E = I × R | ![]() |
I = E ÷ R | |
R = E ÷ I | |
Remember: | |
E = voltage, in Volts I = current, in Amperes R = resistance, in Ohms |
Rule: In a series circuit the total voltage drop equals the sum of the individual voltage drops.
Rule: There is only one path for current to flow in a series circuit, therefore the same current flows everywhere in the circuit and is always the same as the total current.
There are two formulas.
R1 × R2 RT = --------- R1 + R2
1 RT = ----------------------- ...... 1 1 1 ---- + ---- + ---- R1 R2 R3
Note: The total resistance in parallel is always less than the value of the smallest resistor.
ET = E1 = E2 .... = E N
Note: Voltage is common to all elements in a parallel circuit.
IT = I1 + I2 + ..... + IN
Note: the total current is the sum of all the individual currents flowing in all of the various branches of the parallel circuit.
P = E × I P = I2 × R E2 P = ---- R
Where:
P = power in Watts
E = voltage in Volts
I = current in Amperes
LT = L1 + L2 + .... + LN
Note: same formula as resistors in series
1 LT = ---------------------- 1 1 1 --- + --- + .... + --- L1 L2 LN
Note: same formula as resistors in parallel.
CT = C1 + C2 + .... + CN
Note: same formula as resistors in series.
1 CT = ---------------------- 1 1 1 --- + --- + .... + --- C1 C2 CN
Note: same formula as resistors in parallel.
XL = 2 × PI × f × L
Where:
X = inductive reactance in ohms
2 × PI = 6.28
f = frequency in hertz
L = inductance in henries
1 XC = ----------- 2 × PI × f × C
Where:
X = capacitive reactance in ohms
2 × PI = 6.28
f = frequency in hertz
C = capacitance in farads
1 fR = ----------------- 2 × PI × SQRT (L × C)
Where:
f = resonance frequency
2 PI = 6.28
L = inductance in henrys
C = capacitance in farads
SQRT is short for Square Root
c Wavelength = --- f Wavelength c = ----------- f
Where:
Wavelength is in metres
f = frequency in Hertz
c = speed of light (300 000 000 metres/second)
This can be simplified if we consider frequency in megahertz:
300 f = ----------- Wavelength 300 Wavelength = ----- f
Where:
Wavelength is in metres
f = frequency in Hertz
c = speed of light (300 000 000 metres/second)
Remember: this is for a full wavelength. Divide by two for a half wavelength.
1 frequency = ----- time 1 f = --- t 1 time = ---------- frequency 1 t = --- f
Where:
t is the time, called period in this case
f is the frequency
Example: what is the frequency of a waveform having a period of 0.001 seconds?
1 f = --- t 1 f = ------- 0.001 (sec) f = 1 000 (Hz)
Example: what is the period of a signal of 500 Hz?
1 t = --- f 1 t = ----- 500 (Hz) t = 0.002 (seconds)
(this is the time it takes for 1 cycle to occur.)