Physics 9702 · AS & A Level · Potential difference and power

Potential difference and power — practice question

(a)[1]

Define what is meant by electric field strength.

(b)

A potential difference of $2.5\,\text{kV}$ is applied across a pair of horizontal metal plates in a vacuum, as shown in Fig. 2.1. Each plate has a length of $5.9\,\text{cm}$. The separation of the plates is $4.0\,\text{cm}$. The arrangement produces a uniform electric field between the plates. Assume the field does not extend beyond the edges of the plates. An electron enters the field at point A with horizontal velocity $3.7 \times 10^7\,\text{m s}^{-1}$ along a line mid-way between the plates. The electron leaves the field at point B.

(b(i))[1]

Calculate the time taken for the electron to travel from A to B.

(b(ii))[2]

Calculate the magnitude of the electric field strength.

(b(iii))[2]

Show that the acceleration of the electron in the field is $1.1 \times 10^{16}\,\text{m s}^{-2}$.

(b(iv))[3]

Use the acceleration from (iii) together with your answer in (i) to find the vertical distance $y$ between point B and the upper plate.

(b(v))[1]

Explain why the calculation in (iv) does not need to include the gravitational effects on the electron.

(b(vi))[2]

The electron enters the field at time $t = 0$. On Fig. 2.2, sketch graphs to show how, with time $t$, (1) the horizontal component $v_x$ of the velocity of the electron, and (2) the vertical component $v_y$ of the velocity of the electron, vary. Numerical values are not required.

(iv)[3]

Using the acceleration from (iii) and your result in (i), determine the vertical distance $y$ between point B and the upper plate.

(v)[1]

Explain why the calculation in (iv) does not need to include the gravitational effects on the electron.

(vi)[2]

The electron enters the field at time $t = 0$. On Fig. 2.2, sketch graphs to show the variation with time $t$ of: 1. the horizontal component $v_x$ of the velocity of the electron, 2. the vertical component $v_y$ of the velocity of the electron. Numerical values are not required.

Worked solution & mark scheme

This 18-mark question has a full step-by-step worked solution and mark scheme. One marking point: Gives electric field strength as the force per unit positive charge

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