture, and if one device breaks, the others can keep working normally.

What happens to current as resistance increases?

According to Ohm's law (I=V/R), when voltage is held constant, the amount of current flowing decreases as resistance increases. This is because resistance represents the degree to which something impedes the flow of current.