# Ohm's Law

### From AstroBaki

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### [edit] Reference Material

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

## Contents |

## Ohm’s Law

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

## Resistor

*A typical (330Ω) resistor*

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. If we imagine electric current flowing as water, a resistor would be a narrow pipe. The higher the resistance, the narrower the pipe, and the harder you will have to push to get a liter-per-second of water through it. As per all electronic components, resistors dissipate energy as heat according to the equation:

### Resistors in Series

Resistors in series add because, in the pipe analogy used above, all the water has to go through all of the pipes, and they all contribute drag:

*Resistors in series*

### Resistors in Parallel

Resistors in parallel add reciprocally. In the pipe analogy, water has a choice of which pipe to flow through, and the bulk of the water will be carried by the widest pipe (or for electrons, the lowest-value resistor). Having more paths to choose from will always always reduce drag, but a thin straw next to a firehose isn’t going to do much:

*Resistors in parallel*

### Reading Resistor Values

*Color band locations on resistors*

Resistor values are often encoded on the component using colors. For determining the value of a resistor in Ohms, place the component with the triplet of color bands on the left side, and then read from left to right. For the resistor above, we have red-violet-green.

*Matching colors to values*

Match each color with a digit using the chart above (I remember it as "black, brown, ROYGBIV, grey, white", where ROYGBIV is, of course, rainbow ordering). The first two color bands are a two-digit number (e.g. 27 for red-violet above), and the third number is a power-of-ten multiplier (e.g. 10^{5} for green above). Hence the resistor red-violet-green resistor above is a resistor.