Ohm's Law: V = IR
Ohm's Law, formulated by German physicist Georg Simon Ohm in 1827, describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit: V = I × R.
Voltage (V, measured in Volts): electrical potential difference — analogous to water pressure in a pipe. Current (I, measured in Amperes): flow of electric charge — analogous to the volume of water flowing. Resistance (R, measured in Ohms, Ω): opposition to current flow — analogous to pipe diameter.
From V = IR, the other forms follow: I = V/R (current equals voltage divided by resistance) and R = V/I (resistance equals voltage divided by current). For a 12V battery connected to a 4Ω resistor: I = 12/4 = 3 amperes of current.