The Magnetic Field Patterns of Bar Magnets, Straight Wires and Solenoids

Bar magnets possess a field pattern similar to the Earth’s magnetic field. This field is strongest at the poles of the magnet (the ends) and weakest in the middle.
The pattern of the magnetic field can be depicted using plotting compasses or iron filings where the magnetic field lines go from the North pole to the South pole.
Magnetic fields created by straight wires are circular. The direction of the field changes depending on the direction of the current. This can be determined using the right hand grip rule: if the thumb points in the direction of the current, the fingers show the direction of the magnetic field.
When a wire is coiled into a loop, the magnetic field pattern resembles that of a bar magnet, with distinct North and South poles. However, the strength and direction of the field varies within the loop.
By coiling the wire even more to form a solenoid (a long, cylindrical coil of wire), the magnetic field within the solenoid becomes very nearly uniform, similar to the magnetic field near the centre of a bar magnet.
The field outside a solenoid is similar to that of a bar magnet, and the ends of a solenoid act as the North and South poles. When you view the solenoid from one end and the current in the loops is clockwise, that end acts like the South pole. The opposite end, therefore, is the North pole.
The strength of a magnetic field from a solenoid can be increased by inserting an iron core into the solenoid, which becomes a magnet itself when the current is flowing.
Key principles to remember: The magnetic field is always strongest at the poles; magnetic field lines never cross; the field is always from North to South outside the magnet but inside a magnet it is from South to North.
The right hand grip rule helps to remember the direction of the magnetic field in relation to the current in straight wires and solenoids.