Unit Overview

Theme D: Fields

IB Physics Higher Level· 6 min read 📊 17-19% of overall IB Physics HL exam

1. 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

Introduces gravitational field concepts, potential, and orbital motion calculations.

★★⏱ 12 min

D.2 Electric fields

Covers electric field strength, potential, and force between point charges.

★★⏱ 15 min

D.3 Motion in electromagnetic fields

Explores motion of charged particles in combined electric and magnetic fields.

★★★⏱ 15 min

D.4 Magnetic effects of electric currents

Covers magnetic fields produced by current-carrying wires and moving charges.

★★★⏱ 18 min

D.5 Electromagnetic induction (AHL)

Introduces Faraday's and Lenz's laws and induction in circuits and generators.

★★★★⏱ 20 min

D.6 Capacitance and capacitive circuits (AHL)

Covers capacitor behavior, energy storage, and RC circuit dynamics.

★★★★⏱ 18 min

D.7 Electromagnetic radiation (AHL)

Describes the nature of EM waves, Poynting vector, and radiation properties.

★★★★⏱ 18 min

2. Common Pitfalls

Wrong move:

Confusing potential (field) with potential energy

Why:

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 move:

Always label your quantity, and remember to multiply potential by mass or charge to get total potential energy.

Wrong move:

Getting the direction of induced current wrong with Lenz's law

Why:

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 move:

First identify whether flux is increasing or decreasing, then find the direction of the induced magnetic field that opposes that change.

Wrong move:

Forgetting to reverse magnetic force direction for negative charges

Why:

The standard right-hand rule for magnetic force is defined for positive moving charges.

Correct move:

Always reverse the final force direction when working with electrons or other negative charges.

3. Quick Reference 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	$E = \frac{1}{4 π ϵ _{0}} \frac{q}{r ^{2}}$
Lorentz force on a moving charge	$F = q (E + v \times B)$
Magnetic force on a current-carrying wire	$F = B I L sin θ$
Faraday's law of induction	$ε = - N \frac{d Φ}{d t}$
Capacitance of a parallel plate capacitor	$C = \frac{ϵ _{0} A}{d}$
Speed of electromagnetic radiation	$c = \frac{1}{ϵ _{0} μ _{0}} = f λ$

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.