Magnetic field lines and their properties
Magnetic field lines are continuous closed loops. Outside a magnet they run from N to S, inside from S to N, and they never cross.
Properties of magnetic field lines
- They are continuous closed loops. Outside the magnet they go from N to S; inside they go from S to N.
- The tangent at any point gives the direction of B there. A small compass needle placed there lies along the tangent.
- The number of lines crossing a unit area (held at right angles to them) shows the strength of B. Crowded lines mean a strong field.
- No two lines ever cross. If they did, B would have two directions at that point, which is impossible.
- Parallel, equally spaced lines show a uniform field.
- Field lines are not real paths. A moving charge does not, in general, travel along them.
Magnetic field lines vs electric field lines (of static charges)
Alike in both: the tangent gives the direction of the field, crowding shows its strength, and two lines never cross.
How to draw the field lines of a bar magnet
- Draw a rectangle for the magnet. Write N at one end and S at the other.
- Draw one straight line along the axis coming out of N, and one going into S.
- Draw 3 or 4 curved lines on each side, leaving the N end, bulging out and entering the S end. Keep the pattern symmetrical.
- Draw the lines closer together near the poles and farther apart away from the magnet.
- Continue the lines inside the magnet from S to N, so that each line is a closed loop.
- Put arrows on every line, and make sure no two lines cross.
Closed loops and zero net flux
Electric field lines start and end on charges. Magnetic field lines have nowhere to start or end, because there are no magnetic monopoles. So every line that enters a closed surface must also leave it. The net magnetic flux (lines going out minus lines coming in) through any closed surface is therefore zero: ∮ B·dS = 0. This is known as Gauss's law for magnetism. The same loop pattern is seen for a current-carrying solenoid and a current loop, which is why they act like bar magnets.
Reading a field-line diagram
| What you see | What it means |
|---|---|
| Lines crowded together | Strong field |
| Lines spread far apart | Weak field |
| Straight, parallel, equally spaced lines | Uniform field: the same size and direction everywhere |
| Arrow on a line | Direction of B, where a compass N pole would point |
| Lines curving round | The direction of B changes from point to point |
You can see field lines yourself with a magnet, a sheet of paper and iron filings from a local workshop. Engineers map the same lines to design the magnets in MRI scanners and electric scooter motors.
1 marks: “Why do two magnetic field lines never intersect?” At the crossing point B would have two directions, which is impossible.
2 marks: “Write two differences between magnetic field lines and the electric field lines of static charges.” Magnetic lines form closed loops; electric lines start on positive and end on negative charges. Magnetic lines exist inside a magnet; electric lines are absent inside a conductor. The net magnetic flux through a closed surface is always zero.
2 marks: “Draw the magnetic field lines of a bar magnet. What do the lines inside the magnet show?” Symmetric closed loops from N to S outside, crowded near the poles, with arrows. Inside, the lines run from S to N, which shows that every line is a closed loop.
Magnetic lines never end and never cross. Out of N, into S, then back from S to N inside.