The earth’smagneticfield Ifwestand with loads of magnets on the surface of the earththeywould all pointtheir N poletowards the north! And theirS-pole towards the south. This is because the earth has a magnetic field. The earth’ssouthmagneticpole is near the earth’snorthgeographicpole!!
Magneticfield round a wire The danish scientist Hans Christian Örstednoticed that if you have a current in a wire therewill be a magneticfield. The thumbrule is: Imaginegrabbing the wire with your right hand. Thumb in the currents direction, your finger will thenpoint in the same direction as the fieldlines.
The magneticfield round a wire: The magneticfieldlines are circles. The field is strongestnearest the wire. Increasing the currentincreases the strength of the field.
Coils and solenoids A magneticfield round a wire will be strongerif it is woundinto a coil (spole). A longcoil is called a solenoid. The magneticfieldwillthen look like this:
The magneticfield of a coil: is increased by increasing the current. is increased by the number of turnsaround the coil. is reversedif the currentchangesdirection.
Thumbrule nr 2 Imaginegripping the coil with your right hand so that your fingers are in the same direction as the current. Your thumbwillthenpoint at the north of your electromagnet.
Different uses of a electromagnets: Relay Computer hard drive The curcuitbreaker A magnetic door lock Hugeelectromagnets on cranes Door bells