ETA CETa Exam Common Formulas
Potential Divider
As the name says, we divide the potential or reduce the volt-
age in a circuit with help of potential divider.
Vout = Vin * R2 / (R1 + R2)
Current Divider
It is used to redirect current fl owing in a circuit.
Iout = Iin * R1 / (R1 + R2)
Balanced Wheatstone Bridge
A bridge used to measure resistances.
(R1 / R2) = (R3 / R4)
Voltage gain in decibels
Gain dB = 20 log (Vout / Vin)
Ratio of 2 power levels in decibels
Gain dB = 20 log (Vout / Vin)
Resonant frequency
FR = .159 / LC
P = I * E, the power being dissipated by the resistor is a prod-
uct of the current and the applied voltage.
Resistors in series
R = R1 + R2 + R3...
Resistors in parallel
1 / R = (1 / R1) + (1 / R2) + (1 / R3)...
The resistance of a conductor at a temperature, t, is given
by the equation: Rt = R0(1 + αt + b t2 + y t3) where α, b, y
are constants and R0 is the resistance at 0°C. Note that b & y
are very small hence they can be neglected.
Therefore above equation simplifi es to: Rt = R0(1 + αt) where
α = temperature coeffi cient of resistance.
Inductors connected in series
L = L1 + L2 + L3 +...
Inductors connected in parallel
1 / L = (1 / L1) + (1 / L2)...
Reactance of inductors
XL = 2 * * f * L
where XL is reactance, f is frequency, and L is inductance
I2 x R
E x I
I x R
P x R
P/R
P
I
E
I
E2
P
E2
R
P
I2
P
E
E
R
Ohm’s Law
R
I
E
P
I = Current
P = Power R = Resistance
E = Voltage
Current fl owing in a Capacitor
The current fl owing in a capacitor is proportional to the prod-
uct of the capacitance, C, and the rate of change of applied
voltage.
i = C × (rate of change of voltage[d * V / d * t])
How to Compute Charge or Quantity of Electricity
Q = C * V
where Q is the charge (in coulombs), C is the capacitance (in
farads), and V is the potential difference (in volts).
Energy Storage in a Capacitor
W = ½ C * V2
where W is the energy (in Joules), C is the capacitance (in
Farads), and V is the potential difference (in Volts).
Capacitors connected in parallel
C = C1 + C2 + C3 +...
Capacitors connected in series
1 / C = (1 / C1) + (1 / C2)...
Reactance of capacitors
XC = 1 / (2 * * f * C)
