14. Magnetic Field and Field Lines

A magnetic field is the region of space around a magnet (or a current-carrying conductor) within which its magnetic influence can be detected — that is, within which another magnet or a moving charge would experience a magnetic force. The SI unit of magnetic field strength (magnetic flux density) is the tesla (T), named after the Serbian-American inventor and engineer Nikola Tesla; a smaller, commonly cited unit in older/CGS contexts is the gauss (1 T = 10,000 gauss). The SI unit of magnetic flux (the total 'amount' of magnetic field passing through a given area) is the weber (Wb), named after Wilhelm Eduard Weber.

Magnetic Field Lines of a Bar Magnet

Magnetic field lines (or lines of force) are imaginary lines drawn to represent the direction and relative strength of a magnetic field, and they are extremely useful for visualising how a magnet behaves. By convention, magnetic field lines outside a magnet always emerge from the north pole and enter the south pole, forming continuous closed loops that pass through the interior of the magnet from the south pole back to the north pole (unlike electric field lines, which begin and end on charges, magnetic field lines never begin or end — they always form closed loops, a direct consequence of the fact that isolated magnetic poles do not exist). Some key properties of magnetic field lines, all of which are commonly tested: they never intersect or cross one another (if they did, it would imply two different directions of the field at the same point, which is physically impossible); the field is stronger where the lines are closer together (crowded), and weaker where the lines are spread farther apart; and near the poles of a magnet, the field lines are most densely packed, indicating that the magnetic field is strongest at the poles. Field lines can be traced experimentally using a small compass needle (a plotting compass) moved point by point around a magnet, or, more simply for classroom demonstration, by sprinkling iron filings on a sheet of paper or glass placed over a bar magnet, which align themselves along the invisible field lines.