Describing Alternating Current
Describing Alternating Current
- Mains electricity pushes charge back and forth fifty times a second. One sine equation captures it — and P4 keeps hiding a π inside it.
The equation and its dials
Symbols
- = peak values (the amplitude of the oscillation) (A, V)
- = angular frequency (rad s⁻¹)
- = time (s)
- The same machinery as SHM: , and is the number of cycles per unit time — asked as a bare definition in 2024 (9702/42/O/N/24 Q8(a)).
- Reading the dials is most of the marks. Given with a graph showing peak 12 V and period 20 ms: V and — the unit of carries a mark (9702/41/M/J/25 Q6(b)(i)).
- sin or cos works exactly as in SHM: sin starts at zero (rising), cos starts at the peak. Sketch marks check the phase explicitly — a cos graph must peak at (9702/42/O/N/25 Q7(b)), a sin graph must start at the origin heading upward (9702/42/O/N/24 Q8(b)(ii)).
Common mistake
The recycled trap: . The is already inside, so — not . The identical step appeared in 2024 and again in 2025 (9702/42/O/N/24 Q8(b)(i)) (9702/42/O/N/25 Q7(a)(i)).