IB Physics SL D.2 Electric & magnetic fields Question Bank

Practise applying Coulomb and field-strength relations, conserving charge and interpreting electric or magnetic field diagrams and parallel-plate data.

Exam points

apply Coulomb's law and charge conservation to calculate resultant forces or transferred charge
calculate electric field strength for point charges or parallel plates and interpret field-line density
use magnetic field-line patterns and hand rules to determine direction around currents or magnets

Covered syllabus points

D.2.1—Electric charge forces D.2.2—Coulomb’s law D.2.3—Charge conservation D.2.4—Millikan experiment D.2.5—Charge transfer D.2.6—Electric field strength D.2.7—Electric field lines D.2.8—Field-line density D.2.9—Parallel-plate field D.2.10—Magnetic field lines

[Maximum number: 2]

Ion-thrust engines can power spacecraft. In this type of engine, ions are created in a chamber and expelled from the spacecraft. The spacecraft is in outer space when the propulsion system is turned on. The spacecraft starts from rest.

Figure for Question D.2 Electric and magnetic fields question 1 — IB Physics SL

The mass of ions ejected each second is $6.6 \times 10^{-6} \mathrm{~kg}$ and the speed of each ion is $5.2 \times 10^{4} \mathrm{~m} \mathrm{~s}^{-1}$ . The initial total mass of the spacecraft and its fuel is 740 kg . Assume that the ions travel away from the spacecraft parallel to its direction of motion.

In practice, the ions leave the spacecraft at a range of angles as shown.

Outline why the ions are likely to spread out.

[Maximum number: 8]

(a)

Define electric field strength.

[ 2 ]

(b)

The diagram shows a pair of horizontal metal plates. Electrons can be deflected vertically using an electric field between the plates.

[ 4 ]

(i)

Draw the shape and direction of the electric field between the plates on the diagram.

[ 2 ]

(ii)

Calculate the force on an electron between the plates when the electric field strength has a value of $2.5 \times 10^{3} \mathrm{NC}^{-1}$ .

[ 2 ]

$F=2.5 \times 10^{3} \times 1.6 \times 10^{-19}$ ;
$4.0 \times 10^{-16} \mathrm{~N}$ ;

Marking guidance:

Award [2] for a bald correct answer.

(c)

The diagram shows two isolated electrons, X and Y , initially at rest in a vacuum. The initial separation of the electrons is 5.0 mm . The electrons subsequently move apart in the directions shown.

[ 2 ]

(i)

Show that the initial electric force acting on each electron due to the other electron is approximately $9 \times 10^{-24} \mathrm{~N}$ .

[ 2 ]

(c) (i) use of $F=\frac{\left(1.60 \times 10^{-19}\right)^{2}}{4 \pi \varepsilon_{0}\left(5.0 \times 10^{-3}\right)^{2}}$ or $F=\frac{\left(1.60 \times 10^{-19}\right)^{2}}{\left(5.0 \times 10^{-3}\right)^{2}} \times 8.99 \times 10^{9}$ ;
$9.2 \times 10^{-24} \mathrm{~N} ;$

[Maximum number: 1]

The diagram shows a junction in a circuit.

Figure for Question D.2 Electric and magnetic fields question 3 — IB Physics SL

The currents in the three wires are related by $I_{1}=I_{2}+I_{3}$ .
State the fundamental law of Physics from which this relation is derived.

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Covered syllabus points

D.2 Electric and magnetic fields question 1

D.2 Electric and magnetic fields question 2

Question (a)

Question (b)

Question (i)

Question (ii)

Question (c)

Question (i)

D.2 Electric and magnetic fields question 3