Exam-style Questions: Electric Fields and Forces

  1. Fig. 4.1 shows two large parallel insulated capacitor plates, seperated by an air gap of 4.0 x 10-3 m. The capacitance of the arrangement is 200 pF. The plates are connected by a switch to a 2000 V d.d. power supply. The switch is closed and then opened.

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    Calculate

    a) the magnitude of the electric field strength between the plates givinga suitable unit ofr your answer.

    electric field strength = .................... unit ......

    (2 Marks)

    b) the magnitude in μC of the charge on each plate

    charge = ....................... μC

    (3 Marks)

    c) the energy stored in μJ stored in the capacitor.

    energy = ................ μJ

    (3 Marks)

    (Marks available: 8)

  2. The diagram shows a pair of flat, wide metal plates. They are parallel and connected to a constant 2000 V supply.

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    a) What is the electric field (E) between the plates?

    (2 Marks)

    b) A drop of oil (D), between the plates carries a change of 10 electrons (each 1.6 x 10-19 C).

    What is the force on the drop?

    (2 Marks)

    c) If the drop moves a distance of 2.5 mm towards the positive plate, how much electrical energy is transferred?

    (2 Marks)

    (Marks available: 6)

Answer

Answer outline and marking scheme for question:

  1. a) E = v/d = 2000/4 x 10-3 = 5 x 105; N C-1/ V m-1

    (2 Marks)

    b) Q = CV; = 200 x 10-12 x 2000l = 4 x 10-7 = 0.40 (µC)

    taking p as 10-9 -1 mark / corect conversion of any answer to µC gets final mark

    (3 Marks)

    c) W = ½ CV2 ecf possible = ½ QV;

    = 0.5 x 200 x 10-12 x 4 x 106 ; = 400 (µJ)

    (3 Marks)

    (Marks available: 8)

  2. a)

    E = V = 2000 (1 Mark) = 400 kV m-1 (or kJC-1) (1 Mark)


    d 5 x 10-3

    (2 Marks)

    b) F = qE (1 Mark) = 10 x 1.6 x 10-19x 400 x 103

    F = 6.4 x 10-13 N (1 Mark)

    (2 Marks)

    c) W = Fd (1 Mark) = 6.4 x 10-13 x 2.5 x 10-3

    W = 1.6 x 10-15J(1 Mark)

    (2 Marks)

    (Marks available: 6)