P4 - MAGNETISM AND MAGNETIC FIELDS (9-1 SPECIFICATION)

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Magnets

  • Magnets have a north and south pole and a magnetic field is where magnetic material experiences a force.
  • Field lines show the size and direction of magnetic fields and always point from north to south. The closer the lines, the stronger the magnetic field at that point.
  • You can see magnetic fields lines by placing a magnet under a piece of paper and scattering iron filings on top. The filings will align themselves with the field lines.
  • Like poles repel and opposites attract.
  • When a magnet is brought near a magnetic material, the material acts as a magnet because magnetism has been induced by the original magnet. Induced magnets are temporarily magnetic.
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Electromagnetism

Magnetic field around a straight wire:

  • Concentric circles around wire (direction found with the right-hand rule)
  • The further form the wire, the weaker the magnetic field.

Magnetic field around a flat circular coil:

  • Similar field to a bar magnet
  • Concentric ellipses of field lines around the coil

Magnetic field around a solenoid:

  • Increased magnetic effect when lots of coils of wire are joined together, ends acts like the north and south poles. An electromagnet.
  • Increase magnetic effect of solenoids by increasing current in wire, no. coils, adding an iron core, decreasing solenoid length.
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Magnetic Forces

  • A wire experiences a force when it's placed at 90 degrees between two magnetic poles, as the two magnetic fields affect each other.
  • Force increases when current / magnetic feild strenth increase.
  • You can change force direction by changing direction of current / magnetic field

Image result for flemings left hand rule (http://www.bbc.co.uk/staticarchive/633e87e216d452b81a3c1d75fb281f84b60a5ff7.gif)    Fleming's left-hand rule is used to find the direction of the force on a current-carrying conductor (a wire).

Force = magnetic field strength x current x lenth

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Motors

  • In a d.c. motor, a wire sits between two opposite magnetic poles.
  • The current flows in different directions on each side of the coil so each side experiences forces in the opposite direction. Because the coil is on a spindle, it will rotate.
  • The split-ring commutator swaps the contacts every half turn to keep motor rotating in the same direction.
  • You can reverse the direction of the motor by reversing the current / magnetic field. Use Fleming's left hand rule to work out what direction the coil will rotate.
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Loudspeakers

  • A loudspeaker contains a coil of wire surrounded by a magnet, inside the coil is another magnet.
  • A.c. is fed to the coil of wire wrapped aorund the cone base.
  • The interation between the magnetic field and current in coil forces the coil to move in one direction. As the current is alternating, the current changes direction and so does the coil.
  • As the current continues to alternate, the coil moves back and forth and this makes the cone vibrate. This creates pressures in the air which is sound.
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Electromagnetic Induction

Electromagnetic Induction -> when p.d. is induced across a conductor which is experiencing a change in magnetic field

  • A change in magnetic field is experienced when a conductor passes through magnetic field lines and if the conductor is part of a complete circuit then induced p.d. will result in a current in the circuit.
  • This will occur when a wire and magnetic field move relative to each other or when the magnetic field through a conductor changes.
  • Generators use the motion of a conductor and magnetic field to induce p.d.
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Generators - Alternators and Dynamos

Alternators:

  • Rotate a magnet in a coil of wire. As the magnet spins, p.d. is induced across the ends of the coil which changes every half turn because the fields change as the magnet rotates.
  • This produces alternating current if the coil is part of a complete circuit.
  • You can also generate a.c. by rotating a coil in a magnetic field. Slip rings don't swap every half turn so they produce a.c.

Dynamos:

  • Rotate a coil in a magnetic field. The output p.d. and current change direction every half turn, producing a.c.
  • A split-ring commutator swaps connections every half turn to keep the current flowing in the same direction. This is d.c.
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Microphones

  • Structured like a loudspeaker but with a diaphragm instead of the cone.
  • Sound waves cause the diaphragm to move back and forth when hit by them.
  • As the Diaphragm moves, the coil of wire moves which induces a p.d. across the ends of the coil.
  • The coil is part of a complete circuit so the induced p.d. means there are variations in current in the electrical circuit.
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Transformers

 Transformers change the size of p.d. of an alternating current.

  • Primary and secondary coils are joined with an iron core.
  • When an alternating p.d. is applied across the primary coil, an alternating magnetic field is produced.
  • Because iron is a magnetic material, the core becomes magnetised and because there is an alternating magnetic field, the magnetisation in core alternates.
  • The changing magnetic field induces a p.d. in the secondary coil.
  • Transformers are nearly 100% efficient so power in primary coil = power in secondary coil.

Step-down transformers -> step voltage down, more turns on primary coil.

Step-up transformers -> step voltage up, more turns on secondary coil.

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