Physics · Year 11 · Magnetism and Electromagnetism · Spring Term

Permanent Magnets and Magnetic Fields

Students explore the properties of permanent magnets, mapping magnetic field lines and understanding magnetic poles.

• National Curriculum Attainment Targets
• GCSE: Physics - Magnetism and Electromagnetism
• GCSE: Physics - Magnetic Fields

About This Topic

Magnetic Fields and Electromagnets explores the properties of permanent magnets and the creation of temporary magnetic fields using electricity. Students learn to map field lines around bar magnets and solenoids, understanding how field strength is affected by current, the number of turns in a coil, and the presence of an iron core. This topic is a prerequisite for understanding the motor effect and electromagnetic induction.

The ability to control magnetism with electricity is a cornerstone of modern technology, from simple door latches to MRI scanners. In the GCSE curriculum, students must be able to describe these fields and explain how they can be manipulated for industrial use. This topic comes alive when students can physically model the patterns, using iron filings or plotting compasses to visualize the invisible magnetic flux surrounding their own hand-built electromagnets.

Key Questions

1. Explain the concept of magnetic poles and their interactions.
2. Analyze the pattern of magnetic field lines around different types of magnets.
3. Construct a magnetic field map using a compass and iron filings.

Learning Objectives

• Identify the two types of magnetic poles and predict the force between like and unlike poles.
• Map the direction and pattern of magnetic field lines around a bar magnet and a horseshoe magnet using a plotting compass.
• Construct a visual representation of a magnetic field using iron filings and explain the significance of the pattern observed.
• Compare and contrast the magnetic field patterns of a bar magnet and a horseshoe magnet.

Before You Start

Introduction to Forces

Why: Students need a basic understanding of forces, including attraction and repulsion, to grasp the interactions between magnetic poles.

States of Matter

Why: Understanding that iron filings are solid and how they behave when acted upon by an external influence is helpful for visualizing field lines.

Key Vocabulary

Magnetic Pole	The two ends of a magnet, designated North and South, where the magnetic force is strongest. Like poles repel each other, and unlike poles attract.
Magnetic Field	The region around a magnet where a magnetic force can be detected. It is represented by invisible lines of flux.
Magnetic Field Lines	Imaginary lines used to represent the strength and direction of a magnetic field. They emerge from the North pole and enter the South pole.
Iron Filings	Small particles of iron that align themselves with the magnetic field lines when sprinkled around a magnet, making the field visible.
Plotting Compass	A small magnetic compass used to trace the direction of magnetic field lines by observing where its North pole points at different locations around a magnet.

Watch Out for These Misconceptions

Common Misconception

Magnetic field lines start at one pole and end at the other.

What to Teach Instead

Field lines are continuous loops, though we represent them as going from North to South externally. Using a clear 3D magnetic field viewer with iron filings in oil helps students see the field as a complete, three-dimensional volume.

Common Misconception

All metals are magnetic.

What to Teach Instead

Only ferromagnetic materials like iron, nickel, and cobalt are magnetic. A quick 'sorting' activity with various metal samples (aluminum, copper, brass, steel) helps students identify which materials actually interact with a magnetic field.

Active Learning Ideas

Inquiry Circle: The Electromagnet Strength Test

Teams build their own electromagnets and systematically vary the number of coils and the current. They measure the strength by the number of paperclips lifted, plotting their results to find the mathematical relationship between the variables.

50 min·Small Groups

Gallery Walk: Mapping the Invisible

Students use plotting compasses to map the magnetic field around different configurations: a single bar magnet, two attracting magnets, and a solenoid. They must leave their 'maps' at stations for other groups to critique and improve.

40 min·Small Groups

Think-Pair-Share: The Magnetic Scrap Yard

Students are shown a video of a scrap yard crane. They must discuss with a partner why an electromagnet is used instead of a permanent magnet and explain the physics of how the crane 'drops' the metal, then share with the class.

20 min·Pairs

Real-World Connections

• Geophysicists use magnetometers to map the Earth's magnetic field, which protects us from solar radiation and aids navigation. Understanding field lines helps them study geological formations and the planet's core.
• Engineers designing magnetic levitation (maglev) trains utilize precise control over magnetic fields. They must understand pole interactions and field line patterns to create the powerful forces needed for frictionless, high-speed travel.

Assessment Ideas

Quick Check

Provide students with a diagram showing two bar magnets. Ask them to draw the magnetic field lines between the magnets in two scenarios: first, when the North pole of one magnet faces the South pole of the other, and second, when two North poles face each other. They should label the direction of the field lines.

Exit Ticket

On one side of an index card, have students draw a horseshoe magnet and sketch its magnetic field lines, indicating the direction. On the other side, ask them to write one sentence explaining why iron filings form a visible pattern around a magnet.

Discussion Prompt

Pose the question: 'If you broke a magnet in half, what would happen to its poles?' Facilitate a class discussion where students use their understanding of magnetic poles and field lines to explain why each piece would still have both a North and a South pole.

Frequently Asked Questions

How does a solenoid create a magnetic field?

When current flows through a wire, it creates a circular magnetic field around it. By coiling the wire into a solenoid, these individual fields align and add together, creating a strong, uniform magnetic field through the center of the coil, similar to a bar magnet.

What is the role of an iron core in an electromagnet?

An iron core is a soft magnetic material that becomes magnetized when the current is turned on. It concentrates the magnetic field lines, significantly increasing the overall strength of the electromagnet compared to an air-cored coil.

What is the difference between a hard and soft magnetic material?

Hard magnetic materials (like steel) stay magnetized once the external field is removed, making them suitable for permanent magnets. Soft magnetic materials (like pure iron) lose their magnetism quickly, making them ideal for electromagnets.

How can active learning help students understand magnetic fields?

Magnetic fields are abstract because they are invisible. Active learning strategies like using plotting compasses to 'trace' the field lines allow students to discover the shape of the field for themselves. This discovery-led approach makes the standard diagrams in textbooks much more meaningful and easier to reconstruct during exams.