IB Physics Fields (HL) is built around four connected topics: gravitational fields, electric and magnetic fields, motion in electromagnetic fields, and induction. For exams, the essential skill is translating a diagram or physical situation into the correct force, energy, potential, or flux relationship.
Under the current course, first assessed in 2025, D.1 and D.2 contain additional HL material, D.3 is studied by both levels, and D.4 Induction is HL-only. The IB recommends 38 teaching hours for the HL Fields theme, compared with 19 at SL, according to the official physics subject brief.
What Fields (HL) includes
| Topic | Central exam ideas | HL emphasis |
|---|---|---|
| D.1 Gravitational fields | Gravitation, field strength, Kepler’s laws and orbits | Potential, equipotentials, orbital energy and escape speed |
| D.2 Electric and magnetic fields | Coulomb’s law, field patterns, uniform electric fields and magnetic fields | Electric potential, potential energy and potential gradient |
| D.3 Motion in electromagnetic fields | Forces on charges, conductors and parallel currents | Combined mechanics and field calculations |
| D.4 Induction | Magnetic flux, Faraday’s law, Lenz’s law and generators | Entire topic is HL-only |
The current assessment has Paper 1A multiple-choice questions, Paper 1B data-based questions, and Paper 2 short-answer and extended-response questions. At HL, Paper 1 lasts two hours and Paper 2 lasts two hours 30 minutes; together, external assessment contributes 80% of the final grade. The official IB specimen papers are the best reference for the present question style.
D.1 Gravitational fields
Gravitational field strength is the force per unit mass on a small test mass:
g = F/m = GM/r²
Both g and force are vectors directed towards the attracting mass. In two-body questions, calculate each field separately and combine them with signs or vector components; do not simply add magnitudes.
HL introduces gravitational potential:
- Vg = -GM/r
- Ep = mVg = -GMm/r
- g = -ΔVg/Δr
- W = mΔVg
Potential is a scalar, whereas field strength is a vector. The negative gravitational potential reflects the choice that potential is zero at infinity and energy must be supplied to remove a mass from the attractive system.
Orbits and escape
For a circular orbit, gravity supplies the centripetal force:
GMm/r² = mv²/r, giving v = √(GM/r).
The total energy of a circular orbit is -GMm/(2r). A satellite moved to a higher circular orbit gains total energy but travels more slowly, a frequent source of incorrect explanations. Escape speed follows from setting the final total energy at infinity to zero: vesc = √(2GM/r).
Questions commonly ask students to derive, show that, or explain an orbital result. Begin from a named physical principle, such as gravitational force providing centripetal force or conservation of energy, rather than quoting the final equation. The D.1 gravitational fields resources provide targeted review before attempting calculations.
D.2 Electric and magnetic fields
Electric fields closely resemble gravitational fields, but charge can be positive or negative. Coulomb’s law is F = kq₁q₂/r² in magnitude, while electric field strength is E = F/q for a small positive test charge.
Field lines point away from positive charge and towards negative charge. Between oppositely charged parallel plates, the field is approximately uniform and E = V/d. This equation should not be applied to radial fields around point charges.
At HL:
- Ve = kQ/r
- Ep = kq₁q₂/r
- E = -ΔVe/Δr
- W = qΔVe
Unlike gravitational potential, electric potential can be positive or negative. For multiple source charges, add potentials algebraically because potential is scalar; calculate vector components when finding resultant electric field strength.
Equipotential surfaces are always perpendicular to field lines. No work is done moving a charge along one equipotential because the potential difference is zero. Practise these distinctions in the D.2 electric and magnetic fields Questionbank.
D.3 Motion in electromagnetic fields
An electric field exerts F = qE, so a charged particle in a uniform electric field has constant acceleration a = qE/m. If its initial velocity is perpendicular to the field, resolving the motion into components produces a parabolic path, much like projectile motion.
A magnetic field exerts F = qvB sin θ. The force is perpendicular to the velocity, so it changes direction but does no work and does not change the particle’s speed. For perpendicular entry, equating magnetic and centripetal forces gives r = mv/(|q|B).
Examiners often combine fields. In a velocity selector, undeflected motion requires qE = qvB, giving v = E/B. Always reverse the force direction for a negative charge.
Related conductor equations are F = BIL sin θ and F/L = μ₀I₁I₂/(2πr) for long parallel wires. The D.3 motion in electromagnetic fields Questionbank is useful for practising direction rules alongside calculations.
D.4 Induction
Induction is entirely HL-only. Magnetic flux through one turn is Φ = BA cos θ, where θ is the angle between the magnetic field and the normal to the coil, not the plane of the coil.
Faraday’s law is ε = -NΔΦ/Δt. An emf is produced by a change in flux linkage, which can result from changing B, A, θ, or the relative position of the field and conductor. A large but constant flux does not induce an emf.
The minus sign represents Lenz’s law: the induced effect opposes the change producing it. A complete direction answer should identify the original flux direction, state whether that flux is increasing or decreasing, and determine the induced field that opposes the change.
For a straight conductor moving perpendicularly through a uniform field, ε = BvL. A rotating coil produces sinusoidal emf; flux and emf are a quarter-cycle out of phase because emf depends on the rate of change of flux. The D.4 Induction Questionbank contains focused HL practice.
How examiners turn Fields theory into marks
Typical command terms require different responses:
- Calculate: show substitution, units and a suitable final precision.
- Determine: obtain the result using information that may need interpretation first.
- Explain: connect cause and effect using field principles.
- Show that: present enough algebra to establish the supplied result without circular reasoning.
- Sketch: show shape, intercepts, asymptotic behaviour and labels where relevant.
Common errors include using distance from a planet’s surface instead of its centre, treating potential as a vector, forgetting the sign of charge, claiming magnetic force changes speed, and confusing flux with flux linkage. Another frequent mistake is choosing a formula before identifying what physical quantity changes.
A reliable method is to draw the field and force directions, write the governing principle, substitute only after rearranging, and check units. Then compare your approach with Topic D Fields videos and the Fields Questionbank with worked solutions. Per-question worked and video solutions are particularly valuable because they show how a physical argument is converted into mark-earning steps.
Conclusion
IB Physics Fields (HL) becomes manageable when the topic is organized around four recurring ideas: field strength, potential and energy, forces on moving charges, and changing magnetic flux. Strong answers distinguish vectors from scalars, state the relevant physical principle, and handle directions and signs explicitly.
Use RevisionDojo to move from explanation to application: review the IB Physics data booklet, attempt topic-filtered questions, and use Jojo AI feedback to diagnose missing reasoning. Finish by watching the relevant past-paper-style video solutions and repeating each question without assistance.
Sources and referenced URLs
- Official IB Physics subject brief, first assessment 2025
- Official IB Physics curriculum update
- Official IB Physics specimen papers and markschemes
- RevisionDojo D.1 Gravitational Fields resources
- RevisionDojo D.2 Electric and Magnetic Fields Questionbank
- RevisionDojo D.3 Motion in Electromagnetic Fields Questionbank
- RevisionDojo D.4 Induction Questionbank
- RevisionDojo Topic D Fields videos
- RevisionDojo Topic D Fields Questionbank
- RevisionDojo IB Physics data booklet
