Lecture II. Electricity
- Energy is the ability to do work. joule (J)
- Work is product of distance and component
of force along the distance moved. 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 energy.
- Efficiency is the measure of our success in
converting stored energy into useful energy: < 1.
Types of energy
But: easily-stored not transportable/usable; end-use
||Chemical: coal, oil, nat. gas, food
Gravitational: water behind a dam
Nuclear: uranium, plutonium, hydrogen isotopes
||Electrical, chemical, solar (how?)|
||Kinetic: transportation, industrial
Thermal: residential, industrial, commercial
Radiant: residential, commercial
Energy: Stored to End-Use
Total resources: 99.2 Quads
Total US usage: 97.2 Quads (incl nonfuel but not exports)
Assume "charges" can be moved.
- Charge unit: Coulomb (C) (~1019
- Voltage unit: Volt (V), esp., voltage
- Energy . 1 C in 1 V difference has 1 J energy;
voltage difference cause charge flow: current.
- Current (I) unit: ampere or amp (A) = C/s.
- Resistor allows current flow under voltage diff.
- Ohm found relation: V = R I.
R 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
then (C/s) V = I V is rate of doing work.
By Ohm, power dissipated is I V = I2 R.
This dissipation heats the current-carrying wire.
- Bad news. Transporting electric power involves the
dissipation of energy. Transmission losses lower efficiency.
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.
To cite this page:
Electricity: Chapter 4
[Friday, 21-Sep-2018 19:22:04 EDT]
Edited by: firstname.lastname@example.org on
Wednesday, 24-Sep-2008 17:51:41 EDT