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Important formulas :-

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Some Important Formulas [Electrical]
Ohm's Law for D.C. Circuits
P = EI = I2R =  E2   watts
E = IR = PR P   volts
I =  E P P/R Ampere
R =  E  =  E2  =  P  =  Ohm's
I P I2

Power= EMF X Current
P = X

Resistors in series = R = R1 + R2 + R3 +... Rn
Resistors in parallel =  1 1 1 1 +...  1
R R1 R2 R3 Rn

Capacitors & Electrostatics
Capacitors in parallel = C = C1 + C2 + C3 +... Cn
Capacitors in series =  1 1 1 1 +...  1
C C1 C2 C3 Cn
Capacitance of capacitor C =  Q
Energy stored in capacitor W =  1 CV2
Coulomb's law F =  Q1 Q2
4πԐ0Ԑr r2
Electric field strength E =  Q
4πԐ0Ԑr r2
Electric flux density D =  Q
Electric potential at a point due to point charge q: V =  q
4πԐ0Ԑr r
Potential due to number of charges V =  1 [ q1 q2 +... ]
4πԐ0Ԑr r1 r2
Charging of capacitor i =  V e-t/RC
  q = Q(1 - e-t/RC
Discharging of capacitor i =  V e-t/RC
  q = Qe-t/RC 

Electromagnetic Induction, Magnetic circuit, Inductance
Faraday's law electromagnetic induction e = -N 
Dynamically induced emf = B l v Sinθ
Self induced emf = -L  di
Mutual induced emf = M  di1
Self inductance L = 
Mutual inductance M =  N2 Φ2
Coefficient of coupling k =  M
Time constant for RL circuit T =  L
Energy stored in magnetic field =  1  L I2
Magnetomotive force mmf = NI
Magnetic field strength H =  NI
Reluctance H =  l
µ0µr a
Ohm's law for magnetic circuit mmf = flux X reluctance

Hysteresis loss Ph = ηV.f(Bmax)1.6

Eddy current loss Pe = k.(Bmax)2f2t2V  [note here V is volume not voltage]
Ampere turn for iron parts ATi Bi .li = Hili
Ampere turn for air gap ATg = 0.796 Bglg X 106
Leakage factor =  ΦT
Force experienced by conductor F = BIlsinθ
Magnetic field due to an infinite linear conductor H =  1
Flux density due to circular loop Bz μIR2
2(R2 + Z2)3/2
Flux density at the centre of solinoid Bc μNI
4R2 + I2
Force between two parallel conductors F =  μ0 I I' .I
Magnetic Flux density B =  Φ
Ampere's circuit law ∫H dl = ∑ I 
H =  N.I
Biot-savart's law dH =  I Idl  sinθ

Measuring Instruments
For extension of range of ammeter, shunt resistance Rsh is given by:
Rsh = Rm/(m-1)

For extension of range of voltmeter, series resistance Rs is given by:
Rs = (m-1)Rm

The unknown resistance Rs to be measured by wheatstone bridge method is:
R = (P/Q)S

AC Fundamental
RL series circuit ν = VmaxSin ax
i = ImaxSin (ax - Φ)
rms value of current = I = V/Z
where Z = √R2 + X2L
In inductive circuit, the current lag the applied voltage by an angle Φ

RC series circuit ν = VmaxSin ax
i = ImaxSin (ax + Φ)
rms value of current = I = V/Z
where Z = √R2 + X2C
In capacitive circuit, the current leads the applied voltage by an angle Φ

RLC series circuit Z = √R2 + (XL - XC)2

Active power = V I CosΦ
Apparant power = V I
Reactive power = V I SinΦ
Power factor CosΦ =  Active power R
Apparant power Z
Resonance in RLC circuit occurs at a frequency given by fr 1
At resonance XL = XC
Impedance is minimum
Current is maximum
Power factor is unity
Bandwidth =  R

For ideal transformer IpNp = IsNs ; VpIp = VsIs
Transformer ratio k =  Ep  =  Vp  =  Np  =  Is
Es Vs Ns Ip
Only current I is secondary/primary here

Synchronous Motor
Synchronous speed of rotating magnetic field, Ns 120 X f
Percentage slip, S =  Ns - N

3 Phase System
For 3 phase star connection with balanced load
IL = Iph
VL = √3 X Vph
P = √3 VLILCosΦ
For 3 phase delta connection with balanced load
IL = √3 Iph
VL = Vph
P = √3 VLILCosΦ

DC Machine
EMF of a DC Machine, Eg PΦIaZ
Constant losses = Iron losses + Mechanical losses

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Share how to remember these days & leave your comments below...

Gurlal singh said on : 2015-12-30 21:49:25     

Gurpreet Mann said on : 2015-12-31 08:02:12     
good work done

ROBIN BATRA said on : 2016-01-11 05:19:59     
EMF of a DC Machine,Eg=PZΦN/60A

Preeti said on : 2016-07-14 02:12:18     
Its really helpful

AKSHAY SONAWANE said on : 2016-08-04 08:57:22     
very good....

thank you....

keep it up

Zorawar said on : 2016-10-05 21:06:38     
Thanks ........... NYC work !

Harpreet singh said on : 2017-01-19 08:40:02     
Very helpful

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