motor effect & Flemings LH rule

motor effect

when a conductor (wire) carrying a current is placed in a magnetic field the magnet producing the field and the conductor (wire) exert a force on each other (causing the wire to move)

How is a force produced?

• a current carrying wire creates a magnetic field

• the wire is placed between the poles of a magnet (perpendicular to mag field)

• produces a force causing wire to move

How can we increase the force?

• increase the magnetic field

• increase the current

• force is greatest when the wire is perpendicular to the mag field and the weakest parallel to mag field lines

Fleming’s left hand rule

• thumb = (direction of) movement

• first finger = magnetic field ( north to south)

• second finger = current (+ to - )

How does a loudspeaker use the motor effect?

• the coil (current carrying wire) is attached to a diaphram inside a magnetic field

• as alternating potential difference is applied to coil it moves vibrating the diaphram

• moves the air causing sound waves of the same frequency as the alternating potential difference the coil recieved

what is magnetic flux density measured in ?

Tesla (T)

How do electric motors work?

• current carrying coil (has its own mag field) is in magnetic field (between poles of a magnet) which produces a force

• force is experienced on sides of the coil

• one side move up and the other down causing rotation

• a current is passed through the metal brushes in contact with the split ring commutators

• the arrangement reverses direction of the current around every half turn (180) → the coil always spins in the same direction

How do split ring commutators work ?

• the coil is attached to a split ring commutator → the split ring commutator touches metal or graphite brushes which are attached to the rest of the circuit

• the arrangement reverses the direction of the current around every half turn (180) so the coil always spins inthe same direction and doesn’t keep flipping back and forth

How can we make the motor spin faster by increasing force?

• increasing current

• increasing strength of magnetic field (stronger magnet)

• in addition : use more coils / reduce distance between the coil and magnets

How can we reverse the direction of the motor?

by reversing the direction of the force :

• reversing direction of current

• swapping over the magnets ( so the magnetic field runs the other way)

Exam tip !! : link to moments

• the forces are in different directions but they both act in a clockwise direction

• → clockwise moment > anticlockwise moment (which is 0) so the object rotates

• → constantly changing direction so it’s accelerating even if speed is constant

• unbalanced moment