the motor and generator effect

• a current in a magnetic field experiences...

  a force
• what happens when a current-carrying wire (or any other conductor) is put between magnetic poles?

  the magnetic field around the wire interacts with the magnetic field it has been placed in, causing the magnet and the conductor to exert a force on each other
• motor effect


  a force is exerted on a current-carrying wire in a magnetic field, causing the wire to move
• to experience the full force of the motor effect...

  the wire has to be at 90° to the magnetic field - if the wire runs parallel to the magnetic field, it won't experience any force at all and at angles in between, it'll feel some force
• the force of the motor effect always...

  acts at right angles to the magnetic field of the magnets and to the direction of the current in the wire
• the magnitude of the force of the motor effect increases with...

  the strength of the magnetic field and the amount of current passing through the conductor
• What does the force acting on a conductor in a magnetic field depend on?

  - the magnetic flux density (how many field lines are in a region)

  - the size of the current through the conductor

  - the length of the conductor that is in the magnetic field
• Magnetic flux density equation

  Force (N) = magnetic flux density (T) x current (A) x length (m)

  F=BIl
• what does Fleming's left hand rule show?

  if either the current or the magnetic field is reversed, then the direction of the force will also be reversed
• Fleming's left hand rule

  - left hand

  - thumb: direction of force (motion)

  - first finger: direction of the field

  - second finger: direction of the current
• a current-carrying coil or wire ... in a magnetic field

  rotates
• basic dc motor


  a coil of wire through which the current flows rotates when placed in a magnetic field
• electricity is generated using the ... effect

  generator
• cutting field lines induces a...

  potential difference
• the generator effect


  this induces a potential difference (and current if there's a complete circuit) in a wire which is experiencing a change in magnetic field
• what does the generator effect induce?

  a potential difference in a conductor (and a current if there's conductor is part of a complete circuit)
• how does rotation cause the generator effect?

  by turning a magnet end to end in a coil or turning a coil inside a magnetic field - this is how generators work to produce alternating current or direct current
• the generator effect using rotation

  1) as you turn them magnet, the magnetic field through the coil changes - this change in the magnetic field induces a potential difference, which can make a current flow in the wire

  2) every time the magnetic field through moves through a half turn, the direction of the magnetic field through the coil reverses and when this happen, the potential difference reverses, so the current flows in the opposite direction around the coil of wire

  3) if you keep turning the magnetic field in the same direction, then the potential difference will keep on reversing every half turn and you'll get an alternating current
• induced current ... the change that made it

  opposes
• how does an induced current oppose the change that made it?

  the magnetic field created by an induced current always acts against the change that made it (whether that's the movement of a wire or a change in the field it's in) - it's trying to return things to the way they were
• how can you change the size of an induced potential difference?

  change the rate that the magnetic field is changing: induced potential difference can be increased by...

  - increasing the speed of movement (cutting more magnetic field like in a given time)

  - increasing the strength of the magnetic field (so there are more field lines that can be cut)
• alternators generate ... current

  alternating
• what do alternators do?

  rotate a coil in a magnetic field or a magnet in a coil. as the coil/magnet spins, a current is induced in the coil and this current changed direction every half turn, generating an alternating current
• what do alternators have instead of a split-ring commutator?

  slip rings and brushes so the contacts don't swap every half turn
• split-ring commutator


  rotates with the coil in an electric motor and reverses the direction of the current every half turn
• dynamos generate ... current

  direct
• what do dynamos do?

  generate direct current
• difference between alternators and dynamos

  dynamos have a split-ring commutator instead of slip rings
• split-ring commutator


  rotates with the coil in an electric motor and reverses the direction of the current every half turn to keep the current flowing in the same direction
• how can you use an oscilloscope to see the generated potential difference?

  - oscilliscopes show how the potential difference generated in the coil changes over time

  - for ac this is a line that goes up and down, crowing the horizontal axis

  - for dc the line stays above the axis: the potential is always positive

  - the high of the line at a given point is the generated potential difference at that time

  - increasing the frequency of revolutions increases the overall potential difference and creates more peak
• loud speakers work because of the...

  motor effect
• how do loudspeakers use the motor effect?

  1) an alternating current is sent through a coil of wire attached to the the base of a paper cone

  2) the coil surrounds one pole of a permanent magnet, and is surrounded by the other pole, so the current causes a force on the coil, forcing it to move

  3) when the current reverses, the force acts in the opposite direction, which causes the cone to move in the opposite direction too

  4) so variations in the current make the cone vibrate, which makes the air around the cone vibrate and creates the variations in pressure that cause a sound wave

  5) the frequency of the sound wave is the same as the frequency of the ac, so by controlling the frequency of the ac you can alter the save wave produced
• microphones generate current from...

  sound waves
• how do microwaves generate current from sound waves?

  1) sound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet

  2) this causes the coil of wire to move in the magnetic field, which generated a current

  3) the movement of the coil (and so the generated current) depends on the properties of the sound wave (louder sounds make the diaphragm move further)

  5) this is how microphones can convert the pressure variations of a sound wave into variations in current in an electric current