Quantities describing the magnetic field
A magnetic field exists around a magnet: it acts on other magnets and on current-carrying conductors. The force characteristic of the field is the magnetic induction B (a vector quantity); its direction is the direction shown by the north pole of a magnetic needle placed in the field. The unit of B is the tesla (T). We picture the field with field lines: outside the magnet they run from the north pole to the south pole, they are closed lines, they do not cross, and they are denser where the field is stronger. Iron filings in a magnetic field line up along the lines and show the field’s pattern. When experimenting with filings, protect your eyes and do not breathe them in; keep the magnet away from phones and cards.
Draw the field lines of a bar magnet in your notebook: arrows from N to S, denser near the poles. If you use a magnetic needle, check that its N end points along the field line.