Ch. 6 Electricity and magnetism

1
Ch. 6 Electricity and magnetism

• Electric charge electrons (-) and protons ()
• Coulombs Law
• Voltage is measure of electric potential (energy
  per unit charge)
• Current is moving charge -- 1 C/s is a current of
  1 ampere (1 A)
• Power Current ? Voltage

2
Electrical Circuits

• Electrons in conductors are free to move
• Electrons flowing in a wire encounter some
  resistance (R)
• Resistance causes heating if current flows
• Ohms Law V I ? R
• Complete circuit is required
• Series vs. parallel
• Fuses and circuit breakers

3
Breakdown and sparks

• Spark is an electron avalanche
• Breakdown occurs through normally insulating
  material (air, plastic, etc.)
• Dry air breakdown 30 kV/cm
• Lightning - 10s or 100s of kA with voltage up to
  GV range
• Breakdown is easier at
• sharp corners or edges
• Electron multipliers

4
Superconductivity

• At low temperature, the resistance of some
  materials vanishes.
• Get current started in a loop, it will continue
  forever by itself
• Used in MRI, bending magnets at particle
  accelerators, SQUIDs, magnetic levitation trains
• Currently need liquid nitrogen temperatures or
  colder

5
The strange concept of fields

• Non-contact forces action at a distance?
• Electric charges (and other things) set up
  fields, which exert forces on other charges,
  etc.
• Remove a charge suddenly, and its field does not
  go away as rapidly
• Oscillating fields (waves) can exist long after
  the charges that created them are gone.

6
Magnets and magnetic fields

• No magnetic charges
• Earth, lodestones, and north/south
• Electric current produces a magnetic field --
  electromagnets
• Changing magnetic field creates an electric
  current in nearby loops of wire -- induction,
  motors, generators
• Changing electric field creates a changing
  magnetic field which creates a changing electric
  field.

7
Transformers

• AC current makes changing B-field which makes AC
  current
• Output depends on number of coils
• V multiplied by Nout/Nin