GCSE P3 Chapter 3

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  • Created by: emma998
  • Created on: 22-03-14 16:21

Electromagnets

  • The ends of a magnet are called magnetic poles
  • North pole and south pole
  • Like poles repel, opposite poles attract
  • Magnetic field = region in which iron or steel will be attracted to it
  • Magnetic field lines = lines produced by iron fillings placed near a magnet that form a pattern and loop from one pole to the other
  • A plotting compass placed in a magnetic field will always point along a field line
  • When current flows through a wire a magnetic field is produced around the wire
  • An electromagnet is made by wrapping insulated wire around a piece of iron called the core
  • When the current flows through the wire the iron becomes strongly magnetised
  • When current is switched off the iron loses its magnetism
  • Temporary magnetism is used in scrapyard cranes, circuit breakers, electric bells and relays
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The motor effect

  • When a wire carrying an electric current is placed in a magnetic field a force may be experienced
  • This is called the motor effect
  • The force is maximum when the wire is perpendicular to the magnetic field line and zero when it is parallel
  • Flemming's left hand rule: - thumb represents direction of force - first finger represents magnetic field (north-south) - second finger represents direction of current
  • Size of force is increased by increasing strength of magnetic field or size of current
  • Direction of force is reversed by either reversing direction of curent or magnetic field
  • Motor effect is used in different appliances
  • In a motor the speed is increased by increasing size of current
  • Direction of motor can be reversed by reversing direction of current
  • When a current passes through the coil, the coil spins because a force acts on each side of the coil due to motor effect and force on one side is opposite direction to force on other side
  • Split ring commutator reverses direction of current around the coil every half turn. Because sides swap over each half-turn, the coil is always pushed in same direction
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Electromagnetic induction

  • If an electrical conductor cuts through field lines a potential difference is induced across the ends of the conductor
  • If a magnet is moved into a coil of wire a pd is induced across its ends
  • This is called electromagnetic induction
  • If the wire or coil is part of a complete circuit a current passes through it
  • If direction of movement is reversed or polarity of magnet is reversed, the direction of the induced pd is also reversed
  • pd is only induced whilst there is movement
  • Size of pd induced is increased if:
    • Speed of movement is increased
    • Strength of magnet is increased
    • Number of turns on the coil is increased
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Transformers

  • A transformer = two coils insulated wire wrapped around different ends of iron core
  • The coils are called the primary and secondary coils
  • When an alternating curent passes through the primary coil an alternating magnetic field is produced in the core
  • This alternating magnetic field continually expands and collapses
  • The alternating magnetic field lines pass through the secondary coil
  • This induces an alternating potential difference across the ends of the secondary coil
  • If the secondary coil is part of a complete circuit an alternating current is produced
  • The coils of the wire are insulated so that the current does not short across either the iron core or the adjacent turns of the wire but flows around the whole coil 
  • Coil is made of iron because it is easily magnetised
  • Step up transformer - more coils on secondary coil so pd induced is greater
  • Step down transformer - fewer coils on secondary coil so reduces pd induced
  • Switch mode transformers have a ferrite core. Use very little power when no device is connected across output terminals, lighter, smaller, operates at higher frequencies
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Transformers in action

  • National grid uses transformers
  • Step up transformers used from power stations because higher pd so less enegy wasted
  • Step down transformers used at connection points for towns to make it safe for consumers
  • Vp / Vs  =  Np / Ns
  •    (pd)         (turns)
  • Transformers are almost 100% efficient
  • To be 100% efficient:
  • Vp X Ip  =  Vs X Is
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Physics case study

  • Applications of physics used in hospitals for diagnosis and therapy
  • ECG or electrocardigram is used to measure potential difference produced by heart
  • Electronic devices used to measure blood pressure
  • Digital thermometers to measure temperature
  • Endoscopes to look inside the body without large incisions
  • X-rays to build up digital pictures of cross sections through the body
  • M\r scanners which use radiowaves to produce detailed digital pictures of the body
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