• The?motor effect?occurs:

When a wire with current flowing through it is placed in a magnetic field and experiences a force

• This effect is a result of?two?interacting?magnetic fields
  • One is produced around the wire due to the current flowing through it
  • The second is the magnetic field into which the wire is placed, for example, between two magnets

   

• As a result of the interactions of the two magnetic fields, the wire will experience a?force

The motor effect is a result of two magnetic fields interacting to produce a force on the wire

Simple Motors

• The?motor effect?can be used to create a simple?d.c?electric motor
• The simple d.c. motor consists of a coil of wire (which is free to rotate) positioned in a?uniform magnetic field:

A simple d.c. motor consisting of two magnets, a coil and a split ring commutator to control the direction of the current

• When the current is flowing in the coil at 90^o?to the direction of the magnetic field:
  • The?current?creates a?magnetic field?around the coil
  • The magnetic field produced around the coil interacts with the field produced by the magnets
  • This results in a?force?being exerted on the coil
  • The direction of the force can be determined using?Fleming's left-hand rule
  • As current will flow in?opposite?directions on each side of the coil, the force produced from the magnetic field will push one side of the coil?up?and the other side of the coil?down

   

• This will cause the coil to?rotate, and it will continue to rotate until it is in the vertical position
  • When the coil is in the vertical position there will be a force acting upwards and a force acting downwards

   

Forces acting on the coil in the vertical position

• The?split ring commutator?swaps the contacts of the coil
  • This reverses the direction in which the current is flowing

   

• Reversing the direction of the current will also reverse the direction in which the forces are acting
  • As a result, the coil will continue to?rotate

   

Forces on coil after commutator has reversed the direction of the current

• The commutator reverses the direction of the current in the coil every?half turn
  • This will keep the coil rotating continuously as long as the current is flowing

   

Factors Affecting the D.C Motor

• The?speed?at which the coil rotates can be increased by:
  • Increasing the?current
  • Increasing the strength of the?magnetic field

   

• The?direction of rotation?of coil in the d.c motor can be changed by:
  • Reversing the direction of the?current
  • Reversing the direction of the magnetic field by reversing the?poles?of the magnet

   

• The?force?supplied by the motor can be increased by:
  • Increasing the?current?in the coil
  • Increasing the strength of the?magnetic field
  • Adding?more turns?to the coil

   

Loudspeakers

• Loudspeakers and headphones convert electrical signals into sound
  • They work due to the?motor effect

   

• They work in the opposite way to microphones
• A loudspeaker consists of a?coil of wire?which is wrapped around one pole of a?permanent magnet

Diagram showing a cross-section of a loudspeaker

• An?alternating current?passes through the coil of the loudspeaker
  • This creates a?changing magnetic field?around the coil

   

• As the current is constantly changing direction, the direction of the magnetic field will be?constantly changing
• The magnetic field produced around the coil?interacts?with the field from the permanent magnet
• The interacting magnetic fields will exert a?force?on the coil
  • The direction of the force at any instant can be determined using?Fleming’s left-hand rule

   

• As the magnetic field is constantly changing direction, the?force?exerted on the coil will?constantly change direction
  • This makes the coil?oscillate

   

• The oscillating coil causes the speaker cone to oscillate
  • This makes the air oscillate, creating?sound waves

   

Step 1: Draw arrows to show the direction of the magnetic field lines

• • These will go from the north pole of the magnet to the south pole of the magnet

   

Step 2: Draw arrows to show the direction the current is flowing in the coils

• • Current will flow from the positive terminal of the battery to the negative terminal

   

Step 3: Use Fleming’s left hand rule to determine the direction of the force on each side of the coil

• • Start by pointing your?First?Finger in the direction of the (magnetic)?Field
  • Now rotate your hand around the first finger so that the seCond finger points in the direction of the?Current
  • The?THumb will now be pointing in the direction of the?THrust (the force)

   

Step 4: Use the force arrows to determine the direction of rotation

• • The coil will be turning?clockwise

   

• Magnetic forces are due to interactions between?magnetic fields
  • Stronger?magnetic fields produce?stronger?forces and vice versa

   

• For a current carrying conductor, the size of the force exerted by the magnetic fields can be?increased?by:
  • Increasing the amount of?current?flowing through the wire
    • This will increase the magnetic field around the wire

     

  • Using?stronger magnets
    • This will increase the magnetic field between the poles of the magnet

     

  • Placing the wire at?90^o?to the direction of the magnetic field lines between the poles of the magnet
    • This will result in the maximum interaction between the two magnetic fields

     

   

• Note: If the two magnetic fields are?parallel?there will be no interaction between the two magnetic fields and therefore?no force?produced

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