To sketch a magnetic field, draw smooth lines that leave the north pole, curve round and enter the south pole, with an arrow on each line and closer spacing where the field is stronger. The same rules work for a bar magnet, two poles facing each other, a straight wire and a coil.
This skill appears in magnetism and fields as a drawing question, often worth a few marks for arrows, spacing and shape.
What does a field line actually show?
A magnetic field is the region where a magnetic material or a current feels a force. A field line is a map of it. At any point, the line’s direction is the direction a small free north pole would be pushed.
Three rules follow.
Lines never cross, because the field has one direction at each point. Lines are closer together where the field is stronger. Lines leave north and enter south, so the arrow always runs from N to S outside the magnet.
How do I sketch the common patterns?
- Bar magnet. Draw 4 to 6 lines on each side, looping from the north end to the south end. Crowd them at the poles and let them spread out in the middle of the loop.
- Two unlike poles facing each other. Lines run straight across the gap from N to S, evenly spaced and parallel in the middle. That region is a uniform field. Near the edges the lines bulge outwards.
- Two like poles facing each other. Lines from each pole curve away from the other. Midway between them the field is zero, called a neutral point. Do not let lines cross there.
- Straight wire carrying current. Lines are circles around the wire, getting further apart as you move away. Use the right-hand grip rule: thumb along the current, fingers curl in the field direction.
- Solenoid (long coil). Inside, lines are close, straight and parallel. Outside, they loop round like a bar magnet. The end where the current looks anticlockwise is a north pole.
Worked example
Sketch and label the field pattern of a solenoid when the current flows clockwise as you look at its left end. (Invented situation.)
Step 1, find the poles. Clockwise seen from the left end means that end is a south pole. The right end, where the current looks anticlockwise, is the north pole.
Step 2, draw the inside. Draw straight, evenly spaced lines along the inside from the left end (S) to the right end (N). Arrows point right. The field inside is strong and uniform.
Step 3, draw the outside. Draw lines that leave the right end, curve round and return to the left end. Arrows on the outside point from right to left, like a bar magnet.
Step 4, check. Every line is a closed loop, none crosses, and lines are closest inside the coil and at the ends.
Notice that the arrows outside run from north to south, while the arrows inside run from south to north. Together the line is one continuous loop.
The mistake to watch for
A common slip is to put the arrows pointing into the north pole.
Mistaken answer: a bar magnet with arrows running from south to north on the outside.
The student remembered “lines join the poles” but not the direction.
The correction is to say “north is where lines start outside.” Think of the field lines as leaving N and arriving at S. A quick check is to read the sketch aloud: “out of N, into S”.
A second slip is ending a line in empty space. Every line must join one pole to the other, or run off the edge of the drawing in the direction it was heading.
Check yourself
1. Two bar magnets face each other with north poles nearest. Describe the field at the point exactly halfway between them.
Show answer
The field is zero at that midpoint, a neutral point. The two fields point in opposite directions and have equal size there, so they cancel.
2. In a sketch, the lines are closer near point A than near point B. Which point has the stronger field?
Show answer
Point A. Closer lines mean a stronger field.
3. A solenoid has a north pole at its right end. As you look at the right end, is the current clockwise or anticlockwise?
Show answer
Anticlockwise. The end that looks anticlockwise is the north pole.
Where this leads next
Once your sketches are reliable, move on to explaining induced magnetism, which uses the same field lines to show why a magnet attracts iron. Then test everything with the magnetism and fields practice set.
Some students can draw a bar magnet but lose marks on the coil or the neutral point. That is the kind of pattern our teachers look for in online one-to-one Physics tuition.