4.1.4Electricity and Magnetism
Plotting Magnetic Fields — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 9 parts Lesson 8 of 65 1 quiz
Lesson summary
4.1.4 Plotting Magnetic Fields — IGCSE PHYSICS notes: Describe how to plot the magnetic field around a bar magnet with iron filings and with a plotting compass.Iron filings — show the whole pattern at oncePlotting compass — shows the field direction point by pointPlace a piece of…
- Why it works
- Limitation
- No magnet nearby
- Near the magnet
- Far from the magnet
- Neutral point
Part 1 of 7Two methods
Iron filings
- Iron filings show the whole pattern at once
Plotting compass
- Plotting compass shows the field direction point by point
Part 2 of 7Plotting with iron filings
Plotting with iron filings
Place a piece of paper or card on top of the magnet.
Gently sprinkle iron filings thinly and evenly on the paper.
Tap the paper gently so the filings settle along the field lines.
Why it works
- Why it works each filing becomes a tiny induced magnet and lines up with the field
Pattern
- Pattern filings are dense where the field is strong
Limitation
- Limitation it does not show direction because filings have no north end
Part 3 of 7Plotting with a compass
1
Place the magnet on a piece of paper and mark its poles.
2
Draw a dot at one end of the magnet near its corner.
3
Place a plotting compass next to the dot so one end of the needle points towards it.
4
Draw a new dot on the other side of the compass needle.
5
Move the compass so it points towards the new dot and repeat.
6
Keep repeating until a chain of dots runs from one end of the magnet to the other.
7
Remove the compass and join the dots with a smooth curve.
8
Repeat from other starting points to draw more field lines.
- Direction
- the north end of the compass points along the field, from the north pole of the magnet to its south pole
Part 4 of 7The Earth's field and the compass
No magnet nearby
- No magnet nearby a plotting compass lines up with the Earth's field and points to geographic north
Near the magnet
- Near the magnet the magnet's field is much stronger than the Earth's, so the effect is small
Far from the magnet
- Far from the magnet the Earth's field can distort the plot
Neutral point
- Neutral point where a magnet's field cancels the Earth's field the compass points in no particular direction
Part 5 of 7Interpreting patterns
| Arrangement | Pattern |
|---|---|
| Single bar magnet | Curved lines from north to south, densest at the poles |
| Unlike poles facing | Lines cross the gap from north to south, nearly parallel in the middle |
| Like poles facing | Lines bend away from each other with a neutral point between |
| Soft iron ring in a field | Lines pass through the ring, leaving the space inside almost empty |
- Strength
- dots close together after many steps mean closely spaced lines and a strong field
Part 6 of 7Improving the plot
Dots
- Dots use a sharp pencil and mark dots as close to the compass ends as possible
Interference
- Interference keep other magnets and iron objects away
Several lines
- Several lines start from several points around the north pole
Smooth curve
- Smooth curve join dots smoothly and add an arrow for direction
Part 7 of 7More from the specification
More from the specification
Ferromagnetic Materials and Magnetic Induction
- Ferromagnetic metal materials,
- including iron, steel, nickel,
- and cobalt, experience an attractive magnetic force toward a magnet and can be induced to form temporary or permanent poles.
Permanent Magnets vs Electromagnets
- A permanent magnet retains its magnetization continuously,
- (commonly made of magnetically hard steel)
- whereas electromagnets utilize coils wound around a soft iron core that magnetizes only while carrying current.
Magnetic Poles and Field Directions
- Magnetic poles exert non-contact forces where like poles repel and opposite poles attract;
- field lines illustrate the direction of magnetic force experienced by a free north pole from north to south.
Ferromagnetic Materials and Magnetic Induction
- Ferromagnetic metal materials,
- including iron, steel, nickel,
- and cobalt, experience an attractive magnetic force toward a magnet and can be induced to form temporary or permanent poles.
Permanent Magnets vs Electromagnets
- A permanent magnet retains its magnetization continuously,
- (commonly made of magnetically hard steel)
- whereas electromagnets utilize coils wound around a soft iron core that magnetizes only while carrying current.
Magnetic Poles and Field Directions
- Magnetic poles exert non-contact forces where like poles repel and opposite poles attract;
- field lines illustrate the direction of magnetic force experienced by a free north pole from north to south.
Electromagnet
- Magnet made by current through a coil around a magnetic core.
Attract
- Pull together.
Force
- A push or pull measured in newtons.
Attraction
- The force between unlike charges
Permanent magnet
- Magnet whose field is always there.
Magnetic material
- Material attracted to a magnet.
Magnetic force
- The force on a moving charge in a magnetic field
Key terms
| Term | Meaning |
|---|---|
| Plotting compass | Small compass used to find the field direction at a point |
| Iron filings | Tiny iron pieces that line up along field lines |
| Field line | Line showing the direction of the magnetic force |
| Neutral point | Point where the resultant magnetic field is zero |
| Induced magnet | Material temporarily magnetised in a magnetic field |
Avoid these mistakes
1
Saying iron filings show the direction of the field — they show the pattern but a compass is needed for direction.
2
Joining the dots with straight segments — join them in a smooth curve.
3
Leaving other magnets near the experiment — they distort the plot.
4
Forgetting an arrow on each plotted line — the direction must be shown.
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