Physical Science: Electricity and Magnetism, Waves, Light, and Sound
This domain spans electric circuits, magnetism, wave behavior, optics, and sound.
Electricity and circuits
Coulomb's law: F = kq1q2/r². Ohm's law: V = IR. In series, current is the same everywhere, resistances add, and voltage divides. In parallel, voltage is the same across branches, 1/Rtotal = Σ1/R, and current divides. Power P = IV = I²R. Capacitors store charge Q = CV.
Magnetism and induction
Moving charges create magnetic fields; a charge moving through a field feels F = qvB sinθ, perpendicular to both (right-hand rule). Faraday's law: changing magnetic flux induces an emf; Lenz's law says the induced current opposes the change. Generators and transformers depend on this.
Waves and sound
v = fλ. Transverse waves oscillate perpendicular to travel; longitudinal waves (sound) oscillate along it. Reflection, refraction, diffraction, and interference are universal wave behaviors; standing waves occur at resonant frequencies. The Doppler effect shifts observed frequency as source and observer approach or recede. Sound needs a medium (about 343 m/s in air at 20 °C) and is described by frequency (pitch) and amplitude (loudness).
Light and optics
Light is an electromagnetic wave with c ≈ 3.0 × 10⁸ m/s in vacuum. Snell's law: n1 sinθ1 = n2 sinθ2; light bends toward the normal entering a denser medium, and total internal reflection occurs beyond the critical angle. Lenses and mirrors form real (inverted) or virtual images. Double-slit interference supports the wave model; the photoelectric effect (E = hf) supports the photon model.
Worked example
Two 6 Ω resistors in parallel across a 12 V battery. Equivalent resistance: 1/R = 1/6 + 1/6 = 1/3, so R = 3 Ω. Total current I = V/R = 12/3 = 4 A, splitting 2 A per branch. Check: each branch carries 12 V / 6 Ω = 2 A. Reasoning: reduce to equivalent resistance, then apply Ohm's law.
Traps
Saying current is "used up" in a series circuit; adding parallel resistances directly; believing sound travels through a vacuum; thinking frequency changes on refraction (it does not; speed and wavelength do); forgetting real images are inverted.