Magnetism and Electromagnetism

• Solenoid: a wire wrapped into the shape of a coil

  Electromagnet: a solenoid with an iron core inside
• • a permanent magnet produces its own magnetic field

  o always magnetic, always has poles

  • an induced magnet is a material that becomes a magnet when it is placed

  in a magnetic field

  o induced magnetism always causes a force of attraction

  o when removed from the magnetic field an induced magnet loses most/all of its magnetism quickly

  o e.g. iron, nickel , cobalt
• magnetic field: region around a magnet where a force acts on another magnet or magnetic material (e.g. iron)
• • the strength of the magnetic field depends on the distance from the magnet (decreases with distance)

  o the field is strongest at the poles of the magnet

  • the direction of the magnetic field at any point is given by the direction of the force that

  would act on another north pole placed at that point

  o the direction of the magnetic field line is always from north to south

  o the closer the field lines the stronger the magnetic field
• • a magnetic compass contains a small bar magnet

  o the compass needle points in the direction of the Earth’s magnetic field
• • the Earth has a magnetic field

  o the core is magnetic and creates a large magnetic field around the Earth as the Earth spins causing the molten iron in the outer core to spin around the solid inner core (like an electromagnet)

  o geographic north is the magnetic south pole (as opposites attract)
• • the strength of the magnetic field produce depends on the:

  o current though the wire

  o distance from the wire

  • the field is made up of concentric circles perpendicular to the wire

  • use the right-thumb rule to work out the direction of the field
• • shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire, the magnetic field is strong and uniform

  • the magnetic field around a solenoid has a similar shape to that of a bar magnet

  o adding an iron core increase the strength of the magnetic field of the solenoid
• • increase the strength of an electromagnet’s magnetic field

  o add more coils

  o increase length

  o increase cross-sectional area

  o increase current
• the motor effect: when a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a magnetic force on each other
• The Motor Effect:

  o magnetic field around a wire is circular but the field between 2 magnets is straight

  o when the 2 interact the wire is pushed away from the field (at right angles to the

  wire direction and field direction)
• • Fleming’s Left-Hand Rule represents the relative orientation of the force, current in the conductor and the magnetic field (each is 90° to each other)

  o therefore if either the direction of current or the magnetic field is reversed the force is flipped
• • the factors affecting the size of the force on the conductor are:

  o strength of the magnetic field

  o amount of current flowing

  o the length of wire in the magnetic field
• • a coil of wire carrying a current in a magnetic field tends to rotate – this is the basis of an electric motor

  • dc motors work using a spindle, 2 permanent magnets and a split-ring communicator

  o the current flows from the positive to negative terminal

  o the force acts upwards on one wire and downwards on

  the other side due to Fleming’s left-hand rule

  o the split-ring communicator swaps the contacts every

  half turn to keep the motor rotating in the same

  direction

  o to swap the direction you swap the polarity of the dc

  supply (reversing the current)
• • loudspeakers and headphones

  • an ac is sent through a coil attached to the base of a large paper cone

  • the coil of wire surrounds the south pole which is surrounded by the north pole

  • the current causes a force of the wire causing the coil and cone to move

  • when the current reverses the force reverses so the cone

  moves in the opposite direction too

  • variations in current cause the cone to vibrate creating

  variations in pressure that cause a sound wave

  o the frequency of the sound wave is the same as the

  frequency of the ac
• • the generator effect: if a conductor moves relative to a magnetic field or if there is a change in the magnetic

  field around a conductor a potential difference is induced across the ends of the conductor

  o if the conductor is part of a complete circuit a current is induced in the conductor

  o the induced current always opposes the change it was produced by
• • an induced current generates a magnetic field that oppose that opposes the original change, either:

  o movement of the conductor

  o change in the magnetic field

  • this can be achieved in 2 ways

  o moving a magnet in a coil of wire

  o moving a wire in a magnetic field (cutting the magnetic field lines)
• • if you reverse the direction of the magnet or wire the potential difference or current is reversed

  o by constantly moving the magnet or wire back and forth you produce an alternation current

  • factors affecting the size of the induced current:

  o increase the speed of the movement (cuts more magnetic field lines in a shorter time)

  o increase the strength of the magnetic field (more field lines to be cut
• • the generator effect in an alternator (produced ac)

  o a coil rotates in a magnetic field due to a turbine

  o as the coil spins a current is induced which changes direction every half turn

  o instead of a split-ring communicator, ac generators use slip rings and brushes, so the contacts don’t swap every half turn

  o this produces an alternating potential difference
• • the generator effect in a dynamo (produces dc)

  o a coil rotates in a magnetic field (same setup as a motor)

  o as the coil spins a current is induced which changes direction every half turn

  o they use a split ring communicator which swaps the connection every half turn to keep the current flowing in the same direction

  o this produces a direct potential difference
• • oscilloscopes show the generated potential difference

  o they show how the potential difference in a coil changes over time

  o for ac: a line oscillates crossing the horizontal axis

  o for dc: the line isn’t straight but it always stays above the axis (it is always positive)

  o the height of the line is the potential difference

  o increasing the frequency of the revolutions increasing the potential difference and number of oscillations

  
• • microphones are loudspeakers in reverse

  o soundwaves hit a flexible diaphragm that is attached to a coil of wire wrapped around a magnet

  o pressure variations in the sound waves cause the coil to move

  o current is induced in the coil as it cuts the magnetic field produced between the magnet it is wound around and the magnet surrounding it

  o the current produced varies in proportion to the variations in the soundwaves
• • a basic transformer consists of a primary coil and a secondary coil wound around an iron core

  o iron is used because it is easily magnetised

  • transformers change the potential difference of an alternating current
• Transformers:

  • ac in the first coil creates a changing magnetic field in the iron core

  o the changing magnetic field cuts through the secondary coil inducing an alternating current

  • a dc wouldn’t work as the magnetic field in the core would remain constant
• o in a step-up transformer: Vs > Vp

  ▪ potential difference increases because the secondary coil has more turns than the primary

  o in a step-down transformer: Vs < Vp

  ▪ potential difference decreases because the secondary coil has less turns than the primary
• • the national grid uses transformers

  o a step-up transformer is used when transmitting electrical power

  ▪ high potential difference (400,000V) means a low current can be used while transporting high

  amounts of electrical power (high currents waste energy by heating the surroundings)

  o a step-down transformer is used when electrical power is transferred from the overhead cables to homes so potential difference isn’t dangerously high for home appliances etc.