Physics P3 Keeping things moving

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  • Physics P3 GCSE AQA- Keeping things moving
    • The Motor Effect
      • When a current flows through a wire a magnetic field is produced around the wire
        • Electromagnets are used on cranes for lifting iron/steel.
      • Flemings left hand rule- thumb=force, first finger=field and the middle finger=current
      • The size of the force can be increased by:
        • increasing the size of the current
        • Increasing the size of the magnetic field
      • The conducter will not receive a force if it is parallel to the magnetic field.
      • The direction of the force is reversed if either the direction of the current of the direction of the magnetic field is reversed.
    • Transformers
      • If an electrical conducter cuts through a magnetic field a potential difference is induced across the ends of the conductor.
      • If a magnet is moved into a coil of wire a potential difference is induced across the ends of a coil.
      • A transformer has an iron core with a primary coil and secondary coil on either side.
      • An alternating coil in the primary coil produces a changing magnetic fieldin the iron coreand the secondary coil
        • This induces an alternating potential difference across the ends of the secondary coil.
      • In a step up transformer the potential difference across the secondary coil is greater than the potential difference across the primary coil.
      • In a step down transformer the potential difference across the secondary coil is less than the potential difference across the primary coil
      • The potential difference across the transformer is Vp/Vs=np/ns (Vp is the difference across the primary coil in volts V, Vs is the potential difference across the secondary coil in volts V, np is the number of turns on the primary coil and ns is the number of coils on the secondary coil.
      • Transformers are asssumed to be 100% efficient, so we use this equation Vp*Ip=Vs*Is (Vp is the potential difference across the primary coil in volts V,Ip is the currentin the primary coil in amps A, Vs is the potential difference across the secondary coil in volts V and Is is the current in the secondary coil in amps A)
      • Switch mode transformers operate at a high frrequency, between 50 and 200kHz.
      • Switch mode transformers are lighter and smaller than a traditional transformer working form a 50Hz supply
        • This means they are useful for mobile phone chargers etc.
      • Switch mode transformers use very little power when they are switched on but no load is applied.

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