Lecture II. Electricity
- Energy is the ability to do work. Unit: joule (J)
- Work is product of distance and component
of force along the distance moved. Unit: joule (J).
- Power is the rate of doing work or use of energy.
Unit: watt (W) = joule/sec (J/s).
- Energy Conserved: energy is conserved.
Difficulty is conserving useful
- Efficiency is the measure of our success in
converting stored energy into useful energy: .
Electricity - page 2
Types of energy
Easily-stored energy is not
end-use energy is not storable.
||Chemical: coal, oil, nat. gas, food
Gravitational: water behind a dam
Nuclear: uranium, plutonium,
||Electrical, chemical, solar(?)|
||Kinetic: transportation, industrial
Thermal: residential, industrial,
Radiant: residential, commercial
There are more energy graphs.
Assume "charges" can be moved.
- Charge unit: Coulomb (C)
- Voltage unit: Volt (V), esp., voltage
- Energy 1 C across 1 V difference is 1 J energy;
voltage difference drives charge flow: current.
- Current (I) unit: ampere or amp (A) = C/s.
- Resistor allows current flow under voltage
- Georg Ohm found relation (Ohm's law):
V = R I. Unit: Ohm (Ω) = V/A.
- 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 =
- Bad news.
This dissipation heats the current-carrying wire.
Transmitting electric power loses energy in heat:
How to create voltage difference
- Magnets exist (don't ask how).
- Faraday discovered current-carrying wire
moving in a magnetic field produces voltage.
- Motors reverse process. Wire in magnetic field
with applied voltage difference is moved.
- 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.