physics paper 2

• Magnets


  Produce magnetic fields
• Magnetic fields


  • Exert a force on magnetic materials such as iron and steel
  • The force may be attractive or repulsive
• Magnets


  • Have a north pole and a south pole
  • Bar magnet has north pole on one side and south pole on the other
• Magnetic fields


  • Represented using field lines
  • Field lines flow from north pole to south pole
  • Arrows show direction of magnetic force
• Closer together the field lines

  Stronger the magnetic field
• Interaction between two magnets

  • Unlike poles (north and south) attract
  • Like poles (two norths or two souths) repel
• Permanent magnets


  Always magnetic
• Induced magnets


  • Become magnetic when placed in a magnetic field
  • Quickly lose magnetism when field is taken away
  • Magnetic materials can be turned into induced magnets
• Compass


  • Uses Earth's magnetic field for navigation
  • Contains a tiny bar magnet that aligns with Earth's magnetic field
  • Can be used to investigate magnetic field surrounding a magnet
• Using a compass to investigate magnetic field

  1. Place magnet on paper

  2. Use compass to find direction of magnetic field

  3. Repeat in range of positions

  4. Use results to draw field lines
• Electromagnetism


  Magnetic fields affect electrical charges and electrical charges affect magnetic fields
• Creating magnets with an electrical current

  1. Flowing current through a wire

  2. Producing a magnetic field surrounding the wire
• Magnetic field around a wire

  • Represented by field lines in concentric circles around the wire
  • Direction depends on direction of electric current
  • Strength depends on size of current and distance from wire
• Using the right-hand rule

  1. Point thumb in direction of current

  2. Fingers curl in direction of magnetic field
• Solenoid


  • Coil of wire
  • Magnetic field similar to bar magnet
• Magnetic field inside solenoid

  • Uniform
  • Very strong
  • Same direction at every point
• Increasing strength of magnetic field in a solenoid

  1. Increasing current flowing through wire

  2. Placing iron core in centre
• Electromagnet


  • Solenoid with iron core
  • Magnetic field disappears when current switched off
• Uses of electromagnets


  • Cranes to attract and pick up magnetic objects
  • Switches that operate by electromagnet attracting iron contact
• Motor effect


  A force is exerted on a current-carrying wire due to the presence of a magnetic field
• Motor effect


  1. Current-carrying wire placed in magnetic field

  2. Magnetic field produced by current interacts with external magnetic field

  3. Wire experiences a force
• Motor effect


  • Greatest force when wire perpendicular to magnetic field
  • No force when wire parallel to magnetic field
  • Smaller force when wire at angle between perpendicular and parallel
• Force


  Always perpendicular (at right angles) to both the magnetic field and the current
• Fleming's Left-hand Rule


  • Thumb is in the direction of the force
  • First finger is the direction of the magnetic field
  • Second finger is the direction of the current
• F


  Force in Newtons (N)
• B


  Magnetic flux density (strength of magnetic field) in tesla (T)
• I


  Current in amps (A)
• l


  Length of wire in metres (m)
• Electric motor


  1. Magnetic field exerts force on loop of wire

  2. Current travelling in opposite directions on each side of loop

  3. Forces in opposite directions cause loop to rotate

  4. Split-ring commutator reverses current as coil rotates

  5. Keeps wire loop in constant rotation
• How an electric motor works

  1. Magnetic field exerts a force on the loop of wire

  2. Current travelling in opposite directions in each side of the loop, so forces are in opposite directions

  3. Loop of wire rotates

  4. Split-ring commutator rotates with the coil, contacts with positive and negative voltage swap

  5. Current in the wire loop reverses

  6. Forces on the wire swap

  7. Wire loop in constant rotation
• Fleming's Left-hand Rule


  Used to work out which way the loop will rotate