IB Physics SL D.2 Electric and magnetic fields Questions

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Practise IB Physics SL D.2 by analysing charge interactions, charge transfer, electric-field patterns and uniform plate fields, alongside magnetic field-line direction.

58 questions in this syllabus scope, with question parts counted together.

Exam points & syllabus

Exam points

Identify electrostatic attraction or repulsion from charge signs; distinguish the physical interaction from its centripetal role in a supplied model.
Use Coulomb’s inverse-square law with centre separation and both charge magnitudes, including the given medium permittivity when relevant.
Scale the mutual force on both charges consistently when one charge or the separation changes; the pair retains equal force magnitudes.
Determine resultant force directions by adding each signed source contribution, with the actual geometry rather than charge magnitude alone.
Use charge conservation in fully charge-labelled reactions or circuits; explain a junction through charge conservation rather than a law’s name alone.
Track electron transfer and the order of grounding and rod removal: a positive rod can leave a nearby grounded plate negatively charged.
Define E as force per unit positive small test charge, and identify the force and test-charge symbols precisely when asked.
Keep electric field distinct from force: use source charge in E=kQ/r², then F=qE and F=ma for a test particle’s force or initial acceleration.
Describe motion in a non-uniform repulsive field: speed increases while acceleration decreases as the particle moves farther from the source.
Apply a supplied central-charge sphere model at its surface using the radius and SI prefixes; do not replace the stated model with extra assumptions.
Combine collinear electric fields with directions and separate source distances; fields can reinforce between opposite charges and oppose between like charges.
Locate possible zero fields by balancing magnitudes: like charges cancel between them, unequal opposite charges outside on the weaker-charge side.
Infer an unknown charge from a resultant or balance condition by including every source distance and direction in the field equation.
Use symmetry and vector components to determine multi-charge fields, missing signs or rankings; cancel equal opposing contributions first.
For an electric-to-gravitational field ratio at the same radius, write both source fields before cancelling r² and estimating the order of magnitude.
Draw isolated positive-charge lines radially outward; for a negative metal sphere use inward radial lines and show no field inside the metal.
Distinguish separately labelled source-field arrows from a resultant; use cancellation and relative distances when interpreting paired or multiple sources.
For equal opposite charges, check each labelled observation point: fields reinforce between the charges and oppose at exterior points.
Check shape and arrow consistency when selecting a two-charge field pattern; like-positive patterns diverge and static electric lines have no closed loops.
For a fixed charge, use closer field-line spacing to identify a greater force and reverse the field direction when the charge is negative.
Draw the plate field with even central spacing, lines touching both plates, edge fringing and arrows from the positive plate towards the negative plate.
Calculate a uniform plate field with E=V/d, converting the full plate separation to metres and giving N/C or V/m.
Draw a straight wire’s magnetic field as concentric circles in perpendicular planes, with outwardly increasing spacing where required.
Use conventional current in the right-hand rule and reverse electron flow first; into-page current gives clockwise magnetic field lines.
For two wires or a wire in an external field, add local field vectors and name where they cancel or reinforce, specifying sides when required.
Recognize magnetic field lines as a model for visualizing a field rather than a physical discovery or a theory of its cause.
Choose a coil separation using the field over the entire finite needle; a larger peak field is insufficient if the field is not uniform across it.
Orient coil and Earth fields perpendicularly so the needle’s deflection measures their components; relate the tangent to the actual labelled angle.

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

Exam points

Explain an ion beam spreading by stating that its ions have the same sign of charge and therefore repel one another.
For opposite charges, draw attractive force arrows towards the other charge; distinguish each force direction from the electric-field arrow.

[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.

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.

Exam points

State Coulomb’s law for point charges: force magnitude is proportional to their charge product and inversely proportional to separation squared.
Calculate force with consistent charge and length units, squaring the full separation; show substitution when a result is given as a target.
When one charge is halved at fixed separation, halve the mutual force on both charges; the two interaction forces remain equal in magnitude.
In the supplied dielectric model, combine inverse-permittivity and inverse-square distance factors; halving separation and quadrupling ε cancel.
Distinguish point-charge force from field strength: calculate force with both charges, then divide by the test charge to obtain the source field.
For zero resultant force between two sources, make opposing forces balance: sources have the same sign and the farther source must have greater magnitude.
Locate a zero-field point by equating charge divided by distance squared and using the full source separation; a 4q source has twice the distance of q.
When inferring charge from a zero-field position, use each source-to-point distance, determine its sign and give the requested significant figures.
For several point charges, find each force direction from charge signs, scale magnitudes with |Q|/r² and add vectors rather than force magnitudes.

[Maximum number: 2]

This question is in two parts. Part 1 is about simple harmonic motion and forced oscillations. Part 2 is about electric and magnetic force fields.

Part 1 Simple harmonic motion and forced oscillations
The graph shows the variation with time of the displacement of an object undergoing simple harmonic motion.
displacement / mm

time / ms

Part 2 Electric and magnetic force fields

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.

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

(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 $F=\frac{\left(1.60 \times 10^{-19}\right)^{2}}{\left(5.0 \times 10^{-3}\right)^{2}} \times 8.99 \times 10^{9}$ ;
$9.2 \times 10^{-24} \mathrm{~N} ;$

Exam points

Balance total charge on both sides of a labelled reaction to infer the unknown charge, giving its sign and the appropriate elementary-charge unit.
Check a proposed reaction by summing signed charges before and after; a change from zero to +e identifies a violation of charge conservation.
Name conservation of electric charge as the fundamental basis of a junction-current relation; Kirchhoff’s law alone or an extra energy-law list is insufficient.

[Maximum number: 1]

The diagram shows a junction in a circuit.

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.

Exam points

For a positive rod near a grounded plate, identify a negative net plate charge both before and after the grounding wire is removed first.

[Maximum number: 1]

A positively charged rod is near a metal plate that is grounded as shown.

The grounding wire and then the rod are removed. What is correct about the overall charge on the plate before and after grounding is removed?

Charge on plate before

grounding is removed

Charge on plate after

grounding is removed

neutral

negative

neutral

negative

Exam points

Define electric field strength as force per unit positive small or test charge at the specified point, rather than force alone.
Calculate a point-source field with E=k|Q|/r², using source charge and centre distance rather than the charge of the particle placed there.
For a supplied equivalent central-charge sphere model, use its radius for the surface field and convert millimetres and nanocoulombs before substitution.
Use F=qE to calculate electric force and combine it with F=ma for initial acceleration; use charge magnitude when only a magnitude is requested.
Give both field magnitude and direction for a positive point source: divide the force by the test charge and direct the field away from the source.
Add electric fields as vectors, checking charge sign and each source distance; between opposite charges the two contributions can reinforce.
For like charges at unequal distances, subtract the opposing fields and retain the direction of the larger contribution, not the larger source charge alone.
Distinguish zero-field regions: like charges can cancel between them, while unequal opposite charges cancel outside on the weaker-charge side.
When ranking fields at several points, use actual grid distances and vector addition or subtraction for every source contribution.
Infer an unknown source charge from a stated resultant by writing the full field sum; an external point beyond two positive sources receives reinforcing fields.
Use symmetry to cancel equal components at a point equidistant from opposite charges; the resultant points from the positive side towards the negative side.
For a symmetric multi-charge arrangement, cancel equal opposing contributions first, then calculate and state the remaining field’s magnitude and direction.
Infer missing charge signs by testing how their centre-field vectors combine with known charges to give the stated resultant direction.
In a show-that field calculation, give a complete substitution or enough significant figures to demonstrate the supplied rounded target.

[Maximum number: 4]

time / ms

(a)

Part 2 Electric and magnetic force fields

[ 4 ]

(i)

Define electric field strength.

[ 2 ]

(ii)

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

Figure for Question (ii) — IB Physics SL

[ 2 ]

(i)

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.

Exam points

Draw a proton’s field as symmetrical radial lines with outward arrows; satisfy both the pattern and direction requirements in the markscheme.
For a negatively charged metal sphere, draw radial lines inward and no field inside the metal; include enough lines to show the complete pattern.
For two equal positive charges, choose an outward field pattern whose lines bend away from the region between the charges.
Distinguish the field due to each source from the resultant: draw separately labelled arrows away from positive and towards negative charges.
Use symmetry to cancel equal opposing field components before choosing a resultant direction in a triangle or an opposite-charge pair.
For a square charge arrangement, include the diagonal source as well as adjacent sources; its greater distance gives a weaker field contribution.
For equal opposite charges, determine field direction at each stated point: contributions reinforce between them and oppose outside the pair.
Draw evenly spaced plate-field lines touching both plates, with fringing at the edges and arrows from the positive plate towards the negative plate.
When choosing a two-charge field pattern, check curved-line shape and arrow consistency; static electric-field lines do not form closed loops.

[Maximum number: 2]

time / ms

Part 2 Electric and magnetic force fields

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

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

Exam points

Compare local field-line spacing to find the greatest force on a fixed charge, and reverse the field direction when the charge is negative.

[Maximum number: 1]

The diagram shows electrostatic field lines. The source of the field is not shown.

At which position in the field would a negative point charge experience the greatest force to the right?

Exam points

Use E=V/d for the uniform plate field, convert the plate separation to metres, and give the result in N/C or V/m.

[Maximum number: 1]

Two oppositely charged parallel plates are a distance 8.0 cm apart. The potential difference between the plates is 120 V . An alpha particle is placed on the positively charged plate and released from rest. Gravity is ignored.

Calculate the electric field between the plates.

$E=\ll \frac{120}{8.0 \times 10^{-2}}=>1.5 \times 10^{3} N C^{-1} \boldsymbol{O} \boldsymbol{R} \mathrm{Vm}^{-1}$

[1]

Exam points

Draw the field around a straight current-carrying wire as concentric circles centred on the wire, in planes perpendicular to it.
Use conventional current with the right-hand rule: current into the page gives clockwise field lines; reverse the sense for outward current.
When electron motion is given, reverse it to obtain conventional current before using the right-hand rule for the magnetic field.
For a wire-field sketch, show increasing line separation away from the wire when the markscheme requires the weakening field to be represented.
For two wires, determine each local field direction separately and combine the vectors, using equal distances and currents to cancel components.
Explain stronger and weaker fields beside a wire by naming the sides and showing where its own field reinforces or opposes the external field.
Recognize magnetic field lines as a model used to visualize the field, rather than a physical discovery or a theory of its cause.

[Maximum number: 2]

On the diagram below, sketch the magnetic field pattern around the long straight current-carrying conductor. The direction of the current is into the plane of the paper.

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