# Lecture II. Electricity

### Review

• 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 energy.
• Efficiency is the measure of our success in converting stored energy into useful energy: .

## Electricity - page 2

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

There are more energy graphs.

## 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.