Electric Fields Revision Questions

 

  3.     (a)     appropriate shape; lines perpendicular to and touching plate and sphere; (2)
arrows towards negative sphere (1)                                                                     3

  (b)   (i)      By moments, e.g F cos 20 = W sin 20 / by triangle of forces /
by resolution of forces / other suitable method; i.e. justification needed (1)
F = 1.0 × 10–5 tan 20; = 1.0 × 10–5 × 0.364; (= 3.64 × 10–6 N) (2)
triangle of forces gives W/F = tan 70, etc (1)                                              3

  (ii)   E = F/Q; = 3.64 × 10–6 / 1.2 × 10–9 = 3.0 × 103;N C–1 / V m–1                        3

  (c)   E = (1/4πεo)Q/r2; 3.0 × 103 = 9 × 109 × 1.2 × 10–9/r2; (2)
or use F = (1/4πεo)Q2/r2; r2 = 3.6 × 10–3 giving r = 6 × 10–2 (m) (1)                         3

(d)     field line sketch minimum of 5 lines symmetrical about line joining
centres with arrows; (1)
Fig 1 sketch matches RHS of Fig 2/plate analogous to mirror/AW
relating to symmetry (1)                                                                                             2

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(i)      Suitable recognisable pattern around (not just between) the charges                             B1
Quality mark: symmetry, spacing, lines joined to charges                                   B1
Consistent arrows toward B on some lines                                                        B1

  (ii)   Use of E = (1/4πε0)Q/r2                                                                                    C1
Sum of two equal terms
E = 2 × 9 × 109 × 1.6 × 10–19 / (2.0 × 10–10)2                                                     C1
E = 7.2 × 1010 N C–1 or V m–1                                                                          A1

  (iii)  The separation between the ions because this has an effect on the
breaking force. (Allow the size of ionic ‘charges’)                                             B1

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