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Magnetism

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