7.2 The Motor Effect

• What is produced around a conducting wire when current flows through it?

  A magnetic field

  
• How can the shape and direction of the magnetic field around a wire be investigated?

  Using plotting compasses
• What does the magnetic field around a current-carrying wire look like?

  • Made up of concentric circles
  • Indicates no poles
  • Strength decreases with distance from the wire
• What does the right-hand thumb rule help determine?

  The direction of the magnetic field

  
• What happens to the magnetic field when the current direction is reversed?

  The direction of the magnetic field reverses
• What occurs if there is no current flowing through the conductor?

  No magnetic field is produced
• How does increasing the current affect the magnetic field strength?

  It increases the strength of the magnetic field
• What happens to the magnetic field lines when the current increases?

  • Field lines become closer together
  • Indicates stronger magnetic field
• What is the magnetic field around a solenoid like?

  • Similar to that of a bar magnet
  • Strong and uniform inside the solenoid
  • Has distinct north and south poles
• How can the polarity of a solenoid be determined?

  By viewing the current direction from the end
• What indicates a south pole in a solenoid?

  Current traveling clockwise
• What indicates a north pole in a solenoid?

  Current traveling anticlockwise
• What happens to the magnetic field if the current changes direction in a solenoid?

  The north and south poles will be reversed
• How can the strength of the magnetic field around a solenoid be increased?

  • Increase the current size
  • Increase the number of turns in the coil
  • Add an iron core through the coils
• What happens to the iron core when current flows through the coils of a solenoid?

  It becomes an induced magnet
• What is an electromagnet?

  A solenoid with an iron core
• How can the magnetic field of an electromagnet be controlled?

  By switching the current on and off
• What happens to the magnetic field when the current is switched off in an electromagnet?

  No magnetic field is produced
• How does changing the current affect the strength of an electromagnet?

  • Increasing current increases magnetic field strength
  • Decreasing current decreases magnetic field strength
• What happens when the electromagnet in a scrapyard crane is switched on?

  It attracts magnetic materials
• What occurs when the electromagnet in a scrapyard crane is switched off?

  It drops the magnetic materials
• How does an electric bell operate?

  • Current passes through electromagnet
  • Creates magnetic field attracting iron armature
  • Hammer strikes the bell
  • Movement breaks the circuit, stopping the current
• What is the motor effect?

  A force on a wire in a magnetic field
• What causes the motor effect?

  Interaction of two magnetic fields
• What factors affect the force experienced by a current-carrying wire?

  • Amount of current flowing
  • Strength of the magnets
  • Angle of the wire to the magnetic field

  
• What is the equation to calculate the force on a current-carrying wire?

  F = BIL

  
• What does each variable in the equation F = BIL represent?

  F = force, B = magnetic flux density, I = current, L = length

  
• When is the magnetic force on a conductor maximum?

  When field lines are perpendicular
• What happens to the force when the wire and field are parallel?

  The force is zero
• What is Fleming's Left-Hand Rule used for?

  • Determines direction of force
  • Determines direction of magnetic field
  • Determines direction of current

  
• How are the force, magnetic field, and current oriented according to Fleming's Left-Hand Rule?

  All are perpendicular to each other
• What should you look for in an electromagnetic device?

  • A coil or solenoid acting as an electromagnet
  • A piece of iron attracted to the solenoid
• What does Fleming's Left-Hand Rule help determine?

  Directions of force, magnetic field, and current

  
• How are the force, current, and magnetic field oriented in Fleming's Left-Hand Rule?

  All three are perpendicular to each other
• What happens to the force on a wire in a magnetic field?

  It can be into or out of the page
• What are the steps to use Fleming's Left-Hand Rule?

  1. Point the First Finger in the direction of the magnetic field.
  2. Rotate your hand so the Second Finger points in the direction of the current.
  3. The Thumb will point in the direction of the thrust (force).

  
• What is the role of the split ring in a d.c. motor?

  It connects the coil to the current flow

  
• What causes the coil in a d.c. motor to rotate?

  Forces acting in opposite directions on each side
• What happens when the coil in a d.c. motor reaches a vertical position?

  No force acts on the coil
• How does the coil continue to rotate after reaching vertical?

  The momentum of the coil keeps it rotating