Three ideas that run the whole show
A circuit is just a closed loop that lets electric charge flow continuously. Understanding one is mostly a matter of keeping track of three related quantities:
- Current (I) — how much charge flows past a point per second, measured in amps
- Voltage (V) — the “push” driving that charge around the circuit, measured in volts
- Resistance (R) — how much a component resists that flow, measured in ohms
A useful (if imperfect) analogy: think of voltage like water pressure, current like the rate water flows through a pipe, and resistance like how narrow the pipe is. More pressure pushes more flow; a narrower pipe resists flow more.
Ohm’s law
These three quantities are tied together by one of the most-used equations in all of physics:
V = IR
If you know any two of voltage, current, and resistance, you can always solve for the third. This single relationship is behind the design of nearly every circuit, from a simple flashlight to a smartphone’s internal wiring.
Series vs. parallel
Components (like resistors or light bulbs) can be wired two basic ways:
Series — components are connected one after another, so the same current flows through all of them, but the total voltage splits between them. If one component fails (breaks the connection), the whole circuit stops working — like old-style string lights where one dead bulb kills the whole strand.
Parallel — components are connected across the same two points, so they each get the full voltage, but the current splits between them. If one path breaks, current can still flow through the others — this is how the wiring in your house works, so one broken lamp doesn’t cut power to the whole room.
Why this matters
Every electronic device you own is, at its core, a carefully designed circuit built from exactly these principles. Understanding current, voltage, and resistance is the first real step from “physics as abstract equations” to “physics as the reason your phone charger actually works.”