# Understanding Circuits

## Highlights

•     A circuit is a closed loop through which electrons flow
•     A current is a measure of the flow rate of the electrons
•     Voltage is the electron potential difference or simply electron imbalance
•     Resistance is a measure of how difficult it is for electrons to flow through a circuit.
•     In a circuit, voltage is always equal to the product of current and resistance (V=IR)
•     Be careful to never connect the anode and the cathode (− and + terminals of a battery or other voltage source) without a resistor!
•     Building a circuit without proper knowledge and planning can lead to fire or electrocution!

## Notes

An electrical circuit is a setup in which charged particles flow through a closed loop. A power source (like a battery) produces charged particles (electrons), which are moved through a conductor (a wire), and the energy harvested from these charged particles is used to perform some task (like turning on a light bulb). The flow of charged particles is called a current. Current is measured in Amperes, or Amps.

Circuits can be very simple or very complex based on the effect desired, but are typically made up of the same basic set of electrical components, such as power sources, resistors, inductors, capacitors, and switches. The way these components are assembled determines the behavior of the circuit. The most basic circuit that can be created consists of a power source and resistor. This lesson will focus on understanding a basic circuit. The power source is responsible for initiating and maintaining the flow of electrons through the circuit. In general, there are two types of current, Alternating Current (AC) and Direct Current (DC). Alternating currents change the direction of the electron flow periodically with time, while direct currents maintain a constant electron flow direction.

This lesson focuses on batteries, which generatee a direct current. A battery works by creating an electric potential difference (called voltage). This difference causes electrons to be forced through the circuit.

The battery has an anode (−), where a chemical reaction causes a build-up of electrons. The electrons want to move to a region with fewer electrons, called the cathode (+). The anode and the cathode are separated by a solution called an electrolyte. The electrolyte prevents the electrons from moving directly from the anode to the cathode. Instead, these electrons can only move through the circuit, thus creating the flow of electrons that powers the circuit. When the chemical process is not able to create more electrons, which occurs when the number of electrons in the cathode and anode are balanced, the battery runs out of power.

Voltage measures the amount of work to move a charge between two points. The higher the voltage of the battery, the more work the battery is able to do, and more current can be pushed through the battery. Voltage is typically measured in Volts. For example, a standard AA battery is 1.5 Volts, while the standard power from a wall outlet in the US is 120V.

The second component in a basic circuit is a resistor. Resistors act to reduce the current flowing through a circuit. They do this by impeding the flow of electrons. Think of analogy to rolling a ball across a floor. On a hard, polished floor this is very easy. However, if the floor were covered in think mud, it would be much more difficult to roll the ball. Resistors provide a similar effect on the current. Resistance is amount of current flow reduction caused by the resistor, and is measured in Ohms. Resistors come in many sizes and types, and are often used to control the current through a circuit.

A light bulb is an example of a resistor. In a light bulb, the current is forced to flow through the filament, which is a wire that is so thin that it is hard for the electrons to force their way through it. As the electrons work to move through the filament, it heats up the filament so much that the filament actually glows, producing the light we see from a light bulb.

Ohm’s Law gives us a fundamental understanding of the relationship between the battery (voltage) and the resistor (resistance). Ohm’s Law given below states that the voltage must equal the current times the resistance. Therefore, the amount of current flowing through a circuit can be calculated by dividing the battery’s voltage by the resistor’s resistance. $V = I R$

For a simple circuit, this is very straightforward. If a circuit has a 12 V battery, and a 6 ohm resistor, then the current flowing the circuit can be calculated by dividing 12 by 6, giving 2 Amps. However, this becomes more complicated in a circuit with multiple batteries or multiple resistors. The next lesson, parallel and series circuits, will explain how to assemble and analyze more complex circuits.