P26: Magnets and Magnetic Fields
Permanent and induced magnets, magnetic fields
Magnets And Magnetic Fields
📋 Key Definitions
Permanent magnet: A magnet that produces its own magnetic field all the time. It always has a north pole and a south pole. Examples: bar magnets, fridge magnets, compass needles.
Induced magnet: A material that becomes magnetic when placed in the magnetic field of a permanent magnet. It loses most or all of its magnetism when removed from the field. Induced magnets are always attracted towards the permanent magnet (never repelled).
Magnetic field: The region around a magnet where a magnetic force can be experienced. The field is strongest near the poles of the magnet.
Magnetic field lines: Lines that show the direction and strength of a magnetic field. They always go from north to south (outside the magnet). The closer the lines, the stronger the field.
📝 Types of Magnets
| Property | Permanent Magnet | Induced Magnet |
|---|---|---|
| Produces own magnetic field? | Yes — always magnetic | No — only magnetic in another magnet's field |
| Can be switched off? | No | Yes — loses magnetism when removed from field |
| Attracted or repelled by permanent magnet? | Attracted OR repelled (depending on pole) | Only attracted (never repelled) |
| Examples | Bar magnet, compass needle, fridge magnet | Iron nail near a bar magnet, paper clip attracted to magnet |
Magnetic Materials
Only four elements are magnetic at room temperature:
- Iron — soft magnetic material (easily magnetised and demagnetised)
- Steel — hard magnetic material (an iron alloy; harder to magnetise but keeps magnetism)
- Nickel
- Cobalt
Important: Not all metals are magnetic. Copper, aluminium, gold and silver are NOT magnetic. Never assume a material is magnetic just because it is a metal.
🧲 Magnetic Poles
Rules for magnetic poles:
• Like poles repel (N–N or S–S)
• Unlike poles attract (N–S)
• A magnet always has two poles — it is impossible to have a single pole (a monopole). If you cut a magnet in half, you get two smaller magnets, each with N and S poles.
• Like poles repel (N–N or S–S)
• Unlike poles attract (N–S)
• A magnet always has two poles — it is impossible to have a single pole (a monopole). If you cut a magnet in half, you get two smaller magnets, each with N and S poles.
📝 Magnetic Fields
Field Line Rules
- Field lines always go from north to south outside the magnet.
- Field lines never cross.
- The closer together the lines, the stronger the magnetic field.
- The field is strongest at the poles of the magnet.
- Arrows on field lines show the direction a free north pole would move.
Plotting Magnetic Fields
Using iron filings: Sprinkle iron filings around a magnet on a flat surface. The filings align along the field lines, showing the shape of the magnetic field. This is quick but not precise.
Using a compass: Place a small plotting compass near the magnet. The compass needle points along the field line. Mark the position of each end of the needle. Move the compass so the tail of the needle is at the marked head position. Repeat to plot the complete field line. This method shows the direction of the field.
Field Between Two Magnets
- Unlike poles facing (N–S): Field lines connect from one magnet to the other. Strong, uniform field between the poles.
- Like poles facing (N–N or S–S): Field lines push apart. No field lines connect between the magnets — they repel.
🌍 Earth's Magnetic Field
Earth's magnetic field: The Earth's core acts like a giant magnet, creating a magnetic field that extends into space. The geographic North Pole is actually near the magnetic south pole (because the north pole of a compass points towards it — it attracts the north-seeking pole). Compasses align with Earth's field and point towards magnetic north.
Over geological time, the Earth's magnetic poles have reversed (swapped positions). This is called a magnetic reversal.
🧮 Worked Examples
Example 1: Identifying poles
Bar magnet A repels the north pole of bar magnet B. What is the pole of magnet A facing magnet B?
Solution:
Like poles repel. Since magnet B's north pole is repelled, magnet A must also have its north pole facing magnet B.
Example 2: Induced magnetism
An iron nail is placed near a bar magnet. It becomes magnetic and is attracted towards the magnet. Explain why it is attracted and not repelled.
Solution:
The iron nail is an induced magnet. When placed in the magnetic field of the permanent magnet, it becomes magnetised. Induced magnets are always attracted towards the permanent magnet — they are never repelled. This is because the end of the nail nearest the permanent magnet's pole becomes the opposite pole (e.g. if near the N pole, the nail's nearest end becomes S), and unlike poles attract.
Example 3: Plotting field lines
Describe how you would use a plotting compass to map the magnetic field around a bar magnet.
Solution:
1. Place the bar magnet on a sheet of paper and draw around it. Mark the N and S poles.
2. Place the plotting compass near the north pole and mark the position of each end of the needle with a dot.
3. Move the compass so the tail of the needle is at the position of the previous head dot. Mark the new head position.
4. Repeat until you reach the south pole. Join the dots with a smooth line and add an arrow pointing from N to S.
5. Repeat from different starting points to map several field lines.
Example 4: Magnetic vs non-magnetic
A student says "all metals are magnetic." Is this statement correct? Explain your answer.
Solution:
The statement is incorrect. Only four elements are magnetic at room temperature: iron, nickel, cobalt and steel (an iron alloy). Many common metals are non-magnetic, including copper, aluminium, gold, silver, zinc and lead. Being a metal does not mean a material is magnetic.
❓ Practice Questions
Q1: Foundation Name the four magnetic elements.
Q2: Foundation State the rule for what happens when two north poles are brought close together.
Q3: Higher Explain the difference between a permanent magnet and an induced magnet.
Q4: Foundation In which direction do magnetic field lines point?
Q5: Higher Describe how you would use iron filings and a plotting compass to show the shape and direction of the magnetic field around a bar magnet.
✅ Answers
- Q1: Iron, nickel, cobalt and steel (an alloy of iron).
- Q2: Like poles repel — two north poles will repel each other.
- Q3: A permanent magnet always produces its own magnetic field and always has north and south poles. An induced magnet only becomes magnetic when placed in the field of a permanent magnet and loses its magnetism when removed from that field. Induced magnets are always attracted to the permanent magnet, while permanent magnets can attract or repel each other depending on which poles are facing.
- Q4: Magnetic field lines point from north to south (outside the magnet).
- Q5: To show the shape: sprinkle iron filings evenly around the magnet on a flat surface. The filings will align along the field lines, showing the shape of the field. To show the direction: place a plotting compass near the north pole of the magnet. The compass needle points along the field line (its north-seeking end points from N to S). Mark the positions and move the compass along to trace the field line direction.
🎯 Exam Tips
- Remember: like poles repel, unlike poles attract — this comes up every year.
- Induced magnets are always attracted — they can never be repelled by a permanent magnet.
- Field lines go from north to south outside the magnet. Inside the magnet, they go from south to north to form complete loops.
- Field lines never cross — if you draw them crossing, you've made a mistake.
- Be careful with Earth's magnetic field: the geographic North Pole is actually a magnetic south pole (it attracts the north-seeking end of a compass needle).
- When asked to describe plotting a field, mention BOTH the compass method (for direction) and iron filings (for shape) if the question is open-ended.
🔢 Maths Skills
Mathematical Skills
Interpreting magnetic field diagrams: the closer the field lines, the stronger the field. Field line density is proportional to field strength. When comparing magnets, count the number of field lines per unit area near the poles to compare strengths qualitatively.
Maths Example
Two bar magnets are placed with unlike poles facing each other. The field lines between the poles are evenly spaced and parallel. If the distance between the poles is 5 cm and there are 10 field lines crossing a 2 cm wide region perpendicular to the field, the field line density is 10 ÷ 2 = 5 lines per cm. If the magnets are moved closer to 3 cm apart, the same number of lines now cross a narrower region, so the density increases to 10 ÷ (2 × 3/5) = 8.3 lines per cm, indicating a stronger field.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: All metals are magnetic Correct: Only iron, nickel, cobalt and steel (an iron alloy) are magnetic — copper, aluminium, gold and silver are NOT magnetic
2. Wrong: An induced magnet can be repelled by a permanent magnet Correct: Induced magnets are always attracted towards a permanent magnet — they can never be repelled because the nearest end always becomes the opposite pole
3. Wrong: If you cut a magnet in half, you get a separate north pole and a separate south pole Correct: Cutting a magnet in half produces two smaller magnets, each with its own north and south pole — magnetic monopoles do not exist
✍️ 6-Mark Question
Extended Answer
6 marks: Describe how you would use a plotting compass to map the magnetic field around a bar magnet. Explain what the field lines tell you about the strength and direction of the field.
Place the bar magnet on a sheet of paper and draw around it, marking the north and south poles. Place a small plotting compass near the north pole of the magnet. The compass needle aligns with the field line at that point. Mark the two ends of the needle with dots on the paper. Move the compass so that the tail end of the needle is at the position where the head was previously marked. Mark the new head position. Repeat this process, moving the compass along the field line, until you reach the south pole. Join the dots with a smooth curve and add an arrow pointing from north to south. Repeat from different starting points near the north pole to map several field lines. The field lines show the direction of the magnetic field — arrows point from north to south, showing the direction a free north pole would move. The strength of the field is shown by the spacing of the lines — where lines are closer together (near the poles), the field is stronger; where they are further apart, the field is weaker.
Mark scheme: 1 mark — place magnet on paper and draw around it, 1 mark — use compass to mark needle positions and move along the field line, 1 mark — join dots and add arrow from N to S, 1 mark — repeat from different starting points for several lines, 1 mark — direction shown by arrows (N to S), 1 mark — strength shown by line spacing (closer = stronger)
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A student tests whether different materials are magnetic by bringing each one close to a strong bar magnet. Results: iron nail — attracted, copper coin — not attracted, steel paper clip — attracted, aluminium can — not attracted, gold ring — not attracted, cobalt disc — attracted.
(a) List the materials that were attracted and identify the common property they share.
(b) The student says "the copper coin and aluminium can are not magnetic because they are not metals." Evaluate this statement.
(c) The student then brings the iron nail close to the north pole and it sticks. They turn the magnet around and bring the nail close to the south pole. Predict what happens and explain why.
Answers: (a) Iron, steel and cobalt were attracted. They are all magnetic materials. (b) The statement is incorrect — copper and aluminium ARE metals, but they are non-magnetic metals. Being a metal does not mean a material is magnetic. Only iron, nickel, cobalt and steel are magnetic. (c) The iron nail will also stick to the south pole. Iron is an induced magnet — it becomes magnetised when placed in any external magnetic field and is always attracted to the permanent magnet regardless of which pole is used. Unlike permanent magnets, induced magnets cannot be repelled.
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