Chapter 10 to 12

• Coulomb's law


  Electrostatic force
• Electrostatic force


  • Charge attraction and repulsion force
  • Cosmic scale attraction force
• Current


  Power = Current x Voltage
• Charge


  Energy = Charge x Voltage
• North pole (geographic) = South pole (magnetic)
• Compass needle points to north
• Less voltage


  Less burns
• Sound waves


  Particles move
• Radio waves


  Vacuum medium, no particles move
• Water waves


  Water particles move
• Electromagnetism


  The force that underlies the periodic table of elements, giving rise to all of chemistry and, through this, much of biology, that brings live with it. It is the force which binds atoms together into solids and liquids. And it is the force which is responsible for the incredible range of properties that different materials exhibit.
• Electromagnetism at the atomic scale

  • Governs the interactions between atoms and molecules
• Electromagnetism at the macroscopic scale


  • Manifests itself in the familiar phenomena that give the force its name
• Electricity


  Moving electric charges produce a magnetic field
• Magnetism


  Changing a magnetic field moves electric charges
• Electromagnetic field


  A dynamical quantity which takes a value at every point in space and time, described by the electric field E and the magnetic field B
• Magnetic fields from electricity

  1. Static distribution of charges produces an electric field

  2. Charges in motion (electrical current) produce a magnetic field

  3. A current-carrying conductor produces a magnetic field around it
• Magnetic field pattern around a straight wire

  • The resulting magnetic field lines form concentric circles around the wire
• Right-Hand Grip rule


  Can be used to predict the direction of the magnetic field
• Electromagnets


  1. Arranging wire in a coil and running a current through produces a magnetic field that looks a lot like a bar magnet

  2. Putting a real magnet inside, can shove the magnet back and forth depending on current direction: called a solenoid
• Magnetic field direction


  • Outside a magnet is North to South
  • Within a magnet is South to North
• Solenoid


  A cylindrical coil of wire acting as a magnet when carrying electric current
• Increasing magnetic field strength in a solenoid

  1. Increasing the current

  2. Increasing the number of turns per unit length of the solenoid

  3. Placing a soft iron core within the solenoid
• Induced Current


  A changing magnetic field produces an electric current in a loop surrounding the field, called electromagnetic induction or Faraday's Law
• Changing magnetic field


  Produces an electric field
• Moving electric field


  Produces a magnetic field
• Electric and magnetic fields

  Can produce forces on charges
• Accelerating charge


  Produces electromagnetic waves (radiation)
• Transformer


  A device used to increase or decrease voltage
• Where transformers are used


  • In the national grid and household appliances
• Step-up transformer


  A transformer that increases voltage
• Step-down transformer


  A transformer that decreases voltage
• How a transformer works


  When the electricity changes in the first coil an electric voltage is created across the second coil
• Transformers are used to change the voltage, resistance and current of the electricity that comes from NamPower's high voltage transmission
• Only AC voltage can be transformed from one voltage to another, this is why mains electricity needs to be AC
• An electric voltage is only induced when the magnet is moving. We need a changing magnetism to make electricity
• Less turns in a transformer


  Means less voltage
• Transformer example 1

  • A step-down transformer is required to transform 240 V a.c. to 12 V a.c. for a model railway. If the primary coil has 1000 turns, how many turns should the secondary have? T2 = 1000 x (12/240) = 50 turns
• Transformer example 2


  • A transformer is designed to have 2500 primary turns and 5000 secondary turns. What is the output voltage if the input voltage is 120 V? V2 = 240 V
• Transformer example 3


  • A transformer has 100 turns on its primary coil. It has an input voltage of 35V and an output voltage of 175 V. How many turns are on the secondary coil? 500 turns