# Important Concepts

We’re going to dive deeper into resistors and resistor networks later, but:

*Resistors*are electronic components made of materials that restrict the flow of electricity.- Resistance is measured in
*ohms*, often denoted as`Ω`

. *Ohm’s law*states that the resistance provides a proportional reduction of current flow and is codified as:`Voltage = Amperage * Resistance`

or`V = I*R`

`Amperage = Voltage / Resistance`

or`I = V/R`

`Resistance = Voltage / Amperage`

or`R = V/I`

- Where
`I`

is amps,`R`

is ohms, and`V`

is voltage.

- Resistors have a
*power rating*which specifies the amount of power (wattage) they can safely handle. - In addition to the standard definition of a watt (
`volts * amps`

), power can be calculated when only volts or amps and resistance is known:- When amperage (current) and resistance is known:
`P = I^2 * R`

- When voltage (force) and resistance is known:
`P = V^2 / R`

- When amperage (current) and resistance is known:
- Resistors wired in
*series*(end-to-end) have a total resistance which is the sum of the resistance of each resistor. *Conductance*is the reciprocal of resistance,`(1/R)`

, and is measured in siemens (S), but most often abbreviated as`G`

.- Resistors wired in
*parallel*have a total resistance which is the sum of the conductance (in siemens) of each resistor, and then converted back to resistance. - Axial resistors are color coded to aid in identifying them. See this chart for reference.
- Resistors have a tolerance which specifies a range that their actual resistance falls into.
- Resistors come in a set of values that covers the range of possible values, adjusted for tolerance, with only small overlap.
*Breadboards*reduce prototyping complexity by allowing you to create circuits without soldering.- A
*pull-up*resistor provides a default`HIGH`

value at an input in a circuit by connecting the input to a voltage source and allowing a small amount of current to flow. - A
*pull-down*resistor is the opposite of a pull-down, and provides a default value of`LOW`

at an input in a circuit. - Pull-up and pull-down resistors are one of the most common tools in circuit design.
- Pull-ups and pull-downs with smaller resistance values are known as
*strong*pull-ups and pull-downs, and act quicker than*weak*pull-ups and pull-downs, which have a larger resistance value.

## Part 5 - DC Circuit Analysis

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