Definition
A steady electric current generates a magnetic field around the conductor carrying it — a moving charge is always accompanied by a magnetic field, which is why the field appears only while current flows and vanishes the instant it's switched off. This shows electricity and magnetism are deeply linked, not separate effects.
Around a straight current-carrying wire, the magnetic field forms concentric circles centred on the wire.
Right-hand grip rule: if you grip the wire with your right hand, thumb pointing in the direction of conventional current, your curled fingers point in the direction of the magnetic field.
Reversing the current's direction reverses the magnetic field's direction. Increasing the current's magnitude increases the field's strength (more current, more circles of force, stronger field at a given distance).
Remember
The field around a current-carrying wire can be demonstrated by placing a compass (or scattering iron filings) near the wire: with current flowing, the compass needle deflects to align with the circular field instead of pointing north, and iron filings arrange themselves in concentric circles around the wire — both effects disappear the moment the current is switched off.
Rule
A current-carrying conductor placed in an external magnetic field experiences a force, because its own magnetic field interacts with the external one. This force is greatest when the wire is placed perpendicular to the field, and zero when placed parallel to it — the angle between the wire and the field directly determines how strong the force is, which is exactly why motor coils are designed to stay as close to perpendicular to the field as possible for maximum turning effect.
( = magnetic field strength, = current, = length of wire in the field, = angle between wire and field)
Definition
A D.C. motor converts electrical energy into mechanical (kinetic) energy — current-carrying coil sides in a magnetic field experience forces in opposite directions (one up, one down), creating a turning effect (torque) that spins the coil.
Definition
The commutator is a split-ring contact that reverses the direction of current through the coil every half-turn. Without it, the coil would turn only to the point where it's parallel to the field (the position) and then stop, because the force on each side would then act straight along the coil's axis with no turning effect at all — the commutator's timely current reversal keeps the torque acting in the same rotational direction, letting the coil complete a full continuous rotation instead of oscillating back and forth.
Remember
To find the poles of an unmarked magnetized steel bar, bring one end near the known north pole of a compass or another magnet: if it repels, that end is a north pole (like poles repel); if it attracts, that end is a south pole (unlike poles attract) — attraction alone can't identify a pole with certainty, since a magnet always attracts an unmagnetized piece of steel too, but repulsion is only ever seen between two genuine like poles.
Definition
The leading theory (the geodynamo theory) attributes the Earth's magnetic field to convection currents of molten, electrically conducting iron and nickel in the Earth's outer core, combined with the Earth's rotation — moving conducting fluid generates electric currents, and those currents generate the magnetic field, in a continuously self-sustaining process.
The two main layers of the core involved are the liquid outer core (where the convecting, conducting fluid flows) and the solid inner core (whose heat and structure help drive the outer core's convection).
Quick Recap
- ●Current magnetic field around the wire; field only exists while current flows
- ●Right-hand grip rule: thumb current direction, curled fingers field direction
- ● — force is greatest perpendicular to the field, zero when parallel
- ●D.C. motor: current-carrying coil in a field turns; commutator reverses current every half-turn to keep it spinning
- ●Unmarked magnet's poles: repulsion (not attraction) confirms a genuine like pole
- ●Earth's magnetic field: convection of molten conducting outer core Earth's rotation (geodynamo theory)
