Magnetic Effects of Electric Current · Lesson 2 of 8
Magnetic Field and Field Lines
“Invisible magnetic fields leave very visible clues when iron filings join the choreography.”
• Define a magnetic field and explain how it can be detected. • Use iron filings and a compass to represent the field around a bar magnet. • Determine magnetic-field direction from the orientation of a compass north pole. • Explain the major properties of magnetic field lines. • Interpret field-line spacing as an indication of relative field strength. • Draw a scientifically correct field-line pattern around a bar magnet.
Place a compass near one end of a bar magnet and its needle swings into a new direction. Move the compass and it turns again. The magnet is not touching the needle, yet its influence is detectable throughout the surrounding region. To describe this influence clearly, we use the ideas of a magnetic field and magnetic field lines.
A compass needle is itself a small bar magnet. Its end that points approximately north is called the north-seeking pole or north pole, while the other end is the south-seeking pole or south pole. Like poles repel and unlike poles attract. Near the north pole of a bar magnet, the compass south pole is attracted towards the magnet and its north pole points away. This response helps reveal the field direction.
The region surrounding a magnet in which the force of the magnet can be detected.
A magnetic field has both magnitude and direction. Its direction at a point is defined as the direction in which the north pole of a compass needle would move if placed there. The field cannot be seen directly, but its action on a compass or iron filings makes its pattern observable.
Activity
To reveal the overall magnetic-field pattern around a bar magnet with iron filings.
Fix a white sheet on a drawing board and place a bar magnet at its centre. Sprinkle iron filings uniformly around the magnet and tap the board gently. The tapping reduces friction so the filings can turn and settle. Keep the filings away from the eyes and collect them carefully after the observation.
The filings arrange themselves in curved chains around the magnet. Each filing becomes temporarily magnetised and experiences a turning effect, so many filings align along the local field direction. The resulting pattern does not mean that the field exists only where filings are present; the filings merely make selected directions visible.
Imaginary lines used to represent a magnetic field, with the tangent at any point giving the field direction at that point.
Activity
To trace individual magnetic field lines around a bar magnet using a small compass.
Place the magnet on paper, trace its boundary and mark its poles. Put the compass near the north pole and mark the positions of both ends of the needle. Move the compass so that its south pole occupies the previous position of its north pole, then mark the new needle position. Continue step by step until the south pole of the bar magnet is reached. Join the marked points with a smooth curve and repeat from several starting points.
The compass north pole shows the magnetic-field direction at each position. By convention, field lines emerge from the north pole and enter the south pole outside a magnet. Inside the magnet they run from south to north. Consequently, a complete magnetic field line has no free beginning or end: it is a closed curve.
Field-line spacing communicates relative field strength. Where the lines are crowded, a magnetic pole placed there experiences a greater force, so the field is stronger. The lines crowd near the poles of a bar magnet and spread farther apart away from it. The count of drawn lines is a representation choice, but within one diagram their relative spacing is meaningful.
No two magnetic field lines can cross. If two lines intersected, each line would assign a different tangent and therefore a different magnetic-field direction to the same point. A compass north pole at that point would then be required to point in two directions simultaneously, which is impossible. The non-intersection rule follows from the field having one definite direction at each point.
| Feature | Meaning |
|---|---|
| Arrow direction | Direction in which a compass north pole would point |
| Outside a bar magnet | Field runs from north pole to south pole |
| Inside a bar magnet | Field runs from south pole to north pole |
| Closed curve | The line continues through and outside the magnet |
| Closer spacing | Relatively stronger magnetic field |
| No intersections | Only one field direction exists at a point |
Problem
A compass is placed just outside the north pole of a bar magnet. In which general direction does the compass north pole point?
- 1.Outside a bar magnet, field lines emerge from its north pole.
- 2.The compass north pole points along the local magnetic-field direction.
- 3.Therefore it points generally away from the magnet’s north pole.
- 4.Its south pole points towards the magnet because unlike poles attract.
Problem
Two regions of the same field-line diagram contain equally sized areas. The lines are much closer in region A than in region B. Compare the fields.
- 1.Field-line density represents relative magnetic-field strength within the diagram.
- 2.Closer lines in region A mean the field is stronger there.
- 3.Wider spacing in region B means the field is weaker there.
- 4.A small test magnet would experience a greater magnetic effect in region A.
Problem
A drawing shows two magnetic field lines crossing. Explain precisely why the drawing cannot represent a real magnetic field.
- 1.At the crossing, each field line has a different tangent direction.
- 2.The magnetic-field direction is the direction a compass north pole would take.
- 3.The crossing would therefore require one compass needle to align in two directions at the same point.
- 4.Because the field direction at a point is unique, the crossing is impossible and the drawing must be corrected.
Field lines are representations, not material threads. Iron filings do not create the magnetic field; they reveal its pattern. The direction convention follows a compass north pole, and the lines continue through the magnet rather than ending at its poles.
Quiz
What is the direction of a magnetic field at a point?
Why are field lines closer near the poles of a bar magnet?
Which statement about magnetic field lines is correct?
Outside a bar magnet, field lines are conventionally directed from
What would crossing field lines incorrectly imply?
Practice Problems
- Explain why a compass needle deflects near a bar magnet. Answer: The compass needle is a small magnet. The bar magnet’s field exerts a turning effect that aligns the needle with the local field direction.
- Describe how iron filings reveal a magnetic-field pattern. Answer: Each filing becomes temporarily magnetised and turns under the magnetic influence. After gentle tapping, many filings align in curved chains that trace the field-line pattern.
- Draw a bar magnet and state the required arrow directions. Answer: Outside the magnet arrows must run from north to south; inside they must run from south to north. Each complete line must form a closed curve.
- A learner draws widely spaced lines near a pole and crowded lines far away. Identify the error. Answer: The relative strength has been reversed. A bar magnet’s field is stronger near its poles, so the lines should be closer there and generally farther apart at greater distances.
- Prove by compass reasoning that field lines cannot intersect. Answer: A compass north pole aligns with the field direction. At an intersection two tangents would specify two directions, requiring the same needle to point two ways at once, which is impossible.
Key Takeaways
• A magnetic field is the surrounding region in which a magnet’s force can be detected. • A compass needle is a small magnet and reveals the local field direction. • Outside a bar magnet, field lines run from north to south; inside they return from south to north. • Magnetic field lines are closed curves. • Closer field-line spacing represents a stronger field. • Two magnetic field lines never intersect because the field has one direction at each point. • Iron filings reveal an overall pattern, while a compass can trace individual field lines.