Common Errors and Traps

#	Trap	Wrong Thinking	Correct Thinking
1	Resonance impedance	"Z = 0 at resonance"	Z = R (minimum, not zero). Z = √( $R^{2}$ + 0) = R
2	Current/Turns in transformer	"I ∝ N_s/N_p (same as voltage)"	I ratio is INVERSE: I_s/I_p = N_p/N_s
3	RMS vs Mean confusion	"V_mean = $V_{0}$ /√2"	V_rms = $V_{0}$ /√2 ≈ 0.707 $V_{0}$ ; V_mean = 2 $V_{0}$ /π ≈ 0.637 $V_{0}$
4	Zero EMF with zero flux	"EMF = 0 when Φ = 0"	EMF = −dΦ/dt; EMF is max when Φ = 0 (maximum rate of change)
5	Adding LCR voltages	"V = V_R + V_L + V_C"	V = √(V_ $R^{2}$ + (V_L − V_C)^{2}) — phasor addition!
6	Lenz's law direction	"Induced I aids the flux change"	Induced I OPPOSES the flux change (otherwise energy would be created from nothing)
7	Motional EMF condition	"ε = Bvl always"	Only when v, B, l are mutually perpendicular. Use ε = Bvl cos θ otherwise.
8	Step-up transformer current	"Current also increases"	Current DECREASES (P = VI = constant; V up → I down)
9	Average power in full AC cycle	"Average V × Average I"	Must use V_rms × I_rms × cos φ; average of sinusoid over full cycle is 0
10	Lamination reduces hysteresis	"Lamination reduces hysteresis loss"	Lamination reduces EDDY CURRENT loss; soft iron reduces HYSTERESIS loss