Paper 4 Alternating Current Generators Answers

Q1 Model Generator Investigation

1. Axes correctly oriented and labelled [1]; sensible scales [1]; six accurate points [2]; best-fit straight line [1].

2. Gradient $\approx 0.50\mathrm{V}\mathrm{H}{\mathrm{z}}^{-1}$. Accept graph-consistent values. [2]

3. Since the gradient is $2\pi NBA$,

   $$B=\frac{0.50}{2\pi (100)(4.00\times 10^{-3})}\approx 0.20\mathrm{T}.$$

   [3]

4. The points lie close to a straight line whose intercept is consistent with zero, supporting direct proportionality within scatter. [2]

5. Scatter: speed fluctuation, brush/contact variation or oscilloscope reading uncertainty [1]. Control any one of $N$, $A$, $B$, coil orientation geometry or load/input impedance [1].

Q2 Comparing Slip Rings and Commutator

1. Slip rings give alternating output; a split-ring commutator gives unidirectional pulsating output. [2]
2. Slip rings keep each coil end connected to the same external terminal; commutator reverses connections every half-turn. [2]
3. Keep loose wires/clothing away from rotating parts or use low voltages. [1]