Ohm's Law

Short Topical Videos

• Ohm's Law (by MAKE magazine)
• The Resistor (by MAKE magazine)

Reference Material

• Horowitz & Hill, The Art of Electronics, 2nd Ed., Ch. 1

Ohm’s Law

$V=IR,\,\!$

where $V$ is voltage, measured in Volts ($V$), with typical values ranging from $mV$ (into an oscilloscope) to $kV$ (power lines, severe arcing danger); $I$ is current, measured in Amperes ($A$), typical values ranging from $mA$ (relatively safe for bench-top work) to $A$ (very dangerous); $R$ is resistance, measured in Ohms ($\Omega$), typical values ranging from $\Omega$ (power resistors dissipating a lot of power) to $M\Omega$ (almost a no-connect).

Resistor

Symbol for a resistor in schematics

A resistor resists the flow of electrons, such that a potential (i.e. voltage) is required to produce a current, as described by Ohm’s Law above. As per all electronic components, resistors dissipate energy as heat according to the equation:

$P=IV\,\!$

Resistors in Series

Resistors in series add:

Resistors in series

$R=R_{1}+R_{2}+\dots +R_{n}\,\!$

Resistors in Parallel

Resistors in parallel add reciprocally:

Resistors in parallel

$R={\frac {1}{{\frac {1}{R_{1}}}+{\frac {1}{R_{2}}}+\dots +{\frac {1}{R_{n}}}}}\,\!$

Reading Resistor Values

Color band locations on resistors

Matching colors to values