y12 Electricity revision question on Christmas tree bulbs and 2 lamps

 

1.       (a)     either   (If in parallel) when one bulb fails, other bulbs stay on
or        (If in parallel) can identify which bulb has failed; (1)                                1

  (b)   (i)      P = VI (1)
0.5 = 240 I
I = 2.1 × 10–3 A 1 s.f. in answer (–1) once only (1)                                      2

  (ii)   R = V/I (1)
= 240/(2.1 × 10–3)
= 1.14 × 105 Ω or 1.15 × 105 Ω ans
accept (1.1 to 1.2) × 105 Ω. (1)                                                                  2

  (iii)  A = ρ l / R (1)
= 1.1 × 10–6 × 6.0 × 10–3 / (1.14 × 105) (= 5.79 × 10–14 m2)
A = πr2 (1)
5.79 × 10–14 = πr2 so r = 1.4 × 10–7 m (1)                                                   3

  (iv)  filament too thin / fragile to be manufactured / used without damage;
allow ecf from (iii). (1)                                                                              1

 

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