Lecture II. Electricity


Electricity - page 2

Types of energy

Easily-stored energy is not transportable/usable;
end-use energy is not storable.

Energy "Form"Examples
Easily-stored Chemical: coal, oil, nat. gas, food
Gravitational: water behind a dam
Nuclear: uranium, plutonium,
    hydrogen isotopes
Easily-transported Electrical, chemical, solar(?)
End-use Kinetic: transportation, industrial
Thermal: residential, industrial,
Radiant: residential, commercial

Energy: Stored to End-Use

Electricity Fundamentals.

Assume "charges" can be moved.

  1. Charge unit: Coulomb (C) (~1019 electrons)
  2. Voltage unit: Volt (V), esp., voltage difference.
  3. Energy 1 C across 1 V difference is 1 J energy; voltage difference drives charge flow: current.
  4. Current (I) unit: ampere or amp (A) = C/s.
  5. Resistor allows current flow under voltage difference.
  6. Georg Ohm found relation (Ohm's law):
    V = R I. Unit: Ohm (Ω) = V/A.
  7. Power dissipated in a resistor is work done in moving charge thru voltage difference; if C V is energy, then (C/s) V = I V is rate of doing work. By Ohm's law, power dissipated is
       I V = I2 R.
  8. Bad news. This dissipation heats the current-carrying wire. Transmitting electric power loses energy in heat: lower efficiency.

How to create voltage difference

  1. Magnets exist (don't ask how).
  2. Faraday discovered current-carrying wire moving in a magnetic field produces voltage.
  3. Motors reverse process. Wire in magnetic field with applied voltage difference is moved.
  4. Engineers have found efficient ways to
    • wind many-wire coils; thus increasing voltage or force.
    • transform voltage from low to high. At constant power -- note: P = I V -- lower current cuts dissipation.