# Circuit

## What is a Circuit?

### Definition

A circuit is a closed loop through which current can travel. It is made up of individual components such as resistors, capacitors, transistors, and inductors. Components can be assembled on a breadboard, or entire integrated circuits (ICs) can be manufactured on a printed circuit board (PCB).

### Symbols for common circuit elements

### What are the types of circuits?

Circuits can be divided into categories of series vs. parallel, closed vs. open or short, and DC vs. AC.

**Series circuits and parallel circuits**

Series circuits have equal current flowing through all components in the circuit. Adding resistors in series increases the total equivalent resistance following the equation R_{eq}=R_{1}+R_{2}+...+R_{n}. The supply voltage is equal to the sum of all individual voltage drops.

Parallel circuits have equal voltage across all components in the circuit. Adding resistors decreases the total equivalent resistance, following the equation 1/R_{eq}=1/R_{1}+1/R_{2}+...+1/R_{n}. The total current is equal to the sum of individual branches of current.

**Closed circuits, open circuits, and short circuits**

A closed circuit has a complete path between the positive and negative terminals of its power source. An open circuit has an incomplete path due to an open switch, burned out or broken component, a frayed wire, etc. A short circuit contains a pathway of (near) zero resistance, called a short, usually due to unintended contact between components. All current will flow through the short, causing the circuit to malfunction.

**DC circuits and AC circuits**

DC (direct current) circuits have current only flowing in one direction. AC (alternating current) circuits have a power source that periodically switches the direction of the current.

### What is in a simple circuit?

A simple circuit has just the most basic components for a working circuit: a source of voltage, a path for current to travel, and a resistor. These cover the three values necessary to satisfy Ohm’s Law, V=IR (voltage=current x resistance).