Lesson 7.1 • Competency 7.2Complete Lesson

Magnetic Fields & Terrestrial Magnetism

Magnetic flux Φ and flux density B = Φ/A (Tesla), magnetic field line geometry, neutral points between interacting fields, and Earth’s magnetic field elements (declination, inclination/dip, horizontal and vertical components).

Marking a lesson complete represents finishing your study of the topic; it does not denote tested mastery.

Learning Objectives

Define magnetic flux Φ (Weber) and magnetic flux density B (Tesla), solving problems using Φ = B · A cos θ.
Map magnetic field lines, locate neutral points where the external and local magnetic fields cancel, and explain why field lines form continuous closed loops.
Define magnetic meridian, geographic meridian, angle of declination, and angle of dip θ, deriving relations B_H = B cos θ, B_V = B sin θ, and tan θ = B_V / B_H.

Core Theory & Concepts

A magnetic field is a region of space around a magnet, electric current, or moving charge where magnetic forces are exerted. The Earth behaves as a giant magnetic dipole tilted roughly 11° from its rotational axis, providing a crucial terrestrial reference field for navigation and scientific instrumentation.

Magnetic Flux & Flux Density

Magnetic flux Φ measures the total field passing through an area: Φ = B · A = B A cos θ. Flux density B represents flux per unit normal area: B = Φ / A.

\Phi = B A \cos\theta \quad \Longleftrightarrow \quad B = \frac{\Phi}{A}

Earth’s Field Components & Dip Angle

At any point on Earth, the total field B resolves into horizontal component B_H and vertical component B_V: B_H = B cos θ, B_V = B sin θ, tan θ = B_V / B_H.

B_H = B \cos\theta, \quad B_V = B \sin\theta, \quad \tan\theta = \frac{B_V}{B_H}

Common Misconceptions

Misconception

A magnetic compass needle points towards True Geographic North.

Physical Clarification:The compass needle aligns with the local magnetic meridian, which deviates from true geographic north by the angle of magnetic declination. Furthermore, near the magnetic poles, vertical dip renders a horizontal compass useless.

Syllabus Topic Outline

Competency 7.2

1Concept of magnetic poles, field lines, and flux Φ (Weber)
2Magnetic flux density B = Φ / A (Tesla, T = Wb m⁻²)
3Bar magnets and neutral points in Earth’s magnetic field
4Terrestrial magnetism: Geographic vs Magnetic Meridians
5Magnetic elements: Declination, Dip (inclination), and components B_H, B_V