# Formulas

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Summary of basic formulas

A formula is a concise way of expressing information symbolically (as in a mathematical or chemical formula), or a general relationship between quantities. Some basic formulas below:

## Ohm's Law Formulas for D-C Circuits

Ohm's law is one of three fundamental laws which begin the study of electronics.

For Voltage (E)
• $E = I * R$
For Current(I)
• $I = E / R$
For Resistance (R)
• $R = E * I$

## Resistors in Series

If R1 measures 10 ohms, R2 measures 20 ohms, and R3 measures 30 ohms to solve for total resistance you would sum (add) together all three resistances. For example;

• $Rtotal = R1 + R2 + R3 + ....$

so, if we insert the values measure for R1 (10), R2 (20), and R3 (30) as Rtotal = 10 + 20 + 30 then...

## Resistors in Parallel

If R1 measures 10 ohms, R2 measures 20 ohms, and R3 measures 30 ohms to solve for total resistance you perform division to all three resistances as shown in this formula. For example;

• $Rtotal = 1 / 1/R1 + 1/R2 + 1/R3 + ...$

so, if we insert the values measure for R1 (10), R2 (20), and R3 (30) as steps…

Step one: Rtotal = 1/10 + 1/20 + 1/30

Step two: 1/10 equals .10 and 1/20 equals .05 and 1/30 equals .033333 (repeating decimal). So, if using a calculator first enter “1” next, enter “/” or division symbol, and lastly enter “10” that should give you a decimal answer of .10.

Step three: Add up all values .10 + .05 + .033333 equals .1833333

Step four: Divide 1 / .183333 then …

The answer is 5.455 (rounded is 5.46) ohms.

## Two Resistors in Parallel

• $Rtotal = (R1 * R2) / (R1 + R2)$

## Sinusoidal Voltages and Currents

• $Effective value = .707 * peak$
Effective value is also known as Root-Mean Square (RMS) value
• $Average value = .637 * peak$
• $Peak value = 1.414 * Effective value$
• $Effective value = 1.11 * Average value$

## Voltage/Current Phase Rule of Thumb

'Remember "ELI the ICE man"

• ELI: Voltage (E) comes before (leads) current (I) in an inductor (L)
• ICE: Current (I) comes before (leads) Voltage (E) in a capacitor (C)

## Conductance

• $G = 1 / R$

## Reactance

• Capacitive Reactance $XC = 1 / 2 PIfC$
• Inductive Reactance $XL = 2 PIfL$

## Common Laws

The following are common engineering laws:

## Reference

1. sengpielaudio.com. "Formulas". http://www.sengpielaudio.com/FormulaWheel-ElectricalEngineering.htm