The Motor Effect (Edexcel IGCSE Physics)

Magnetic force on a current-carrying wire

• 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

• When no current is passed through a conductor in a magnetic field, however, it will experience no force

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

The D.C. motor

• The motor effect can be used to create a simple d.c. electric motor

  • The force on a current-carrying coil is used to make it rotate in a single direction

• The simple D.C. motor consists of a coil of wire (which is free to rotate) positioned in a uniform magnetic field

• The coil of wire, when horizontal, forms a complete circuit with a cell

  • The coil is attached to a split ring (a circular tube of metal split in two)

  • This split ring is connected in a circuit with the cell via contact with conducting carbon brushes

Forces on the horizontal coil in a D.C. motor

Forces acting in opposite directions on each side of the coil, causing it to rotate. The split ring connects the coil to the flow of current

• Current flowing through the coil produces a magnetic field

  • This magnetic field interacts with the uniform external field, so a force is exerted on the wire

• Forces act in opposite directions on each side of the coil, causing it to rotate:

  • On the blue side of the coil, current travels towards the cell so the force acts upwards (using Fleming's left-hand rule)

  • On the black side, current flows away from the cell so the force acts downwards

• Once the coil has rotated 90°, the split ring is no longer in contact with the brushes

  • No current flows through the coil so no forces act

Coil in the vertical position

No force acts on the coil when vertical, as the split ring is not in contact with the brushes

• Even though no force acts, the momentum of the coil causes the coil to continue to rotate slightly

• The split ring reconnects with the carbon brushes and current flows through the coil again

  • Now the blue side is on the right and the black side is on the left

• Current still flows toward the cell on the left and away from the cell on the right, even though the coil has flipped

  • The black side of the coil experiences an upward force on the left and the blue side experiences a downward force on the right

  • The coil continues to rotate in the same direction, forming a continuously spinning motor

Forces on the coil when rotated 180°

Even though the coil has flipped, current still flows anticlockwise and the forces still cause rotation in the same direction

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

Factors affecting magnetic force

• 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