Capacitance and magnetism

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Capacitance and magnetism

• Capacitance
• Electromagnetism
• How electromagnetism is used on the automobile
• Electromagnetic interference suppression (EMI)

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Capacitance

• The ability of an object to store an electrical
  charge
• Consists of two conductive plates with an
  insulator between them

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Capacitance

• When a capacitor is placed in a closed circuit,
  electrons collect on one plate while the other
  plate loses electrons
• When the circuit is opened, the capacitor will
  hold its charge until it is placed in an external
  closed circuit where it will discharge

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Capacitance

• Capacitors used on the automobile
• Voltage spike suppression
• Used as a shock absorber for voltage surges (ex
  ignition coil)
• Radio interference suppression
• Can be used to block DC current while allowing AC
  current direct path to ground
• Can be used to store charges for electronics

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Electromagnetism

• Magnetism is a form of energy that is caused by
  electron motion in some materials
• Electromagnetism is a magnetic field created by
  current flowing through a conductor

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Electromagnetism

• An electromagnetic field can be strengthened by
  increasing current flow through the conductor
• As the current flow decreases or stops, the
  magnetic field collapses
• Electricity can create magnetism
• Magnetic fields can induce current flow

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How it is used

• Electric motors use magnetic fields to convert
  electrical energy into mechanical energy
• Two conductors carry current in opposite
  directions between two opposing magnets
• The magnetic fields of the conductors attract and
  repel with the magnetic fields of the poles
  causing the center of the motor to rotate

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How it is used

• Electromagnets are made with a coil of wire
  around an iron core
• When current flows through the coil, the magnetic
  fields concentrate in the iron core
• This concentration of force magnifies the
  strength of the magnetic field

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How it is used

• Relays use electromagnets to control a high
  current circuit with a low current circuit
• When the coil is energized, the force from the
  electromagnet pulls another set of contacts
  closed allowing current flow in that circuit

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How it is used

• Electromagnetic induction magnetic fields create
  or induce a voltage in a conductor if either the
  magnet or the conductor is moving

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How it is used

• Electromagnetic Induction
• Voltage is induced when magnetic fields are
  broken by a conductor
• This can be accomplished by a conductor moving
  across a stationary magnetic field or a magnetic
  field moving across a stationary conductor
• Can be used for speed and position sensors

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How it is used

• Mutual induction used in ignition coils
• Two coils of wire are wound around the same iron
  core
• One coil is connected to the battery through a
  switch (the ignition module or igniter)
• The second coil is connected to an external
  circuit (the spark plug)

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How it is used

• Mutual Induction
• When the switch closes and current flows through
  the primary coil, a magnetic field builds up
  around both coils
• When the primary field expands across the
  secondary winding, a current is built up in the
  secondary winding
• When the switch opens and primary current stops,
  the magnetic field collapses and causes a high
  voltage surge in the secondary winding

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EMI

• Whenever there is current in a conductor, an
  electromagnetic field is created
• When current starts and stops, field strength
  changes and an electromagnetic signal is created
• Electromagnetic signals or waves can interfere
  with radio signals or electronic systems on the
  car (example your car stereos reaction just
  before your cell phone goes off)

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EMI

• Ways to reduce or suppress EMI
• Adding resistance to conductors in order to
  suppress signal radiation
• Using capacitors to reduce the inducing of EMI
• Metal shielding around conductors to reduce
  signal radiation
• Metal ground straps to bypass unwanted signals to
  ground

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• Often, automakers will install ground straps
  between body parts such as a hood and fender,
  where no electrical current exists
• The straps only job is to suppress EMI without
  it, the cars hood could function as a capacitor