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

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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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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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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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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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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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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