Physics
XL < XC frequency rule — In AC circuits, inductive reactance XL = 2πfL increases with frequency; capacitive reactance XC = 1/(2πfC) decreases with frequency. At resonance XL = XC, impedance is minimum (pure resistance).
Right-hand palm rule for force — For F = qvB sinθ, stretch right palm flat: fingers point in velocity direction, curl toward B-field direction; thumb shows force direction on positive charge (reverse for negative).
VIBGYOR for increasing energy — Electromagnetic spectrum in order of *increasing* frequency/energy: Radio, Microwave, Infrared, Visible (VIBGYOR itself), Ultraviolet, X-ray, Gamma. Remember "RM-I-VUXG."
Capacitor blocks DC, passes AC — At steady DC (f=0), XC = ∞, so capacitor acts as open circuit. At high AC frequency, XC → 0, capacitor becomes short circuit. Inductor does opposite.
Photoelectric threshold trick — Maximum KE = hf - φ (work function). If frequency < threshold, *no* emission occurs regardless of intensity. Stopping potential Vs = KEmax/e, independent of intensity.
Gauss law symmetry shortcut — Use Gauss only for spherical, cylindrical, or infinite planar symmetry. For sphere: E·4πr² = Qenc/ε₀. For cylinder: E·2πrL = λL/ε₀. For plane: E·2A = σA/ε₀.
Chemistry
LEO says GER — Loss of Electrons is Oxidation; Gain of Electrons is Reduction. Oxidizing agent itself gets reduced; reducing agent itself gets oxidized (swap roles).
Alcohols: Lucas test timing — Lucas reagent (ZnCl₂/HCl) with alcohols: Tertiary gives turbidity *immediately*, Secondary in 5 min, Primary doesn't react at room temp. Remember "3-2-1, instant-medium-never."
Aldehydes vs Ketones tests — Aldehydes give positive Tollen's (silver mirror), Fehling's (red ppt), and iodoform if CH₃CO- present. Ketones only give iodoform test if methyl ketone (CH₃COR). Aldehyde = reducing; ketone = not.
Basicity order of amines — In gas phase: 3° > 2° > 1° > NH₃ (more alkyl = more basic). In water: 2° > 1° > 3° > NH₃ (3° bulky, poor solvation). Aromatic amines (aniline) much less basic due to resonance.
Aufbau order: 1s-2s-2p-3s-3p-4s-3d — Filling order by (n+l) rule: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p. When (n+l) equal, lower n fills first. 4s fills before 3d.
SN1 vs SN2 mechanism — SN1: carbocation intermediate, racemization, favored by 3° carbons, polar protic solvent. SN2: backside attack, inversion (Walden), favored by 1° carbons, polar aprotic solvent. Remember "1 = carbocation, 2 = backside."
Zero-order kinetics trap — Rate = k (independent of concentration). Half-life t½ = [A₀]/(2k) – depends on initial concentration. Unlike 1st order (t½ = 0.693/k, concentration-independent). Common in enzyme saturation, surface catalysis.