Paper 2 Alternating Current Generators Answers

Q1 Peak emf

1. $\omega =2\pi f=2\pi (25)=157\mathrm{rad}{\mathrm{s}}^{-1}$. [1]
2. ${\mathcal{E}}_0=NBA\omega =80(0.45)(3.0\times 10^{-3})(157)=17.0\mathrm{V}$. [3]
3. Increase $N$, $B$, $A$ or rotation speed. [1]

Q2 Flux and emf Phase

1. At maximum flux, the flux-time graph has zero gradient, so $d(N\Phi )/dt=0$. [2]
2. At zero flux in uniform rotation, the flux is changing fastest, giving maximum gradient magnitude. [2]
3. $90^{\circ }$ or a quarter cycle. [1]

Q3 Slip Rings

1. They maintain electrical contact between the rotating coil and external circuit. [2]
2. As the coil rotates, the direction of flux-linkage change reverses every half-turn, so induced emf reverses. [2]