ELECTRICITY FUNDAMENTALS

Electricity is an apparent force in nature that exists whenever there is a net electrical charge between any two objects.

Basics of Electrostatics:

• Electrical charges are either negative (electrons) or positive (protons)

• The unit of charge, q , is called the coloumb.

• When there are equal numbers of positive and negative charges there is no electrical force as there is no net charge. This is the case for a neutral atom.

• Electrical force is created when electrons are transferred from one material to another (e.g. rubbing a wool cloth with a plastic comb).

• Electrical charge is conserved; charge is neither created nor destroyed

Properties of Electricity:

CURRENT: denoted by I and measured in amperes. Current flows from negatively charged material to positively charged material and is essentially the number of electrons per second that are carried through a conductor. Current is measured in units of amps. 1 amp = 1 coloumb/sec = 6.2 x 10¹⁸ electrons per second!

VOLTAGE: Potential difference between a negatively charged object and a positively charged one (like two terminals on a battery). Potential difference is measured in units of Volts ( V )which represents the work done per unit charge to move electrons between the positive and negative terminals. If a potential difference exists, then energy can be extracted.

Imagine that you have two opposite charges that you want to separate. It takes work to separate the charge and thus the separated charges store energy. The amount of stored energy is given by:

E = qV where V is the voltage or electric potential of some system.

The units of voltage or Volts: 1 Volt = 1 Joule/Coulomb

If the separated charges get back together, work/energy can be extracted from the system. If there is some pathway for the charges to flow then we get a current. Current is denoted by I and is in units of amperes or amps 1 Ampere = 1 coulomb/second

RESISTANCE: Property of material that helps prevent the flow of electrons in it. Metals are good conductors due to low resistance. Wood is a poor conductor due to high resistance. Resistance, R , is measured in ohms and depends upon both the type of material and its size. Long wires have more resistance than short wires; thin wires have more resistance than thick wires. R is also temperature dependent.

OHM's LAW

Is there a relation between I, V, and R ? Let's do an experiment:

In the above circuit there is a battery (V), some resistors (R), and a light bulb that can only be activated if the right number of amps reach it. We can control this buy putting the right resistance in the circuit for a given battery Voltage.

Experimental results then lead to Ohms law:

V = R * I

This is a linear relation. If you double the voltage (V) then for the same value of R you get twice the current. If you want to keep the current the same value after doubling V, you would have to double the resistance (R).

Example:

• Standard US household voltage is 120 Volts.
• The heating element in your toaster has R = 15 ohms.
• What is the current flowing through your toaster?
  I = V/R = 120/15 = 8 amps

Your electricity bill essentially measures the amount of current that you use but you use this current as Power .

Power = V * I

So the toaster has a power of 120x8 = 960 Watts.

Energy = Power * Time (and its energy kilowatt hours that you pay for - a 100 watt light bulb left on 10 hours = 1 kilowatt hour. )

If you leave your toaster on for one hour, than that would also be approximately 1 KWH (960 watt-hours if you want to nit pick).

1000 Watt-hours = 1 Kilowatt hour (KWH); A KWH will be our basic unit of energy in this class. You purchase KWHs from the electric utility whenever you use power in your home.

Now its important to get a sense of scale, regarding watts, kilowatts, megawatts, etc.

• 100 Watts is a convenient starting place. This is the power of a standard incandescent light bulb. Its also the power consumption of a standard laptop computer.

• 1 Kilowatt (KW) = 1000 Watts This is the typical power consumption for a toaster, a washing machine motor, etc.

• A typical 2000 square foot house with normal applicances in it requires an average power of 2 KW. Over a course of a 24 hours day, the house has used 2 KW x 24hrs = 48 KWH of electricity. At cost of 10 cents per KWH, this house used $4.80 of electricial energy.

• 1 Megawatt = 1000 Kilowatts = 1 million watts. Megawatt power consumption levels are now in the large building domain. For instance, the entire UO campus uses 20 MW of power (including space heating). The city of Eugene uses 310 MW on average with Peak loads (winter time) of 500-559 MW Note: in 2006 only 11.9% of the annual power use was produced by EWEB owned facilities What are the implications of this?

• 1 Gigawatt = 1000 MW This is the unit capacity of a typical large scale power plant (usually coal fired). The largest single energy generating facility in the world is this one:
  

  

  With total capacity of 18,000 MW. To scale to the US this is equivalent to the US bringing 198,000 MW of new power on line.