Lesson Note on Magnetic Field: Electromagnets, Screening and Applications for SSS 2

Class: Senior Secondary School 2 (SS2 / SSS2)
Term: Third Term
Week: 6
Age: 16 years
Duration: 45 minutes
Subject: Physics
Curriculum Theme: Magnetic Phenomena
Previous Lesson: Magnetic Field: Meaning, Field Around Magnets and Making Magnets.
Topic: MAGNETIC FIELD
Subject Matter: Magnetic properties of iron and steel (differences), magnetic screening or shielding (reducing field effect), electromagnets and application of electromagnet (lifting, relays, and more), temporary magnet (electric bell, telephone earpiece).

Specific Objectives

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

Cognitive Domain:

• Define magnetic field.
• Differentiate between the magnetic properties of iron and steel.
• Explain the concept of magnetic screening or shielding.
• Describe the working principle of an electromagnet.
• List various applications of electromagnets.

Affective Domain:

• Appreciate the importance of magnetic shielding in protecting sensitive equipment.
• Show interest in the practical applications of electromagnets in daily life.

Psychomotor Domain:

• Investigate and observe the magnetic field around a current-carrying conductor.
• Identify components and sketch simple diagrams of an electromagnet and its applications.

Social Domain:

• Collaborate with peers in observing and discussing magnetic phenomena.

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
• https://www.physicsclassroom.com/class/circuits/Lesson-4/Magnetic-Field-Around-a-Current-Carrying-Wire
• https://www.britannica.com/science/electromagnet

Instructional Materials

The teacher will teach this lesson with the aid of:

• Textbook
• Charts showing magnetic fields and electromagnets
• Equipped laboratory with a compass needle
• Iron filings
• Insulated copper wire
• Battery
• Switch
• Soft iron core
• Electric bell (model)
• Telephone earpiece (dissected model)

Rationale for the Lesson

This lesson helps pupils understand the fundamental principles of magnetism and electromagnetism, which are essential in many technological devices. It enables them to grasp how materials respond to magnetic fields and how temporary magnets are used in various practical applications.

Prerequisite/Previous Knowledge

Pupils have a basic understanding of magnets, magnetic poles, and simple electric circuits.

Lesson Content/Board Summary

MAGNETIC FIELD

Magnetic Field around a Current-Carrying Conductor

A magnetic field is produced around a conductor when an electric current flows through it. The direction of the magnetic field lines can be determined using the right-hand grip rule, where the thumb points in the direction of the current and the curled fingers indicate the direction of the magnetic field lines.

Magnetic Properties of Iron and Steel

Magnetic materials can be classified based on how easily they are magnetized and demagnetized, and how well they retain magnetism.

Soft Magnetic Materials (e.g., Soft Iron):

• Easily magnetized and demagnetized.
• Have low retentivity (do not retain magnetism strongly).
• Have low coercivity (require little energy to demagnetize).
• Used for temporary magnets, transformer cores, and electromagnets.

Hard Magnetic Materials (e.g., Steel):

• Difficult to magnetize and demagnetize.
• Have high retentivity (retain magnetism strongly).
• Have high coercivity (require significant energy to demagnetize).
• Used for permanent magnets.

Magnetic Screening or Shielding

Magnetic screening or shielding is the process of protecting a region or an object from external magnetic fields. This is achieved by enclosing the region or object within a material that can easily be magnetized, such as a soft iron box or ring.

• Soft iron diverts the magnetic field lines through itself, preventing them from penetrating the enclosed space.
• It is used to protect sensitive electronic instruments from unwanted magnetic interference.

Electromagnets

An electromagnet is a temporary magnet formed when an electric current passes through a coil of wire (solenoid) wound around a soft iron core. The soft iron core enhances the magnetic field produced by the coil.

Factors Affecting the Strength of an Electromagnet:

• Number of turns: More turns increase strength.
• Current: Higher current increases strength.
• Type of core material: Soft iron core is best for strong temporary magnets.

Applications of Electromagnets:

• Lifting magnets: Used in scrap yards to lift heavy iron and steel objects.
• Magnetic relays: Used to switch high currents with a small current.
• Electric bells: An electromagnet attracts an armature, which strikes a gong.
• Telephone earpieces: An electromagnet causes a diaphragm to vibrate, producing sound.
• Circuit breakers: Used to automatically switch off circuits when current exceeds a safe limit.

Temporary Magnets

Electromagnets are examples of temporary magnets because they are only magnetized when current flows through their coil. Once the current is switched off, they lose their magnetism almost instantly.

Working Principle of an Electric Bell:

When the switch is pressed, current flows through the electromagnet, which attracts a soft iron armature. The hammer attached to the armature strikes the gong, producing sound. As the armature moves, it breaks the circuit, demagnetizing the electromagnet. The armature springs back, closing the circuit again, and the process repeats rapidly.

Working Principle of a Telephone Earpiece:

Varying electric currents (representing sound waves) pass through the coils of an electromagnet in the earpiece. The electromagnet’s strength varies, causing a thin soft iron diaphragm to vibrate at the same frequency as the incoming electrical signals. These vibrations produce sound waves heard by the listener.

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 reviews the previous lesson on basic magnetism and asks pupils to identify common magnetic materials. The teacher then introduces the topic of magnetic fields and their applications.
Pupils’ Activity: Pupils recall previous knowledge about magnets and listen attentively to the introduction.
Learning Point: Pupils connect prior knowledge to the new topic and understand the relevance of magnetic fields.

Step 2: Magnetic Field around a Current-Carrying Conductor

Time: 7 minutes
Teaching Skill: Demonstration/Investigation
Teacher’s Activity: The teacher guides pupils to set up an experiment to investigate the magnetic field around a current-carrying conductor using a compass needle and iron filings. The teacher explains observations.
Pupils’ Activity: Pupils observe the setup and the behavior of the compass needle and iron filings, noting the pattern of the magnetic field.
Learning Point: Pupils understand that current-carrying conductors produce magnetic fields.

Step 3: Magnetic Properties of Iron and Steel

Time: 7 minutes
Teaching Skill: Explanation/Discussion
Teacher’s Activity: The teacher explains the differences in magnetic properties between soft iron and steel, focusing on their ease of magnetization/demagnetization, retentivity, and coercivity.
Pupils’ Activity: Pupils listen, ask questions, and take notes on the distinguishing characteristics of iron and steel as magnetic materials.
Learning Point: Pupils can differentiate between soft and hard magnetic materials and their uses.

Step 4: Magnetic Screening or Shielding

Time: 6 minutes
Teaching Skill: Explanation
Teacher’s Activity: The teacher explains the concept of magnetic screening or shielding, how it is achieved using soft iron, and its practical importance in protecting sensitive instruments.
Pupils’ Activity: Pupils listen and understand how magnetic fields can be reduced or diverted from specific areas.
Learning Point: Pupils learn about methods to protect objects from external magnetic fields.

Step 5: Electromagnets

Time: 7 minutes
Teaching Skill: Demonstration/Explanation
Teacher’s Activity: The teacher demonstrates a simple electromagnet, explaining its construction, how it works, and the factors that affect its strength (number of turns, current, core material).
Pupils’ Activity: Pupils observe the demonstration, ask questions, and learn the principles of electromagnetism.
Learning Point: Pupils understand how to create and control temporary magnets.

Step 6: Applications of Electromagnets

Time: 6 minutes
Teaching Skill: Discussion/Illustration
Teacher’s Activity: The teacher discusses various applications of electromagnets such as lifting magnets, relays, electric bells, and telephone earpieces, explaining the basic working principle of each.
Pupils’ Activity: Pupils identify and describe the functions of electromagnets in different devices.
Learning Point: Pupils relate the concept of electromagnets to real-world technologies.

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. Define a magnetic field.
2. State two differences between the magnetic properties of soft iron and steel.
3. Explain what magnetic screening is and why it is important.
4. List three applications of electromagnets.
5. Describe briefly how an electric bell works.

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 summarizes the key learning points on magnetic properties, shielding, and electromagnets. The teacher then assigns homework, which may include drawing and labeling the components of an electric bell.
Pupils’ Activity: Pupils listen to the summary and copy down the assigned homework.
Learning Point: Pupils consolidate their understanding of the lesson and prepare for further study.

Lesson Keywords

• Magnetic field – The region around a magnet or current-carrying conductor where magnetic force is exerted.
• Electromagnet – A temporary magnet created by passing electric current through a coil of wire wound around a soft iron core.
• Retentivity – The ability of a material to retain magnetism after the magnetizing field is removed.
• Coercivity – The resistance of a magnetic material to demagnetization.
• Magnetic shielding – The process of diverting magnetic field lines around a sensitive area or object.
• Solenoid – A coil of wire that acts as a magnet when current flows through it.

Differentiation

For pupils who grasp concepts quickly, the teacher can provide additional questions on the design of more complex electromagnetic devices. For those needing more support, the teacher will provide simplified diagrams and explanations, focusing on identifying key components and basic functions.

Note for teachers using this lesson plan

Ensure all necessary apparatus for the demonstration are available and in working condition. Encourage pupils to actively participate in the investigation of magnetic fields. Emphasize the practical relevance of electromagnets by relating them to everyday devices. Reinforce the differences between permanent and temporary magnets clearly.