P7 - Magnetism & Electromagnetism

  • Created by: Benny52
  • Created on: 16-03-19 12:49

Permanent & Induced Magnets

  • All magnets produce magnetic field - region where other magnets or magnetic materials - e.g. iron, steel, nickel & cobalt - experience non-contact force. Show magnetic field by drawing magnetic field lines: Go from N to S  & show which way force would act on a north pole if put at that point in field. Closer together lines are, stronger magnetic field. Magnetic field & magnetic force strongest at poles of magnet. 
  • Force between magnet & magnetic material always attractive - no matter the pole. If 2 poles are put near each other, they'll each exert force on each other. 
  • Inside compass - bar magnet. North pole of magnet attracted to south pole of nearby magnets - compass points in direction of magnetic field it's in. Can use compass and iron filings to trace magnetic field. When not near magnet, compass points north - Earth generates magnetic field - core of earth magnetic.
  • Permanent magnets - produce own magnetic field. Induced magnets - magnetic materials that turn into magnet when put into magnetic field. Force between permanent & induced - always attractive. When removed from magnetic field, induced magnets lose magnetism & stop producing magnetic field.
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  • When current flows through a wire, magnetic field is created around the wire. Field made up of concentric circles perpendicular to wire, with wire in the centre. Changing direction of current changes direction of magnetic field - use right-hand thumb rule: Point thumb of right hand in driection of current and curl fingers. Direction of fingers = direction of field.
  • Strength of magnetic field changes with current & distance from wire. Larger current or closer you are to wire means stronger field.
  • Increase strength of magnetic field wire produces by wrapping it in coil called a solenoid - field lines around each loop of wire line up with each other, resulting in lots of field lines pointing in same direction, close together, creating stronger field. Magnetic field inside solenoid - strong & uniform - same strength & direction at every point. Outside - magnetic field just like one round bar magnet.
  • Can increase field strength of solenoid by putting block of iron in centre - becomes induced magnet whenever current is flowing. if current stopped, magnetic field disapperars. Solenoid with iron core = electromagnet. Can also increase strength by adding more coils to wire or increasing current.
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The Motor Effect

  • When current-carrying wire or any conductor is put between magnetic poles, the magnetic field around wire interacts with magnetic field it's been placed in, causing magnet & conductor to exert force on each other - Motor Effect - can cause wire to move.
  • To experience full force, wire has to be 90 degrees to magnetic field. If wire runs parallel to field - won't experience any force. Fore always acts at right angles to magnetic field of magnets & the direction of current in wire.
  • To show direction of force, apply current to set of rails in horseshoe magnet. Bar is placed on rails, completing circuit - generates force that rolls bar along rails. 
  • Magnitude of force increases with strength of field. Force also increases with amount of current passing through conductor.
  • Force = Magnetic Flux Density (T/Tesla) x Current x Length                 F = BIl
  • B = how many field lines there are in a region - shows strenght of magnetic field. 
  • Use equation, when current is at 90 degrees to magnetic field.
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Electric Motors

  • Find direction of force with Fleming's left-hand rule: Using left hand, point First finger in directuion of magnetic Field. Point seCond finger in direction of Current. thuMb will point in direction of force - Motion. Rule shows if either current or magnetic field is reversed, direction of force will also be reversed.
  • In a basic dc motor, forces act on the two side arms of coil of wire carrying current. As coil is on a spindle and one force acts upwards and the other downwards, it rotates. Use of split-ring commutator swaps the contacts every half turn to keep the motor rotating in same direction. Direction of motor reversed by swapping polarity of dc supply - reversing current, or swapping magnetic poles - reversing field. Speed of motor increased by increasing current, adding more turns to coil or increaing the mangnetic flux density.
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