Graphs of Potential Difference in the Coil (AQA GCSE Physics)

Potential difference graph for an alternator

• The output of an alternator can be seen on a graph of potential difference (p.d.) against time, or angle of rotation
• The shape of the graph is a sine or cosine curve, depending on the starting position of the coil
  • When it starts from a horizontal position (p.d. is at a maximum), the graph is a cosine curve
  • When it starts from a vertical position (p.d. is zero), the graph is a sine curve

Graph of induced p.d. with angle for an alternator

Alternating p.d. with corresponding positions of the coil relative to the field

• When the coil is vertical at 0°
  • it is moving parallel to the direction of the magnetic field
  • the size of the induced p.d. is zero

• When the coil has rotated by 90°
  • it is now horizontal and moving perpendicular to the direction of the magnetic field
  • the size of the induced p.d. is at a maximum

• When the coil has rotated by 180°
  • it is vertical again and moving parallel to the direction of the magnetic field
  • the size of the induced p.d. is zero

• When the coil has rotated by 270°
  • it is horizontal again and moving perpendicular to the direction of the magnetic field
  • the size of the induced p.d. is at a maximum and in the opposite direction to its position at 90°

• When the coil has completed a full 360° rotation
  • it is back at its starting point where it is moving parallel to the direction of the magnetic field
  • the size of the induced p.d. is zero

Potential difference graph for a dynamo

• The output of a dynamo can be seen on a graph of potential difference (p.d.) against time, or angle of rotation
• The shape of the graph is a sine curve and is always in the same direction

Graph of p.d. with time for a dynamo

Direct potential difference trace showing the position of the coil relative to the magnetic field

• When the coil is vertical at 0° (position 1)
  • it is moving parallel to the direction of the magnetic field
  • the size of the induced p.d. is zero

• When the coil has rotated by 90° (position 2)
  • it is now horizontal and moving perpendicular to the direction of the magnetic field
  • the size of the induced p.d. is at a maximum

• When the coil has rotated by 180° (position 3)
  • it is vertical again and moving parallel to the direction of the magnetic field
  • the size of the induced p.d. is zero

• When the coil has rotated by 270° (position 4)
  • it is horizontal again and moving perpendicular to the direction of the magnetic field
  • the size of the induced p.d. is at a maximum and in the same direction as its position at 90° (i.e. position 2)

• When the coil has completed a full 360° rotation (back to position 1)
  • it is back at its starting point where it is moving parallel to the direction of the magnetic field
  • the size of the induced p.d. is zero

Factors affecting alternator or dynamo output

• The magnitude of the induced p.d. can be increased by:
  • increasing the frequency of rotation of the coil
  • increasing the number of turns on the coil
  • increasing the strength of the magnet
  • inserting a soft iron core into the coil