Magnetism + electromagnetism

• Magnet


  Has a north and south pole, and produces a magnetic field
• Magnetic field


  • Can be shown by drawing magnetic field lines

  Lines go north to south, and show which way a force would act on the north

  The closer together the lines are, the stronger the magnetic field is, and the further apart the lines are, the weaker the magnetic field is

  The field is strongest at the poles
• Force between a magnet and magnetic material

  Always attractive
• Force between two poles

  Repel if the poles are the same, and attract if the poles are different
• Compass


  Inside is a tiny bar magnet, the north of which is attracted to the south of any magnet it's near

  Can be used to trace the position of a magnetic field
• When not near a magnet

  A compass will point north, as the Earth has a magnetic field
• Permanent magnet


  Produces its own magnetic field
• Induced magnet


  Magnetic material that turns into a magnet when put into a magnetic field

  Quickly lose their magnetism and stop producing a magnetic field when the field is removed
• Force between a permanent and induced magnet


  Always attractive
• Electromagnetism


  When a current flows through a wire, a magnetic field is created, made of concentric circles perpendicular to the wire
• Right hand rule


  Thumb is the direction of the current, fingers are the direction of the magnetic field
• Solenoid


  • Wrapping the wire into a coil increases the strength of the magnetic field

  The magnetic field inside the solenoid is strong and uniform

  Outside, the magnetic field is the same as a bar magnet

  Strength can be increased with an iron core, which becomes an induced magnet

  If the current is stopped, the field disappears. A solenoid with an iron core is an electromagnet
• Electromagnet


  Can be switched on and off, used because of this or because of their varying force
• Motor effect


  When a current-carrying wire is put between magnetic poles, the magnetic field around the wire interacts with the field it's been placed in, causing the objects to exert a force on each other, and causing the wire to move

  The force acts at right angles to the field of the magnets and direction of the current
• To experience the full force, the wire must be at a 90 degree angle to the field, if it is parallel, there is no force
• Fleming's left hand rule

  First finger is the direction of the field, second finger is the direction of the current, thumb is the direction of the force
• Electric motors


  • Force acts on the two side arms of a coil of wire carrying a current

  The split ring commutator swaps the contacts every half turn to keep the motor rotating in the same direction

  The direction of the motor can be reversed by swapping the direction of the current or swapping the magnetic poles
• Loudspeakers and headphones


  • Use electromagnets

  An alternating current is sent through a coil of wire attached to the base of a paper cone

  The coil surrounds one pole of a permanent magnet, and is surrounded by the other pole, so the current causes a force on the coil (the cone moves)

  Variations in the current makes the cone vibrate, and the air around it vibrate, creating variations in pressure, causing a sound wave
• Generator effect


  The induction of a potential difference in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic field
• The induced current always opposes the change that made it
• Increasing induced potential difference

  • Increasing the speed of movement

  Increasing the strength of the magnetic field
• Alternators


  • Rotate a coil in a magnetic field

  As the coil spins, a current is produced in the coil, changing direction every half turn

  They have slip rings and brushes so the contacts don't swap, creating an alternating current
• Dynamos


  • Work in the same way as alternators, but have a split ring commutator instead of a slip ring to keep the current flowing in the same direction
• Oscilloscope


  Shows the potential difference generated in the coil changing over time
• Microphones


  • Sound waves hit a flexible diaphragm attached to a coil around a magnet, causing the wire to move in the field, generating a current

  Movement of the coil depends on properties of the sound wave, allowing microphones to convert pressure variations of a sound wave into variations in current
• Transformers


  • Change the size of the potential difference of alternating current

  Have two coils of wire, primary and secondary, joined with an iron core

  When alternating potential difference is applied across a primary coil, the iron core magnetises and demagnetises quickly, inducing alternating potential difference in the secondary coil