Magnetism, Motor effect and electromagnetic induction

• Bar magnet


  A permanent magnet because it is always magnetic
• Magnetic materials


  Metals iron, steel, nickel and cobalt
• Magnetic field

  The space around a magnet where it can attract magnetic materials
• North-seeking pole and south-seeking pole


  The two ends of a bar magnet
• Two magnets are placed close to each other

  North pole on one attracts south pole on the other, north poles or south poles repel
• Induced magnet


  A piece of magnetic material that becomes a magnet when placed in a magnetic field
• Magnets


  • Used in electric motors, generators, loudspeakers and other electrical devices
  • Used for simpler things such as door latches and knife holders
• Magnetic field lines


  • Show how a single north pole would move (from north to south)
  • Field is strongest where the lines are closest together
• Compass needle


  A very small magnet that points to the Earth's North Pole
• The Earth has a magnetic field similar in shape to the magnetic field of a bar magnet
• Earth's magnetic field


  Caused by electric currents in the molten outer core, which is made from a mixture of iron and nickel
• Speakers work because of the magnetic effect of an electric current flowing through a wire
• The direction of the magnetic field depends on the direction of the current
• If the current changes direction

  The direction of the magnetic field changes
• The strength of the magnetic field depends on the size of the current - the higher the current the stronger the field
• The magnetic field is strongest closer to the wire and gets weaker with increasing distance
• Solenoid

  A coil of wire with a current flowing through it, an electromagnet
• Motor effect


  The force produced on a wire carrying a current in a magnetic field
• Any wire carrying a current can experience a force near a magnetic field
• Fleming's left-hand rule


  Shows the relationship between the directions of the magnetic field, current and force
• The size of the force on the wire depends on the magnetic field strength, the current and the length of the wire in the field
• Transformer


  A device used to change the potential difference (voltage) of an electricity supply
• There is no electrical connection between the two coils of wire in a transformer
• Electromagnetic induction means that a voltage in one coil causes (induces) a voltage in the second coil
• The power supplied to a transformer in the primary coil must be equal to the power transferred away from the transformer in the secondary coil if the transformer is 100% efficient
• Step-up transformer


  Increases the voltage and decreases the current
• Step-down transformer


  Decreases the voltage and increases the current
• Electricity is sent from power stations to homes, schools and factories by the national grid
• When electricity flows through a wire, the wire gets warm and there is a significant amount of energy wasted by heating in the transmission lines of the national grid
• Increasing the voltage of the electricity passing through a wire decreases the current, reducing the energy wasted by heating and improving efficiency
• Power stations produce electricity at 25 kV, which is changed to 400 kV by transformers before being sent around the country
• Electromagnetic induction


  A changing magnetic field induces a potential difference (pd) in a wire, which causes a current to flow
• Reversing the direction of the magnetic field reverses the direction of the induced current
• If the potential difference causes a current to flow in a wire, the magnetic field of this current opposes the original change
• Transformers only work with alternating current, when the direction of the potential difference (and so also the current) changes many times each second