Exam-style Questions: Electric Potential

  1. Fig. 5.1 shows electric equipotential lines between a charged sphere and an earthed plate.

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    a) On Fig. 5.1, draw an arroe to show the direction of the electric filed at the point labelled X.

    (2 Marks)

    b) Here is a list of potentials

    +50 V

    0 V

    -10 V

    -50 V

    The potential of the sphere is +50 V.

    State the best value for the potential of the plate.

    (1 Mark)

    (Marks available: 3)

  2. This question is about the ground state energy of a hydrogen atom.

    A hydrogen atom in its ground state has an energy of -2.2 x 10-18 J. The most probable seperation between electron and proton is 5.3 x 10-11 m in this state.

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    a) The electron has electrical potential energy.

    Show that the electrical potential is about +27 V at a distance of 5.3 x 10-11 m from the proton.

    e = 1.6 x 10-19 C, k = 1/4πε0 = 9.0 x 109 N m2 C-2

    (2 Marks)

    b) Show that the potential energy of the electron is about -4.3 x 10-18J at this distance from the proton.

    (2 Marks)

    (Marks available: 4)

Answer

Answer outline and marking scheme for question:

  1. a) at right angles to euipotential through the point (by eye) pointing away from sphere

    ACCEPT curved field lines which have the correct direction at X

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    (2 Marks)

    b) 0 V

    (1 Mark)

    (Marks available: 3)

  2. a) V = kQ/r

    V = 9.0 x 109 x 1.6 x 10-19/ 5.3 x 10-11 = 27.2 V

    (2 Marks)

    b) Ep = QV

    Ep = -1.6 x 10-19 x 27.2 = -4.35 x 10-18 J

    (2 Marks)

    (Marks available: 4)