Electrical components generally operate through the flow of electric current from higher voltage to lower voltage. In the case of an Arduino, this current generally flows from 5V to GND (0V). The rate at which current flows is controlled by the resistance of the branch of the circuit (more on this later). You may have heard that “electricity follows the path of least resistance”. This is somewhat true while at the same time being wrong. Electrical current will flow along all paths from high voltage to GND. However, less current will flow along a high resistance path than a low resistance path. Therefore if the difference in resistance is high enough, then almost no current will flow along the difficult path and electricity will appear to follow the path of least resistance.
Using sensors, switches, transistors or relays to control the flow of current is essential to understanding electrical control systems.
Resistance, Voltage and Current
In simple direct current circuits (like these with the Arduino) the amount of current (I measured in amps (A)) that can flow through a component is calculated using Ohm’s law by dividing the voltage across the component (V measured in volts (V)) by the component’s resistance (R measured in ohms (Ω)).
Technically the resistance of a circuit or component is defined to be the ratio of the voltage applied across a it and the current flowing through it.
For example the current flowing through a 220Ω resistor with 5V across it is calculated as:
Prefixes such as m, k or M appear everywhere in electronics because they make things easier to write down. There are a few prefixes that are handy to try to remember.
Prefix Symbol Prefix Name (pronunciation) Means Press this on your calculator G Giga (gɪgə) × 1 000 000 000 (1 billion) [×10ⁿ] [9] t9 M Mega (mɛgə) × 1 000 000 (1 million) [×10ⁿ] [6] k kilo (kɪlə or kiːloʊ) × 1 000 (1 thousand) [×10ⁿ] [3] – – × 1 m milli (mɪlliː) × 0.001 (1 thousandth) [×10ⁿ] [-3] μ micro (maɪkroʊ) × 0.000 001 (1 millionth) [×10ⁿ] [-6] n nano (nænoʊ × 0.000 000 001 (1 billionth) [×10ⁿ] [-9]
And the current flowing through a 10,000Ω (10kΩ) resistor with the same 5V across it is calculated as:
Your turn
If a 2,200Ω (also called 2.2kΩ or simply 2k2) resistor has 3V across it, what will be the current through it?