# Lecture II. Electricity

### Review

• 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

 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 (how?) End-use Kinetic: transportation, industrial Thermal: residential, industrial, commercial Radiant: residential, commercial

## Energy: Stored to End-Use

Total US usage: 97.2 Quads (incl nonfuel but not exports)

## 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 in 1 V difference has 1 J energy; voltage difference cause charge flow: current.
4. Current (I) unit: ampere or amp (A) = C/s.
5. Resistor allows current flow under voltage diff.
6. Ohm found relation: V = R I.
R 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
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.
8. Bad news. Transporting electric power involves the dissipation of energy. Transmission losses 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.