Simple phenomena of magnetism
Learning Objective 1: Describe the forces between magnetic poles and between magnets and magnetic materials.
Every magnet has two poles: a North pole (N) and a South pole (S). These are the regions where the magnetic force is strongest.
The Fundamental Law of Magnetism:
- Like poles repel each other (N repels N, S repels S).
- Unlike poles attract each other (N attracts S).
This applies to interactions between two magnets. However, magnets also interact with magnetic materials (like iron, nickel, and cobalt). A magnet will always attract a magnetic material, regardless of which pole is used. This is because the magnetic material becomes temporarily magnetised by induction.
Magnetised vs. Unmagnetised:
- A magnetised object produces its own persistent magnetic field and can attract/repel other magnets.
- An unmagnetised magnetic material (like a soft iron nail) does not produce its own field but will be attracted to a magnet due to induced magnetism.
Induced magnetism is the process by which a magnetic material (such as soft iron) becomes a temporary magnet when placed in a magnetic field.
When a piece of unmagnetised magnetic material is brought near a permanent magnet, the magnetic domains within the material align with the external field. This causes the end of the material closest to the magnet's pole to become an opposite pole (e.g., if the North pole of a magnet is brought near iron, the near end of the iron becomes a South pole). This creates an attractive force.
Once the external magnet is removed, the domains in soft magnetic materials randomise again, and the induced magnetism is lost.
Learning Objective 3: State the differences between the properties of temporary magnets (made of soft iron) and the properties of permanent magnets (made of steel).
The key difference lies in retentivity (the ability to retain magnetism).
| Property | Temporary Magnets (Soft Iron) | Permanent Magnets (Steel) |
|---|---|---|
| Magnetisation | Easily magnetised by an external field. | Harder to magnetise. |
| Demagnetisation | Loses magnetism immediately when the external field is removed. | Retains magnetism for a long time (hard demagnetised). |
| Typical Use | Cores of electromagnets, transformer cores. | Compass needles, loudspeakers, holding magnets. |
Learning Objective 4: State the difference between magnetic and non-magnetic materials.
- Magnetic materials (e.g., iron, nickel, cobalt) are attracted to magnets and can be magnetised (experience induced magnetism).
- Non-magnetic materials (e.g., wood, plastic, copper, aluminium) are not attracted to magnets and do not become magnetised because their atomic domains cannot align with an external magnetic field.
A magnetic field is defined as a region in which a magnetic pole experiences a force. It is the space around a magnet where its influence can be detected.
Learning Objective 7: State that the direction of a magnetic field at a point is the direction of the force on the N pole of a magnet at that point.
The direction of the magnetic field vector at any point is defined as the direction in which the North pole of a small compass needle would point if placed at that location. Therefore, field lines always emerge from the North pole and enter the South pole.
Learning Objective 6: Draw the pattern and direction of magnetic field lines around a bar magnet.
Field lines are imaginary lines used to visualise the magnetic field. For a bar magnet:
- Lines emerge from the North pole and enter the South pole.
- Outside the magnet, lines curve from N to S.
- Inside the magnet, lines run from S to N (forming closed loops).
- Lines never cross each other.
- Lines are closest together at the poles, indicating the strongest field.
Learning Objective 11: Know that the relative strength of a magnetic field is represented by the spacing of the magnetic field lines.
The density (spacing) of the field lines represents the strength of the magnetic field:
- Close together: Strong magnetic field (e.g., near the poles).
- Far apart: Weak magnetic field (e.g., further away from the magnet).
Note: The spacing is a visual convention used to represent relative field strength; closer lines indicate a stronger field, while wider spacing indicates a weaker field.
Learning Objective 8: Describe the plotting of magnetic field lines with a compass or iron filings and the use of a compass to determine the direction of the magnetic field.
To map the field pattern:
- Place a bar magnet on a sheet of paper.
- Using Iron Filings: Sprinkle iron filings evenly over the paper. Tap the paper gently. The filings align with the field lines, revealing the pattern.
- Using a Plotting Compass: Place a small compass near one pole. Mark two dots at the ends of the needle. Move the compass so the tail of the needle is on the second dot. Mark the new head position. Repeat until the other pole is reached. Join the dots to form a smooth curve.
- Direction: The arrow on the compass needle points in the direction of the magnetic field (from N to S outside the magnet).
Learning Objective 9: Describe the uses of permanent magnets and electromagnets.
- Permanent Magnets: Used where a constant magnetic field is needed without power, e.g., compasses, electric motors (stator), loudspeakers, fridge door seals.
- Electromagnets: Used where the magnetic field needs to be switched on/off or varied in strength, e.g., cranes lifting scrap metal, relays, circuit breakers, MRI machines. They typically use a soft iron core because it magnetises and demagnetises quickly.
Magnetic forces are not 'action-at-a-distance' but result from the interaction (superposition) of the two magnetic fields in the space between the magnets.
When two magnets are brought near each other, their individual magnetic fields overlap.
- If the fields reinforce each other (lines flow continuously from one N to the other S), the interaction results in attraction.
- If the fields oppose each other (lines push against each other between like poles), the interaction results in repulsion.
Therefore, magnetic forces are due to interactions between magnetic fields.
Correction: Steel is the correct material for permanent magnets because it is 'hard' magnetic material (high retentivity). Soft iron is 'soft' magnetic material; it loses its magnetism immediately when the external field is removed, so it cannot be a permanent magnet. Iron is only suitable for the core of an electromagnet.
Correction: Outside the magnet, field lines always go from North to South. The direction of the field at any point is the direction of the force on a North pole placed there. A compass needle's North end points along the field line.
Why examiners accept this: The definition must specify the region and the force on a pole. Vague answers like 'area around a magnet' are insufficient.
Correct phrasing: "A region in which a magnetic pole experiences a force." or "A region in which a magnetic material experiences a force."
Example:
Q: Define magnetic field.
A: A region where a magnetic pole experiences a force.
Why examiners accept this: They look for specific procedural steps: using a compass, marking positions, and moving the compass. Simply saying 'use a compass' is often too brief for full marks.
Correct phrasing: "Place a plotting compass near the magnet. Mark the position of the needle ends. Move the compass so one end is on the previous mark. Repeat to trace the line." or "Sprinkle iron filings on paper over the magnet and tap gently."
Example:
Q: Describe how you would determine the pattern of magnetic field lines around a bar magnet.
A: Place a sheet of paper over the magnet. Sprinkle iron filings evenly. Tap the paper to allow filings to align with the field. The filings form visible lines showing the pattern.