**Ohm’s Law**

I =E/R

or E = IR

where I = current (amperes)

E = electromotive force (volts)

R = resistance (ohms)

**Conductor Resistivity**

R = ρ*L/a

where ρ = specific resistance (or resistivity) (ohm metres, Ω·m)

L = length (metres)

a = area of cross-section (square metres)

Temperature correction

Rt = Ro (1 + αt)

where Ro = resistance at 0ºC (Ω)

Rt = resistance at tºC (Ω)

α = temperature coefficient which has an average value for copper of 0.004 28

(Ω/ΩºC)

R2 = R1 * (1 +αt1 ) /(1+ αt2 )

where R1 = resistance at t1 (Ω)

R2 = resistance at t2 (Ω)

α Values Ω/ΩºC

copper 0.00428

platinum 0.00385

nickel 0.00672

tungsten 0.0045

aluminum 0.0040

**Alternating Current**

R.M.S. value of sine curve = 0.707 maximum value

Mean value of sine curve = 0.637 maximum value

Form factor of sinusoidal = RMS Value /Mean Value = 1.11

Frequency of alternator =pN/60 cycles per second

Where p = number of pairs of poles

N = rotational speed in r/min

Instantaneous value = Maximum value × Sin (2πft)

Note: calculator must be in radian mode

**Slip of Induction Motor**

((Slip speed of field - speed of rotor)/Speed of field) * 100

**Current in AC Circuit**

Current = impressed volts/impedance

**Power Factor**

p.f. = true watts/(volts x amperes)

also p.f. = cos Φ, where Φ is the angle of lag or lead