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Revision Notes on Alternating Current

  • Alternating Current:-

Alternating CurrentAn alternating current (a.c.) is a current which continuously, changes in magnitude and periodically reverses in direction.`

i = I0 sin ωt = I0 sin (2π/Tt

Here I0 is the peak value of a.c.

(a) Current, I =I0 sin ωt

(b) Angular frequency, ω= 2π  (n is the frequency of a.c.)

(c) I =I0 sin 2πnt

  • Mean value of A.C or D.C. value of A.C.:-

Mean value of a.c. is that value of steady current which sends the same amount of charge, through a circuit, in same time as is done by a.c. in one half-cycle.

(Iav)half cycle = (2/π)I0

Thus, mean value of alternating current is 2/π times (0.637 times) its peak value.

(Vav)half cycle = (2/π) V0Mean value of A.C or D.C. value of A.C.

  • Average value of A.C. over a complete cycle:-

Iav = 0

The average value of a.c. taken over the complete cycle of zero.

  • Root mean square value of a.c. or virtual value of a.c.:-

Root mean square value of alternating current is defined as that value of steady current which produces same heating effect, in a resistance, in a certain time as is produced by the alternating current in same resistance in same time. The r.m.s value of also called its virtual value.

Irms = I0/√2

Root mean square value of alternating current is I/√2 times (or 0.707 times) the peak value of current.

Similarly, VrmsV0/√2

Here V0 is the peak value of e.m.f.

  • Form Factor:-

Form Factor = rms value/average value = (V0/√2)/ (2 V0/π)  = π/2√2

  • Current elements:-

(a) Inductive reactance:- XL = ωL

Here, ω = 2πn, n being frequency of a.c.

L is the coefficient of self-inductance of coil.

(b) Capacitative reactance:- Xc = 1/ωC

Here C is the capacity of the condenser

  • Capacitor in AC circuit:-

Capacitor in AC circuitq = CV0sinωt

I = I0 sin(ωt +π/2)

V0 = I0/ωC

Xc = 1/ωC

  • Inductor in AC circuit:-

Inductor in AC circuitVL(dI/dt) = LI0ω cosωt

I = (V0/ ωL) sinωt

Here, I0 = V0/ ωL

XL = ωL

And the maximum current, I0 = V0/XL

  • R-L circuit:-

R-L circuitI = ε/R [1-e-Rt/L]

V = ε e-Rt/L

  • Graph between I (amp) and t (sec):-

Graph between I (amp) and t (sec)

  • Graph between potential difference across inductor and time:-

Graph between potential difference across inductor and time

  • L-C Circuit:-

L-C Circuitf = 1/2π√LC

q = q0 sin (ωt+ϕ)

I = q0ωsin (ωt+ϕ)

ω = 1/√LC

  • The total energy of the system remains conserved,

    ½ CV2 + ½ Li2 = constant = ½ CV02 = ½ Li02

  • Series in C-R circuit:-

Series in C-R circuit

 V = IZ

The modulus of impedance, |Z |= √R2+(1/ωC)2

The potential difference lags the current by an angle, ϕ = tan-1(1/ωCR)

  • Series in L-C-R Circuit:

Series in L-C-R Circuit

V = IZ

The modulus of impedance, |Z |= √[R2+(ωL-1/ωC)2]

The potential difference lags the current by an angle, ϕ = tan-1[ωL -1/ωC)/R]

  • Circuit elements with A.C:-

Circuit elements

Amplitude relation

Circuit quantity

Phase of V


V0 = i0R


In phase with i


V0 = i0XC


Lags i by 90°


V0 = i0XL

XL = wL

Leads i by 90°

  • Resonance:-

(a) Resonance frequency:- fr = 1/2π√LC

(b) At resonance, XL = XC, ϕ = 0, Z = R(minimum), cosϕ = 1, sinϕ = 0 nad current is maximum (=E0/R)

  • Half power frequencies:-

(a) Lower,  f1 = fr – R/4πL    or      ω1 = ωr – R/2L

(b) Upper,  f2 = fr + R/4πL    or      ω2 = ωr + R/2L

  • Band width:- Δf = R/2πL   or   Δf = R/L

  • Quality Factor:-

(a) Q = ωr/Δω = ωrL/R

(b) As ω = 1/√LC, So Q ∝ √LQ ∝1/R and Q ∝ 1/√C

(c) Q = 1/ωrCR

(d) Q = XL/R   or  Q = XC/R 

(e) Q = frf   

  • At resonance, peak voltages are:-

(a) (VL)res = e0Q

(b) (VC)res = e0Q

(c) (VR)res = e0

  • Conductance, susceptance and admittance:-

(a) Conductance, G = 1/R

(b) Susceptance, S = 1/X

(c) SL = 1/XL and SC = 1/XC = ωC

(d) Admittance, Y = 1/Z

(e) Impedance add in series while add in parallel

  • Power in AC circuits:-

Circuit containing pure resistance:- Pav = (E0/√2)×(I0/√2) = Ev×Iv

Here Ev and Iv are the virtual values of e.m.f and the current respectively.

Circuit containing impedance (a combination of R,L and C):-

Pav = (E0/√2)×(I0/√2) cosϕ = (Ev×Iv) cosϕ

Here cosϕ is the power factor.

(a) Circuit containing pure resistance, Pav = EvIv

(b) Circuit containing pure inductance, Pav = 0

(c) Circuit containing pure capacitance, Pav = 0

(d) Circuit containing resistance and inductance,

Z = √R2+(ωL)

cosϕ R/Z = R/[√{R2+(ωL)2}]

(e) Circuit containing resistance and capacitance:-

Z = √R2+(1/ωC)2

cosϕ R/Z = R/[√{R2+(1/ωC)2}]

(f) Power factor, cosϕ = Real power/Virtual power = Pav/ErmsIrms

  • Transformer:-

(a) Cp = Np (dϕ/dt) and es = Ns (dϕ/dt)

(b) ep/es = Np/Ns

(c) As, epIp = esIs, Thus, Is/Ip = ep/es = Np/Ns

(d) Step down:- eepNsNp and IsIp

(e) Step up:- e>ep, Ns>Np and IsIp

(f) Efficiency, η = es Is/ ep Ip

  • AC Generator:-

e = e0 sin (2πft)

Here, e0 = NBAω