Grade 10 electricity Electrical Machines – Magnetism Notes

Specific Learning Outcomes

1. Explain the magnetic properties of materials (ferromagnetic, paramagnetic, diamagnetic, permeability, coercivity, retentivity).
2. Perform magnetisation and demagnetisation procedures of magnetic materials safely.
3. Draw and describe magnetic field patterns around magnets and wires carrying current.
4. Care for magnets used in equipment at a workplace.
5. Appreciate applications of magnets in day-to-day life and in electrical machines.
6. Identify the five categories of magnetism learning: properties, magnetisation/demagnetisation, field patterns, magnet care, and applications.

Key concepts (simple and local examples)

• Ferromagnetic: Strongly magnetic materials (iron, steel, nickel, cobalt). Common in tools, nails, motor cores.
• Paramagnetic: Weak attraction to magnets (aluminium, magnesium). Effect is tiny — not used for magnets.
• Diamagnetic: Weakly repelled by magnets (copper, bismuth, graphite).
• Permeability: How easily magnetic field lines pass through a material (iron has high permeability).
• Retentivity (remanence): How well a material retains magnetism after magnetising.
• Coercivity: The resistance of a magnetised material to being demagnetised.

Magnetic field patterns — visual examples

Use a small compass or iron filings to reveal the same shapes shown below.

Practical procedures (safe, classroom level)

A. Magnetisation (three simple methods)

1. Stroking method (for small steel tools):
   • Materials: strong bar magnet, soft steel screwdriver/needle, clean cloth.
   • Procedure: hold tool on a table, stroke it repeatedly from one end to the other with the same end of the magnet (always same direction). About 30–60 strokes.
   • Test with small pins or paperclips. If weak, repeat strokes in same direction.
2. Solenoid (electromagnet) method:
   • Materials: insulated copper wire, iron nail, battery (1.5–9 V), switch.
   • Procedure: wind many turns of wire tightly around nail, connect to battery briefly (a few seconds) so current flows in one direction. Remove power; nail becomes magnet.
   • Safety: do not keep battery connected long (wire heats). Use supervision.
3. Hammering in a magnetic field:
   • Materials: steel object, strong permanent magnet.
   • Procedure: hold the steel object near the magnet and gently tap along its length while keeping it aligned—this aligns domains to magnetise.

B. Demagnetisation (three classroom-safe ways)

1. AC coil method (recommended): place magnetised object in a coil connected to an AC source (or an AC electromagnet). Slowly withdraw the object while reducing current or move it out — the alternating field randomises domains and reduces magnetism.
2. Heat method (caution): heating above the Curie temperature removes magnetism. Only demonstration by teacher with proper equipment; many magnets lose strength when overheated.
3. Mechanical shock: strong hammer blows or bending can reduce retained magnetism — not recommended for precision parts or classroom objects unless supervised.

Care of magnets in the workplace

• Store magnets with a keeper (soft iron piece joining poles) to preserve strength and reduce stray fields.
• Keep magnets away from electronic devices (phones, cards, computers) and credit cards.
• Avoid high temperatures and strong impacts (dropping or hammering) — both reduce magnet strength.
• Label strong magnets and provide instructions/warnings where used (pinch hazard).
• Use gloves for large magnets to avoid pinch injuries and eye protection if handling brittle magnets that may chip.

Applications (everyday and electrical machines)

• Everyday: fridge magnets, magnetic clasps, compasses for navigation, magnetic fasteners in bags.
• Household appliances: loudspeakers (voice coil + magnet), electric shavers, magnetic sensors in doors.
• Electrical machines & transport: DC and AC motors, generators, transformers (use magnetic cores), relays, speakers in radios.
• Industrial/medical: cranes (lifting scrap metal), magnetic separation in recycling, MRI machines (advanced).
• Local example: small electric motors used in boda-boda/headlight motors, water pump motors and radios rely on magnetism in their design.

Identify the five categories of magnetism learning

When asked to identify topics related to magnetism, look for words or descriptions under these categories:

1. Magnetic properties (e.g., ferromagnetic, permeability, coercivity).
2. Magnetisation / demagnetisation procedures (e.g., stroking, solenoid, AC demag).
3. Magnetic field patterns (diagrams or compass tests showing field lines).
4. Magnet care (storage, temperature, handling safety).
5. Applications (in daily life and electrical machines).

Suggested Learning Experiences (Activities for the classroom)

1. Demonstration: Teacher shows iron filings or a compass around a bar magnet to reveal field lines. Pupils draw what they observe.
2. Group practical: In groups of 3–4, students magnetise a nail with a coil and battery, then test it by lifting paperclips. Record results and explain why it worked.
3. Investigation: Compare magnets: test how many paperclips different materials (iron nail, steel bolt, aluminium) attract. Classify as ferromagnetic/other.
4. Magnet care role play: Students design a safe storage box for magnets used in a school workshop, label it and present reasons.
5. Field mapping: Using a small compass, map field lines around a bar magnet on paper by marking compass needle direction at multiple points.
6. Local application task: Ask learners to identify five devices at home or in the community that use magnets and explain the role of the magnet.
7. Assessment & reflection: Short quiz on definitions and a practical test: magnetise/demagnetise a small object correctly under supervision.

Safety & teacher notes

• Always supervise experiments with electricity and heating. Use low-voltage batteries for student activities.
• Warn students about pinch hazards from strong magnets and the risk to electronic items.
• Relate lessons to Kenyan context: talk about local tools, motors in water pumps, and how magnetic separators are used in recycling centres.

Quick formative assessment (examples)

1. Explain why iron is used in transformer cores (answer: high permeability concentrates magnetic field and increases flux).
2. Draw the magnetic field pattern around a bar magnet and label N and S.
3. List two safe ways to demagnetise a steel object and explain when you would use each.
4. Give three examples of devices at home or school that use magnets and state the function of the magnet in each.