Physics - P3.3 - Using Magnetic Fields to Keep Things Moving

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P3.3.1 - Electromagnets

  • Magnetic poles: ends of magnet - north pole one end, south pole the other - region around magnet is magnetic field - iron filings placed near magnet form pattern of lines looping from one pole to other - lines of force or magnetic field lines - plotting compass placed in magnetic field points along field lines
  • Like poles repel, unlike poles attract
  • Magnetic field produced around wire when current flows through - elctromagnet made by wrapping insulated wire around piece of iron (core) - iron becomes strongly magnetised when current flows through wire - loses magnetism when current switched off: temporary magnetism is very useful
  • Electromagets used in scrapyard cranes, circuit breakers, electric bells and relays
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P3.3.2 - The Motor Effect

  • Motor effect: place a wire carrying an electric current in a magnetic field it experiences a force - maximum force: wire at 90° angle to magnetic field, force = 0: wire parallel to magetic field
  • Flemings left-hand rule: determines direction of force - thumb and two first fingers held at right angles to each other: thuMb - Movement(direction of force), First finger - magnetic Field, seCond finger - Current
  • Force size increased by: increasing magnetic field strength or current size
  • Direction of force on wire reversed if direction of current or magnetic field is reversed
  • When current passes through coil, it spins as: force acts on each side due to motor effect and force on one side is equal to force on other side
  • 'Split-ring' commutator reverses current direction around coil every half turn - sides swap every half turn: coil always pushed in same direction
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P3.3.3 - Electromagnetic Induction

  • If electrical conductor cuts through magnetic field lines, pd is induced across ends of conductor; if a magnet is moved into wire coil, pd induced across ends of coil: called electromagnetic induction: if a wire or coil passes through a circuit, a current passes through it
  • Direction of movement in wire or coil or polarity of magnet reversed: direction of induced pd reversed - pd only induced while there's movement
  • Size of induced pd increased by increasing: speed of movement, strength of magnetic field, number of turns on the coil
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P3.3.4 - Transformers

  • Transformer: coils of insultated wire: primary and secondary coil wound onto iron core - ac passed through primary coil produces ac magnetic field in core: continually expanding and collapsing
  • Alternating magnetic field lines pass through secondary coil inducing alternating pd - current produced if part of circuit
  • Wire coils insulated: current can't short across core or other turns of wire: flows around circuit - core made of iron: easily magnetised
  • Transformers in national grid: step-up transformer - secondary coil has more turns; higher pd, step-down transformer -  secondary coil has less turns; lower pd
  • Switch mode transformer has ferrite core: its operates at higher frequency, lighter and smaller, uses very little power when no device is connected across output terminals
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P3.3.5 - Transformers in Action

  • National grid steps-up pd from power stations: electrical energy transmitted across grid at higher pd = smaller energy wasted in cables - stepped-down to be used by consumers
  • Vp / Vs = np / ns
    • Vp - pd across primary coil - V
    • Vs - pd across secondary coil - V
    • np - number of turns on primary coil
    • ns - number of turns on secondary coil
  • Step-down transformer: turns on secondary coil < primary coil - step-up transformer: turns on secondary coil > primary coil
  • Transformers almost 100% efficient - for 100% efficiency: Vp × Ip = Vs × Is
  • Ip - primary current - A
  • Is - secondary current - A
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P3.3.6 - A Physics Case Study

  • Applications of physics used in hospitals for diagnosis and therapy - ECG (electrocardiogram) used to measure pds generated by heart
  • Electronic devices used to measure blood pressure
  • Digital thermometers used to measure temperature
  • Endoscope containing bundles of fibre optics used to look inside a body without large incisions
  • X-rays used to take pictures of suspected broken bones
  • CT scanners used to build up digital pictures of cross-section through body
  • MR scaners use radio waves to produce detailed digital pictures of body
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