Waves, Electricity, and Magnetism
This domain combines wave behaviour, simple circuits, and electromagnetic effects. Items ask you to compute wavelength or frequency, find the equivalent resistance or current in a series or parallel circuit, compute power, or explain what happens when a magnet moves near a coil.
Wave rules
- v = fλ; period T = 1/f; amplitude sets energy, frequency sets pitch or colour.
- Transverse waves oscillate perpendicular to travel; longitudinal waves (sound) oscillate along it.
- Waves reflect, refract (bending when speed changes), diffract around gaps, and interfere constructively or destructively.
- The electromagnetic spectrum, from longest to shortest wavelength: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. All travel at c ≈ 3.0 × 10⁸ m/s in vacuum.
Circuit rules
- Series: same current everywhere; voltages add; R_total = R₁ + R₂ + …
- Parallel: same voltage across each branch; currents add; 1/R_total = 1/R₁ + 1/R₂ + …
- Ohm's law V = IR; power P = IV = I²R = V²/R.
- Current is the flow of charge and is not "used up" around a circuit; energy is transferred instead.
- A current-carrying wire has a magnetic field around it; a changing magnetic flux through a coil induces an emf (Faraday). Motors convert electrical to mechanical energy; generators do the reverse.
Worked example
A 12 V supply is connected across a 6 Ω and a 3 Ω resistor in parallel. Find the total current and the current in each branch.
Reasoning: 1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6, so R = 2 Ω. Total current I = V/R = 12 ÷ 2 = 6 A. In parallel each branch sees the full 12 V: through 6 Ω, I = 12 ÷ 6 = 2 A; through 3 Ω, I = 12 ÷ 3 = 4 A. The branch currents sum to 6 A, which checks the answer. Total power P = IV = 6 × 12 = 72 W.
Traps that cost marks
- Swapping series and parallel rules for current and voltage.
- Adding parallel resistances directly instead of adding reciprocals, which would give 9 Ω here.
- Saying current is consumed by the first component in a series loop.
- Confusing frequency with amplitude, or period with frequency.
- Bending light away from the normal when entering a denser medium - it bends toward the normal.
- Treating a light-year as a time or the electromagnetic spectrum as unordered.