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Ohms Law

V = Volts
R = Resistance (ohms, )
I = Current ( Amps)

Formular Example
V = R * I 5 * 2A = 10Vs
I = V / R 10V / 5 = 2A
R = V / I 10V / 2A = 5

Watts

W = Watts
V = Volts
I = Current ( Amps)

Formular Example
W = V * I 10V * 2A = 20W
I = W / V 20W / 10V = 2A
V = W / I 20W / 2A = 10V
W = R * I2 5 * 22 = 20W
W = V2 / R 10V2 / 5 = 20W
R = W / I2 20W / 2A2 = 5
R = V2 / W 10V2 / 20W = 5

 




AC Theory

RMS 0.707 Peak voltage

Impedance

AC current in a capacitive or inductive component. Impedance is the resistance ( measured in ohms ) of a reactive component ( capacitor, inductor ) at a certain AC frequency.

Capacitor Inductor
Xc = 1 / ( 2 * Pi * f * C ) Xl = 2 * Pi * f * L
where Pi = 3.14 where Pi = 3/14
f = frequency in Hz f = frequency in Hz
C = capacitance in Farads L = Inductance in Henry
eg A 100uF cap at 50Hz eg A 20mH inductor at 50Hz
Xc = 1/(2*3.15*50Hx*0.0001F) Xl=2*3.14*50Hx*0.02
Xc = 31.8 ohms Xl = 6.28 ohms

Multipliers.

Pica 0.000,000,000,001 1^-12
Nano 0.000,000,001 1^-9
Micro 0.000,001 1^-6
Milli 0.001 1^-3
Centi 0.01 1^-2
Unit 1 1
Kilo 1,000 1^3
Mega 1,000,000 1^6
Giga 1,000,000,000 1^9
Tera 1,000,000,000,000 1^12

The Metric system.

All measurements are based on a unit, and a multiplier, like those above. ie for distance we use a measure called the meter, and thats the only unit of distance we need to remember. We also use a multiplier, so we call 1000 meters a KiloMeter, or 1/1000 of a meter is a MilliMeter. Easy.

Another easy one to remember is Centigrade as a measurement of temperature. Water freezes at 0 degrees, and boils at 100 degrees. Body temperature is about 32 degrees C.


Some common conversions

To Convert To Mulitply by
Inches MilliMeters 0.03937
MilliMeters Inches 25.4
Feet Meters 3.281
Meters Feet 0.3048
Miles KiloMeters 1.609
KiloMeters Miles 0.6214

For more conversions, visit http://www.simetric.co.uk/si_c.htm


Common copper wire sizes and resistance

AWG Max I * Inch Dia mm Dia per 1000' ** per km **
22 0.918 0.025 0.644 16.46 54
20 1.46 0.032 0.812 10.35 33.95
18 2.32 0.04 1.024 6.51 21.36
16 3.69 0.05 1.291 4.09 13.42
14 5.87 0.064 1.628 2.57 8.43
12 9.33 0.08 2.053 1.62 5.31
10 14.8 0.101 2.588 1.02 3.34
8 23.6 0.128 3.264 0.64 2.1
6 37.5 0.162 4.115 0.4 1.31
4 59.6 0.204 5.189 0.25 0.83
2 94.8 0.257 6.544 0.16 0.52
1 119 0.289 7.348 0.12 0.39
* max safe current in a tightly wound coil
** at 25 degrees C
Source: The Radio Amatures Handbook 1978

Resistance of other materials compared to copper
where copper = 1

Aluminium :
1.6
Brass :
3.7 - 4.9
Gold :
1.4
Silver :
0.94
Iron :
5.68
Lead :
12.8
Steel :
7.6 - 12.7
Tin :
6.7
Zinc :
3.4
Nickel :
5.1
Source: The Radio Amatures Handbook 1978