Physics 9702 · AS & A Level · Electric force between point charges

Electric force between point charges — practice question

A helium atom can be represented by a nucleus with two electrons moving in circular paths on diametrically opposite sides, each with radius $170\,\text{pm}$, as shown in Fig. 2.1.
(a)[2]

State Coulomb’s law for point charges.

(b(i))[1]

State the charge on the nucleus in terms of the elementary charge $e$.

(b(ii))[1]

Show that the electric force acting between the nucleus and one electron is $1.6 \times 10^{-8}\,\text{N}$.

(c(i))[2]

Assume that the force in (b)(ii) is the only force acting on the electrons. Calculate the speed of the electrons in orbit.

(c(ii))[2]

Calculate the period of the orbit of the electrons.

(d(i))[2]

In practice, each electron’s orbit is influenced by the other electron. For the position of one electron, determine the ratio $$\frac{\text{electric field strength due to the other electron}}{\text{electric field strength due to the nucleus}}.$$

(d(ii))[1]

Use your answer in (d)(i) to suggest and explain how the electron’s orbit is changed by the presence of the other electron.

Worked solution & mark scheme

This 11-mark question has a full step-by-step worked solution and mark scheme. One marking point: electric force is directly proportional to the product of the charges

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