# Theme D: Fields

> IB Physics Higher Level · IB Physics HL (2025 syllabus)
> Source: https://www.owlsprep.com/study/ib-physics-hl-u4-overview/
> Weight: 17-19% of overall IB Physics HL exam

This unit explores how fields act as mediators of force across space, covering gravitational, electric, and magnetic fields, plus core electromagnetic phenomena that underpin modern physics and technology.

**Prerequisites:** IB Physics HL Unit 2: Mechanics; IB Physics HL Unit 3: Core Electricity and Circuits

## Learning objectives

- Distinguish between gravitational, electric, and magnetic fields and describe their core fundamental properties
- Calculate the motion of massive objects and charged particles in different static and dynamic field configurations
- Apply principles of electromagnetic induction and capacitance to solve quantitative problems for circuits and field systems
- Explain how propagating electromagnetic fields produce electromagnetic radiation and relate to core wave properties

## Unit at a Glance

This unit follows a logical progression from fundamental static fields to dynamic electromagnetic phenomena. We start with the two most common static fields (gravitational and electric) before moving to magnetic fields and their interaction with moving charges. The AHL extension topics cover induction, capacitance, and the nature of electromagnetic radiation as a propagating field disturbance.

Understanding fields is foundational for astrophysics, particle physics, and all modern electrical and electronic technologies. This unit makes up a large proportion of exam marks, so mastering each sub-topic in order will help you build a consistent, transferable model for all field problems.

This unit includes the following core and AHL sub-topics:
- [D.1 Gravitational fields](https://www.owlsprep.com/study/ib-physics-hl-u4-d-1-gravitational-fields/) — Introduces gravitational field concepts, potential, and orbital motion calculations.
- [D.2 Electric fields](https://www.owlsprep.com/study/ib-physics-hl-u4-d-2-electric-fields/) — Covers electric field strength, potential, and force between point charges.
- [D.3 Motion in electromagnetic fields](https://www.owlsprep.com/study/ib-physics-hl-u4-d-3-motion-in-electromagnetic/) — Explores motion of charged particles in combined electric and magnetic fields.
- [D.4 Magnetic effects of electric currents](https://www.owlsprep.com/study/ib-physics-hl-u4-d-4-magnetic-effects-of/) — Covers magnetic fields produced by current-carrying wires and moving charges.
- [D.5 Electromagnetic induction (AHL)](https://www.owlsprep.com/study/ib-physics-hl-u4-d-5-electromagnetic-induction/) — Introduces Faraday's and Lenz's laws and induction in circuits and generators.
- [D.6 Capacitance and capacitive circuits (AHL)](https://www.owlsprep.com/study/ib-physics-hl-u4-d-6-capacitance-and-capacitive/) — Covers capacitor behavior, energy storage, and RC circuit dynamics.
- [D.7 Electromagnetic radiation (AHL)](https://www.owlsprep.com/study/ib-physics-hl-u4-d-7-electromagnetic-radiation/) — Describes the nature of EM waves, Poynting vector, and radiation properties.

## Common pitfalls

- **Wrong:** Confusing potential (field) with potential energy
  - Why it fails: Potential is defined as energy per unit mass (gravitational) or per unit charge (electric), while potential energy is the total energy for a specific object.
  - Correct: Always label your quantity, and remember to multiply potential by mass or charge to get total potential energy.
- **Wrong:** Getting the direction of induced current wrong with Lenz's law
  - Why it fails: Induced current opposes the change in magnetic flux, not the flux itself. A decreasing flux still produces an induced current in the opposite direction to an increasing flux.
  - Correct: First identify whether flux is increasing or decreasing, then find the direction of the induced magnetic field that opposes that change.
- **Wrong:** Forgetting to reverse magnetic force direction for negative charges
  - Why it fails: The standard right-hand rule for magnetic force is defined for positive moving charges.
  - Correct: Always reverse the final force direction when working with electrons or other negative charges.

## Cheatsheet

| Concept | Key Formula/Relation |
| --- | --- |
| Newton's law of gravitation | $F = G \frac{m_1 m_2}{r^2}$ |
| Electric field from a point charge | $E = \frac{1}{4\pi\epsilon_0} \frac{q}{r^2}$ |
| Lorentz force on a moving charge | $F = q(E + v \times B)$ |
| Magnetic force on a current-carrying wire | $F = BIL \sin\theta$ |
| Faraday's law of induction | $\varepsilon = -N \frac{d\Phi}{dt}$ |
| Capacitance of a parallel plate capacitor | $C = \frac{\epsilon_0 A}{d}$ |
| Speed of electromagnetic radiation | $c = \frac{1}{\sqrt{\epsilon_0 \mu_0}} = f \lambda$ |

## What's next

Begin your study of this unit with the first sub-topic on gravitational fields, which builds the core field model you will reuse for all other field types in subsequent sub-topics. After you complete all sub-topics in Theme D: Fields, continue to the first sub-topic of the next unit on thermal physics.

- [D.1 Gravitational fields](https://www.owlsprep.com/study/ib-physics-hl-u4-d-1-gravitational-fields/)
- [D.2 Electric fields](https://www.owlsprep.com/study/ib-physics-hl-u4-d-2-electric-fields/)
- [D.3 Motion in electromagnetic fields](https://www.owlsprep.com/study/ib-physics-hl-u4-d-3-motion-in-electromagnetic/)

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From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ib-physics-hl-u4-overview/