Lesson Note on Earth’s Magnetic Field: Declination, Dip and Compass Use for SSS 2

Class: Senior Secondary School 2 (SS2 / SSS 2)
Term: 3rd Term
Week: 7
Age: 16 years
Duration: 45 minutes
Subject: Physics
Curriculum Theme: Magnetism and Electromagnetism
Previous Lesson: Magnetic Field: Electromagnets, Screening and Applications.
Topic: MAGNETIC FIELD
Subject Matter: The earth’s magnetic field (basic idea), magnetic elements of a place (angle of declination, angle of dip, horizontal component of earth’s field), bar magnet in earth’s field (neutral point), mariner’s compass (use and principle)

Specific Objectives

By the end of the lesson, pupils should be able to:

Cognitive Domain:

• Explain the basic idea of the Earth’s magnetic field.
• Define angle of declination, angle of dip, and horizontal component of the Earth’s magnetic field.
• Describe the concept of neutral points when a bar magnet is placed in the Earth’s magnetic field.
• State the principle and use of a mariner’s compass.

Affective Domain:

• Appreciate the importance of the Earth’s magnetic field for life on Earth.
• Show interest in understanding magnetic phenomena and their applications.

Psychomotor Domain:

• Suspend a bar magnet to locate the Earth’s North-South direction.
• Draw simple diagrams illustrating magnetic declination and dip.
• Demonstrate the correct use of a mariner’s compass.

Social Domain:

• Discuss with peers the applications of magnetism in everyday life.
• Collaborate in identifying the Earth’s magnetic North-South direction.

Reference Materials

The following resources were used in planning this lesson:

• Senior Secondary Schools Education Curriculum
• State Unified Scheme of Work
• New School Physics for Senior Secondary Schools

Instructional Materials

The teacher will teach this lesson with the aid of:

• Textbook
• Charts showing the Earth’s magnetic field lines
• A freely suspended bar magnet
• A mariner’s compass
• Drawing materials (pen, paper, protractor)

Rationale for the Lesson

This lesson helps pupils understand the invisible forces of magnetism, particularly the Earth’s magnetic field, which is important for protecting our planet. It enables pupils to grasp fundamental concepts like declination and dip, and the practical application of a compass in navigation.

Prerequisite/Previous Knowledge

Pupils should have prior knowledge of magnets, magnetic poles, magnetic fields, and magnetic field lines from previous lessons.

Lesson Content/Board Summary

The Earth’s Magnetic Field

Basic Idea of the Earth’s Magnetic Field

The Earth acts like a giant bar magnet, producing its own magnetic field. This field originates from convection currents of molten iron and nickel in the Earth’s outer core. The Earth’s magnetic North pole is geographically near the South pole, and the magnetic South pole is near the geographic North pole. This field protects the Earth from harmful solar radiation.

Magnetic Elements of a Place

The Earth’s magnetic field at any point can be described by three quantities known as the magnetic elements:

• Angle of Declination (Magnetic Declination): This is the angle between the geographic meridian (true North-South line) and the magnetic meridian (direction shown by a freely suspended magnet) at a particular place. It indicates how far off a compass reading is from true North.
• Angle of Dip (Magnetic Dip or Inclination): This is the angle that the Earth’s resultant magnetic field vector makes with the horizontal at a particular place. At the magnetic poles, the dip angle is 90°, and at the magnetic equator, it is 0°.
• Horizontal Component of Earth’s Magnetic Field (H): This is the component of the Earth’s total magnetic field that acts horizontally at a given location. It is responsible for aligning a compass needle horizontally. The vertical component is responsible for the dip. The total magnetic field (B) is related to the horizontal (H) and vertical (V) components by B² = H² + V².

Bar Magnet in Earth’s Field (Neutral Point)

A neutral point is a point in a magnetic field where the resultant magnetic field strength is zero. This occurs when the magnetic field of a bar magnet exactly cancels out the Earth’s magnetic field. Neutral points are found where the magnetic field lines of the bar magnet are equal in magnitude and opposite in direction to the Earth’s horizontal magnetic field.

Common configurations for neutral points:

• Bar magnet with N-pole facing geographic North: Neutral points are found on the axial line (extension of the magnet’s axis).
• Bar magnet with S-pole facing geographic North: Neutral points are found on the equatorial line (perpendicular bisector of the magnet).

Mariner’s Compass

A mariner’s compass is a device used for navigation and direction finding. It consists of a small, freely pivoted magnetic needle that aligns itself with the Earth’s magnetic field, pointing towards the magnetic North pole.

• Principle: The magnetic needle is a small magnet that experiences a torque due to the Earth’s magnetic field, causing it to rotate until it aligns with the magnetic field lines.
• Use: It is used by sailors, hikers, and aviators to determine direction, particularly the magnetic North, which helps in plotting courses and navigating.

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher asks pupils what they know about magnets and how a compass works, linking it to the Earth’s magnetic field.
Pupils’ Activity: Pupils share their prior knowledge about magnets and compasses.
Learning Point: Pupils recall previous knowledge about magnetism and are introduced to the lesson topic.

Step 2: Explanation of Earth’s Magnetic Field

Time: 8 minutes
Teaching Skill: Explanation/Illustration
Teacher’s Activity: The teacher explains the basic idea of the Earth acting as a giant magnet, its origin, and its importance, using charts to illustrate the field lines.
Pupils’ Activity: Pupils listen attentively, observe the charts, and ask questions for clarification.
Learning Point: Pupils understand the fundamental concept and importance of the Earth’s magnetic field.

Step 3: Magnetic Elements – Angle of Declination

Time: 7 minutes
Teaching Skill: Demonstration/Explanation
Teacher’s Activity: The teacher guides pupils on how to suspend a bar magnet horizontally to locate the Earth’s N-S direction and explains the angle of declination with a simple diagram.
Pupils’ Activity: Pupils participate in suspending the magnet and discuss their observations. They define angle of declination.
Learning Point: Pupils learn to identify the magnetic North and understand the concept of magnetic declination.

Step 4: Magnetic Elements – Angle of Dip and Horizontal Component

Time: 7 minutes
Teaching Skill: Explanation/Illustration
Teacher’s Activity: The teacher explains the angle of dip and the horizontal component of the Earth’s magnetic field, using diagrams to show their relationship.
Pupils’ Activity: Pupils take notes and ask questions about the definitions and diagrams.
Learning Point: Pupils understand the concepts of magnetic dip and the horizontal component.

Step 5: Bar Magnet in Earth’s Field (Neutral Point)

Time: 6 minutes
Teaching Skill: Concept Introduction
Teacher’s Activity: The teacher explains what a neutral point is, how it is formed, and describes its characteristics when a bar magnet is placed in the Earth’s magnetic field.
Pupils’ Activity: Pupils listen and try to visualize the magnetic field interactions.
Learning Point: Pupils grasp the concept of neutral points and conditions for their formation.

Step 6: Mariner’s Compass

Time: 5 minutes
Teaching Skill: Demonstration/Application
Teacher’s Activity: The teacher demonstrates the use of a mariner’s compass and explains its principle and application in navigation.
Pupils’ Activity: Pupils observe the compass demonstration and state its uses.
Learning Point: Pupils understand how a compass works and its practical importance.

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. Explain the basic idea of the Earth’s magnetic field.
2. Define angle of declination and angle of dip.
3. What is a neutral point in the context of a bar magnet in the Earth’s magnetic field?
4. State the principle of operation of a mariner’s compass.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 2 minutes
Teaching Skill: Summarization
Teacher’s Activity: The teacher briefly summarizes the key concepts covered in the lesson and assigns homework.
Pupils’ Activity: Pupils listen and copy down the homework assignment.
Learning Point: The lesson is reinforced, and pupils are prepared for further study.

Lesson Keywords

• Magnetic Field – The region around a magnet or a current-carrying conductor where magnetic forces are exerted.
• Earth’s Magnetic Field – The natural magnetic field generated by the Earth’s core.
• Magnetic Declination – The angle between true North and magnetic North.
• Magnetic Dip – The angle between the Earth’s magnetic field and the horizontal.
• Horizontal Component – The horizontal part of the Earth’s total magnetic field.
• Neutral Point – A point where the resultant magnetic field strength is zero.
• Mariner’s Compass – A device used for navigation that aligns with the Earth’s magnetic field.

Differentiation

For struggling learners, the teacher will use more visual aids and simplified explanations, focusing on defining key terms and identifying the main components. Advanced learners will be encouraged to research further into the origin of the Earth’s magnetic field or explore calculations involving the magnetic elements.

Note for teachers using this lesson plan

Ensure that the bar magnet used for demonstration is properly demagnetized or has clear poles. When demonstrating the compass, ensure there are no strong magnetic materials nearby that could interfere with its readings. Encourage pupils to draw and label diagrams for better understanding of the magnetic elements.