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iGCSE Magnetism (Electromagnetism (Devices based on electromagnets (Relays…

- - - - The switch would be closed to thus cause an electric current to flow in the conductor
        
        A magnetic needle placed above the conductor is deflected
        
        As soon as the current stops flowing, the needle returns to its original position
      - If the direction of the current is reversed, the needle is deflected in the opposite direction
      - This simple experiment was repeated and extended by Ampere and established electric currents produce magnetic fields
        
        It led to the realisation that electricity and magnetism were in fact a single fundamental force of nature
        
        It was named the electromagnetic force.
  - - - Only the lines in the horizontal plane are shown
  - - - The result is that the solenoid produces an overall magnetic field similar to that produced by a bar magnet.
    - - If you grasp the solenoid in your right hand, so that your fingers curl around it in the direction of the conventional current flow, then your thumb will point to the northof the magnet
  - - - This is the principle of the ELECTROMAGNET.
      - The strength of an electromagnet depends on
        
        The strength of the current passing through the coil
        
        The number of turns in the coils
        
        using a core of a soft magnetic material.
  - - - When the S switch is closed, the iron core inside the solenoid becomes magnetised
        
        It attracts the iron armature which is pivoted.
        
        The upper limb of the armature causes the contacts in the secondary circuit XY to close, thus activating that circuit
      - A relay is a device whereby a secondary circuit can be switched on by a primary circuit
    - - When the push button is depressed, current will flow through the circuit and this will magnetise the electromagnet
        
        It will attract the soft iron armature causing the hammer to strike the gong
        
        Contact at the screw will be broken de-magnetising the electromagnet
        
        The flexible metal strip acts as a spring, pulling back the armature and the hammer, and restoring contact at the screw
        
        This cycle is repeated as long as the push button is depressed
    - - A loudspeaker consists of a central pole surrounded by a ring magnet
        
        A movable coil is wound around the centre pole
        
        Attached to this coil, which carries the current is a cardboard cone
        
        When a current passes through the coil, a force acts on the coil forcing it to move in and out
        
        The paper cone which is attached to the coil moves with it setting up sound waves, which reproduce the sounds originally converted to a current
  - - - A simple motor works with DC.
        
        Each end of the coil is connected to half of a split ring of copper
        
        This is a commutator and rotates with the coil
        
        Two carbon blocks (brushes) are pressed lightly against the commutator by springs.
        
        The carbon brushes allow the current to get into the coil without tangling up the wires
        
        When a current carrying conductor is at right angles to a magnetic field, it experiences the maximum force. A wire parallel to the field experiences no force.
      - How it works
        
        When the coil is horizontal the wire on the left experiences an upward force and the wire on the right experiences a downward force
        
        This force rotates the coil until it is vertical. The carbon brushes are then in line with the gaps of the commutator and the current stops
        
        Because of inertia, the coil overshoots the vertical and the commutator hales have changed contact from one brush to the other.
        
        This reverses the current through the coll and also the direction of the forces on its sides
        
        The coil thus carries on rotating
    - - this is the motor effect
      - the force depends on the angle between the wire and the magnetic field lines
      - The force is biggest when the wire is perpendicular to the field lines and none when parallel
        
        The size of the force can be increased by
        
        increasing the strength of the magnetic field
        
        increasing the size of the current
    - - this is used to work out the direction of the force
      - The direction of the force is reversed if either the direction of the current or the direction of the magnetic field is reversed
      - The fingers on your left hand are each assigned a component
        
        First finger shows magnetic Field
        
        ThuMb shows Motion
        
        SeCond finger shows Current
  - - - When a wire moves through a magnetic field, a voltage is induced in the wire
        
        Electricity produced from magnetism is called Electromagnetic Induction
        
        Generators, dynamos and alternators are machines that convert mechanical power into electrical power. It has a coil rotating in the magnetic field
        
        Some generators produce alternating current.
        
        The structure of it
        
        The ends of the coil are joined to two slip rings on the axle
        
        The carbon brushes push against the split rings
        
        When the coil is rotated, it cuts the field lines and a voltage is induced in the coil.
    - - Step-up and Step-down Transformers
        
        A transformer is an electrical device that changes the voltage of an ac supply.
        
        It can change a high voltage supply into a low-voltage one or vice versa.
        
        A transformer that increases the voltage is called a step up transformer. These have more turns on the secondary coil than the primary coil
        
        A transformer that decreases the voltage is called a step down transformer. These have fewer turns on the secondary coil than the primary coil.
        
        The voltages across the primary and secondary coils of a transformer are related through
        
        primary voltage / secondary voltage = primary turns / secondary turns
        
        Since P=IV for 100% efficiency
        
        input power = output power
        
        Step down transformers are used in mains adaptors and rechargers for mobile phones and CD players
        
        The National Grid
        
        Power stations use transformers to produce very high voltages before the electricity is transmitted to where it is needed through the power lines
        
        The national grid relies on transformers
        
        There are at least five transformers between a power station and a domestic use
        
        This is because we use high voltage for carrying electrical current over large distances and we use small voltages in our homes
        
        the high transmission voltages are to reduce the current and thus large losses of heat in the transmission cables
        
        The low voltages in the home are for reasons of safety
        
        If we use a high transmission voltage, then the current that needs to flow to the town will be less (and still deliver the same power).
        
        A smaller current causes less heating in the transmission and so, there is less waste
      - A changing magnetic field will also produce an induced voltage in a coil.
        
        This is how a transformer works.
      - When alternating current is passed through the primary coil, the magnetic field around it is continuously changing
        
        As the size of the current increases the field grows.
        
        As the size of the current decreases the field collapses
      - A secondary coil placed near a primary coil in a changing magnetic field causes a voltage to be induced as the magnetic field cuts through it.
        
        An alternating voltage is produced in the secondary coil
        
        The core is the frame on which the coils are mounted
        
        the core is usually made of laminated soft iron
        
        Soft means that when magnetised, it loses its magnetism as soon as it is turned off
        
        Laminated means that the core is made up of sheets of iron
        
        Each layer is separated by a layer of insulating material.
        
        A current would be produced n the iron core without the lamination
        
        Iron is a magnetic material and so guides the magnetic field from the primary coil to the secondary coil.
        
        Electromagnets have a soft iron core
        
        Permanent magnets are made of hard magnetic material
    - - Electromagnetic induction is a process where a conductor placed in a changing magnetic field (or conductor moving in a stationary magnetic field) causes the production of a voltage across the conductor
      - If a magnet is moved into a coil of wire which is part of a complete circuit, a current is induced in the wire
      - If the magnetic or coil of wire 'cuts through' a magnetic field or vice versa, a voltage is produced between the ends of the wires
        
        This induced voltage causes a current to flow if the wire is part of a complete circuit
        
        The size of the induced voltage increases when
        
        the speed of the movement increases
        
        the strength of the magnetic field is increased
        
        the number of turns on the coil is increased
        
        the area of the coil is greater
- - - - Likewise, further apart magnetic field lines show a weak field.
    - - An easy way to produce a roughly uniform magnetic field is to hold two opposite poles to each other so they attract
      - The diagram shows the magnetic field between two flat magnets.
        
        The magnetic field lines are the same distance apart throughout the field and so it is called a uniform field.
  - - - They always go from N to S and never cross
- - - - Ancient Greeks knew a type of rock with magnetic properties
        
        known as lodestone or magnetite attracted iron
      - The compass has a suspended magnetic which aligns parallel to magnetic field produced by Earth and then points to North Pole
    - - If a magnet is broken in two, it creates two new magnets with N and S poles
      - If a bar magnet is broken in two at the fracture, new N and S poles are formed at the point of fracture
    - - If a N pole is brought close to another N pole, a repulsive force is felt
      - If a S pole is brought close to another S pole, the two magnets will repel each other
  - - - This includes...
        
        iron
        
        nickel
        
        cobalt
        
        compounds and alloys containing these metals
    - - These magnets lost their magnetism quickly
      - They form 'temporary' magnets.
      - They are used in cores of electromagnets or in keepers of permanent magnets.
    - - These materials retain their magnetism
      - they form 'permanent' magnets
        
        these are generally compounds or alloys of the magnetic elements
        
        They are hard to magnetise
        
        They keep their magnetised states for very long periods of time
      - Permanent magnets used to be made from steel but are now more likely to be made from alloys containing iron, and small amounts of nickel, cobalt and aluminium.