Electromagnets

• Motor Effect: current-carrying wire, which creates a magnetic field, interacts with another magnetic field, causing a force that pushes the wire at right angles
• Fleming's left hand rule finds the direction of a motor effect force:

  • thumb: direction
  • index: field (north to south)
  • middle: current (positive to negative)
• The force is greatest when the direction of the current is perpendicular to the direction of the magnetic field
• There is no force if the current is parallel to the magnetic field
• A solenoid carrying a current in a magnetic field experiences a force that causes it to rotate, which can be used to make an electric motor.
• Electric Motor:

  • current in left hand part of the coil causes downward force and right hand part of the coil causes an upward force, causing the coil to rotate anticlockwise
  • when the coil is vertical it is parallel so it is not moved, except for the momentum and the split ring commutator changing current direction
• Headphones contain small loudspeakers. Variations in the electric current cause variations in the magnetic field produced by an electromagnet. This causes the cone to move, creating pressure variations in the air forming sound waves.
• Generator Effect: a voltage is needed to make an electric current flow in a circuit, also called electromagnetic induction
• The induced voltage produces an induced current if the conductor is connected in a complete circuit.
• The generator effect: The induction of a potential difference (and current if there is a complete circuit) in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic field.
• The generator effect creates a pd in a conductor and a current if the conductor is part of a complete circuit.

  This is done by moving a magnet in a coil of wire or moving a conductor (wire) in a magnetic field, which cuts field lines
• If you move the magnet or conductor in the opposite direction, or reverse the polarity of the magnet, then the PD or current will be reversed
• A change in magnetic field induces a current in a wire, but when a current flows through a wire a magnetic field is created. The induced current opposes the change that made it.
• The size of the induced PD can be changed through increasing movement speed or the strength of the magnetic field
• Alternators generate alternating current. They rotate a coil in a magnetic field, inducing a current which changes direction every half turn. slip rings and brushes mean the contacts don't swap every half turn.
• Dynamos generate direct current and rotate a coil in a magnetic field, using a split-ring commutator to swap the connection every half turn so the current flows in the same direction.
• • as you turn a magnet, the magnetic field through the coil changes, inducing a pd, and then a current
  • half a turn of the magnet reverses the direction of the magnetic field through the coil reverses, so does the pd, so the current flows in the opposite direction
  • turning the magnet in the same direction, will make the pd reverse every half turn, creating an alternating current
• Microphones:

  • sound hits diaphragm attached to solenoid around a magnet
  • causes solenoid to move in the magnetic field, generating a current
  • movement of coil and current depends on the properties of the sound wave
• Transformers change the size of the pd of an alternating current. They have two coils of wire joined with an iron core
• When an alternating pd is applied across the primary coil the iron core magnetizes and demagnetizes quickly. This changing magnetic field induces an alternating pd in the secondary coil. If the second coil is part of a complete circuit the current becomes induced
• The ratio between the primary and secondary pd's is the same as the ratio between the number of turns on the primary and secondary coils.
• Step-up transformers increase the pd and have more secondary turns
• Step-down transformers have more primary turns
• A high pd lowers current, so less energy is wasted.