A magnetic field is a region where a magnet, a moving charge or a current feels a force. We picture it with field lines: arrows that show where a compass would point and how strong the field is.
A magnetic field line points the way the north pole of a small compass points. Lines run out of a north pole and into a south pole, never cross, and crowd together where the field is stronger. Around a straight current-carrying wire the lines are concentric circles, with their direction set by the right-hand grip rule.
Switch the source between a bar magnet, a straight wire and a solenoid. Each small compass turns to lie along the local field line, and the arrows show the field direction. Press Reverse to flip the poles or the current and watch every arrow turn around.
However the field is made, the same rules for field lines apply.
Field lines never cross, because the field has a single direction at each point. Closer lines mean a stronger field (greater flux density), so do not draw them evenly spaced near a magnet. The lines are continuous closed loops: outside a magnet they go north to south, inside they go south to north. Reversing the current or flipping the magnet reverses every arrow but keeps the same shape.
Four quick checks on representing magnetic fields. Each correct answer earns XP and lights this skill on your star map.
A magnetic field line shows the direction in which:
The magnetic field pattern around a long straight wire carrying a steady current is:
Two magnetic field lines can never cross because at any point the field has:
Where magnetic field lines are drawn closer together, the field is:
State directions precisely: field lines go north to south outside the magnet. For a wire, name the right-hand grip rule and give the sense (clockwise or anticlockwise when viewed along the current). Remember that a uniform field is shown by parallel, equally spaced lines, as inside a long solenoid.
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