magnetism

• Magnetism


  Describes the ability of magnets to attract and repel other magnets without touching them
• Attraction


  • Magnets have a north and south pole, opposites attract, it is a non-contact force
• Magnetic materials


  • Can be magnetic all the time but sometimes do not act like magnets, such as iron, will never attract non-magnetic material, permanent magnets are always magnetic and produce their own magnetic field
• Induced magnetism


  • Materials that become magnets when they are in a magnetic field, a permanent magnet creates temporary poles in the magnetic material, which attracts the two together
• Strength of magnetic field

  • Depends on the distance from the magnet, strongest at the poles
• Magnetic field patterns


  • The arrows on the field line always go from north to south, the arrows point away from north and towards the south
• Earth magnet


  • The core of the Earth produces a magnetic field, the needle in a compass always points in the direction of the Earth's magnetic field
• Magnetic effect of a current

  • The field lines are in circles around a single conducting wire, they are in loops facing the same direction, the strength depends on the size of the current and the distance from the wire
• Magnetic field around a coil of a wire

  • It is strong and uniform, it has the same strength and direction at every point, the field lines are evenly spaced-out parallel lines inside the coil, outside they are like a bar magnet
• Solenoid


  • Coil of wire with a charge through it, flipping the current direction flips the direction of the magnetic field lines, increasing the current increases the magnetic field strength
• Electromagnet


  • A current carrying a wire is wrapped around an iron core, the iron core increases the magnetic field strength
• Use of electromagnets, magnets and currents


  • Washing machines, scrap yards
• Transformers


  Can change the voltage in wires carrying alternating currents, there are step-up and step-down transformers, the changing magnetic field is induced in the iron core
• Step up transformer


  • The secondary coil has more turns than the primary coil, the potential difference in the secondary coil is greater than in the primary coil, this means the voltage is stepped up, decreases the current
• Step down transformer


  • The secondary coil has fewer turns than primary coil, the potential difference is less than the potential difference across in the primary coil, this means that it is stepped down
• Iron core


  • Iron is used as it is easily magnetized and demagnetized
• High voltage transmissions and transformers

  • Transformers are used to increase the voltage of electricity so it can be transmitted over long distances, high voltage means less current, and less heat energy meaning less resistance
• Power loss in transformers

  • Energy is lost because the current heats the wires, a higher current lead to more heat lost, a step-up transformer increase the voltage so less energy is lost
• Motor effect


  A current flowing through a wire will produce its own magnetic field, such a wire is in a magnetic field and will experience a force, the current and magnetic field needs to be perpendicular
• Measuring motor effect


  Put the magnet on a balance, measure the mass when the current is flowing, convert to N, measure the length of wire in a field and use the F=BIL formula
• Motor effect


  A wire with a current flowing through it is placed in a magnetic field, the magnet and the conductor experience a force which is the motor effect, the effect comes from the field created by the electric current interacting with the magnetic field
• Direction of motor effect

  • Depends on the direction of the magnetic field and the direction of the current flow
• Fleming left hand rule

  Point the first finger in direction of magnetic field, point second finger in direction of current, the thumb is the direction of force, this is where it would move
• Electric motors


  • A coil carrying a current will rotate in a magnetic field, this is caused by the current going up on side is flowing in the opposite direction to the current coming back down, one side will move down, and one will move up
• Split ring commutator


  • In an electric motor, spins the contacts every half term to make the motor spin
• Force acting on a coil in magnetic field

  • Increasing the force makes it spin more, increasing the magnetic field strength, adding more coils and using a higher current increases this
• Loudspeakers


  • As the direction changes the direction of magnetic field induced changes, this magnetic change causes the current the change, making the speakers vibrate to create sound waves
• Induced potential


  It can be induced across the ends of an electrical conductor through a conductor moving in a magnetic field and changed in magnetic field around a conductor, this is either the conductor spinning around a magnet or a magnet spinning around a conductor, faster movement and more coils increase this, this creates a potential difference in wires
• Magnetic field direction


  • Reversing the direction of the magnetic field movement will reverse the direction of any induced current, an induced current opposes the original change
• Forces between electricity and magnets

  With a potential difference induced in a wire, it creates a force acting in the opposite direction to the action of the induced PD, this is due to the motor effect, a wire moved down in a horseshoe magnet
• Generator effect


  A magnet will cause a current carrying wire to move, a wire that moves throughout a magnetic field will gain a current, a potential is produced
• Generator effect


  • No split ring commutator, turn it faster or have a stronger magnet to make it stronger, the current produced in the coil also produces a magnetic field, makes it hard to turn it and needs energy
• Microphone


  • Back and forth generator, sound waves cause the coil to oscillate past magnet, which induces a signal, the magnet vibrates the coil, this induces a potential difference and a current in the coil
• Direct current


  Allows current to flow in one direction, this is what batteries and cells use
• Alternating current


  Repeatedly changes in direction, what UK plug sockets use
• Alternating current generators


  • These produce an AC, there is a rotating magnet and a fixed coil of wire, the magnet rotates and the direction of the field that the coil passes through alternates, there is a soften iron core, coils around it and a spinning magnet
• Dynamo


  • Direct current producer, the coil rotates which produces a potential difference in one direction, the split ring commutator reverses the current to keep the coil rotating, the potential difference is largest when the coil and the magnetic field are parallel to each other