What is a Magnetic Field?
A region of space surrounding a magnet or a current-carrying conductor where a magnetic force can be detected. It is represented by field lines that show the direction and strength of the field.
You have probably played with magnets before — sticking them to fridges, watching them attract or repel each other. That invisible force you feel between magnets exists because of a magnetic field.
Key ideas about magnetic fields:
- Magnetic fields are invisible, but we can detect and map them using iron filings or a compass.
- The field is strongest where the field lines are closest together (near the poles of a magnet).
- Magnetic field lines always travel from the North pole to the South pole outside the magnet.
- Field lines never cross each other.
The symbol for magnetic field (or magnetic flux density) is B, and it is measured in Tesla (T). To give you a sense of scale: Earth's magnetic field is about 0.00005 T, a fridge magnet is around 0.01 T, and an MRI machine uses 1–3 T. At MYP level, you mostly need to understand direction and shape of fields rather than calculate B values.
Magnetic Field Lines — Rules and Representation
Scientists use field lines (also called lines of flux) to visualise magnetic fields. These are imaginary lines, but they follow strict rules that make them very useful for understanding how a field behaves.
Rules for drawing magnetic field lines:
- Lines emerge from the North (N) pole and enter the South (S) pole outside the magnet.
- Lines are continuous loops — inside a bar magnet they travel from S to N, completing the loop. (For a current-carrying wire, the field lines are complete circles around the wire — there are no poles, so lines simply loop around continuously.)
- Lines are closer together where the field is stronger.
- Lines never intersect (cross each other).
- Each line has an arrow indicating direction (N to S outside a bar magnet).
Think of field lines like roads on a map — they show you the direction you'd travel if you were a tiny compass needle placed at that point. Where roads are packed closely, traffic (the field) is intense!
Students often draw field lines crossing each other. This is never correct — if lines crossed, it would imply a magnetic force pointing in two different directions at the same point, which is physically impossible.
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