IB Physics SL A.2.14 Electric force Question Bank

Practise IB Physics SL A.2.14 by identifying electric force and resolving it in simple equilibrium problems.

Exam points

Identify the electric force acting on a charged object in a field or force diagram.
Resolve electric force components and use equilibrium to calculate an unknown force.
State the direction of electric force for a positive or negative charge.

[Maximum number: 3]

A vertical wall carries a uniform positive charge on its surface. This produces a uniform horizontal electric field perpendicular to the wall. A small, positively-charged ball is suspended in equilibrium from the vertical wall by a thread of negligible mass.

Figure for Question A.2.14—Electric force Fe question 1 — IB Physics SL

The thread makes an angle of $30^{\circ}$ with the vertical wall. The ball has a mass of 0.025 kg .

Determine the horizontal force that acts on the ball.

Horizontal force on ball $F=T\sin30^\circ$ .
$T=mg/\cos30^\circ$ .
$F=mg\tan30^\circ=0.025\times9.8\times\tan30^\circ=0.14\ \mathrm{N}$ .

Allow $g=10\ \mathrm{N\ kg^{-1}}$ . Award [3] for a bald correct answer. Award [1 max] for an answer of zero, interpreting the horizontal force as the horizontal component of the net force.

F=mg\tan30^\circ=0.025\times9.8\times\tan30^\circ=0.14\ \mathrm{N}

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A.2.14—Electric force Fe question 1