ELECTRICITY FUNDAMENTALS

Relevant Reading in the Book:

• Pages 10-11
• Pages 85-92
• Pages 105-114

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 coulomb.
• 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 negative material to positive material and is essentially the number of electrons per second that are carried through a conductor.

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.

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 voltages in an outlet is 120 Volts.
• Toaster element is 15 ohms
• I = V/R = 120/15 = 8 amps flow through your toaster

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

  Power = V * I

  (but for most appliances/consumer electronics V is fixed at 120 Volts). So the toaster has a power of 120x8 = 960 Watts.

  A better way to understand your electricity bill:

  Since V = R*I then Power = I*I*R = I²R

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

  Your stove "burners" are made of some material which has some resistance R. You can't change R. To cook your food you need to have so much power radiated by the stove. Since R in the burner material is quite high, the material resists the flow of electrons and the material heats up. The more electrons per second (e.g. I) that you can put through the burner the more power which is radiated away as heat.

  You pay for I, but its I² that determines the power usage that you pay for. The longer you leave the stove burner on, the more energy you have used. For instance, if you leave the toaster on for one hour, you have used 1 KWH of energy.

What Stan Said: (we will be going over this stuff in other lectures as well).

Energy has different forms:

• chemical energy (stored energy in say gasoline)
• potential energy
• kinetic energy
• heat energy

Energy is always conserved:

Energy is neither created or destroyed it can only be converted from one form to another.

Units of Energy:

Energy has units of mass x (length/time)² This is called a JOULE.

Power:

Power is the rate of energy usage or:

Power = Energy/Time
Energy = Power x Time

The basic unit of power is a Watt. 1 Watt = 1 Joule/Sec.

1 Watt-second = 1 Joule.

3600 Watt-seconds = 1 Watt-hour = 3600 Joules

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.

Forces:

A force causes a change in the motion of an object. More specifically, it causes an acceleration.

Work is defined as moving an object some distance against a force. If you climb the 4 flights of stairs in the Willamette Hall atrium you are doing work against a gravitational force. As you increase your height above the Atrium floor (the earth) that work you have done is transferred into potential and kinetic energy.

Note that 1 Joule of energy is equivalent to the work it would take to lift 1 kilogram of material to a height of 10 cm.

Electro-magnetic Force:

This force is similar to gravity in some respects except that it is both a repulsive and attractive force. Electricity is a result of the application of electro-magnetic force to objects.

The basic carrier of electric charge in an atom is the electron. The units of charge are denoted as q, which is the charge of an individual electron.

On standard unit for charge is the Coulomb but this represents a very large charge. 1 Coulomb represents about 6 x 10¹⁹ individual electrons!

Conservation of Charge:

This is similar to conservation of energy. Charge can neither be created or destroyed but just transferred.

When you rub a rod on a wool cloth to create 'static" electricity - you are not creating charge. Your are merely transferring electrons from one object to another to produce a charge imbalance.

Voltage:

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

What do you get if you multiply Voltage by Current?

Joule/coulomb x coulomb/second = Joule/second = Power

Power = V x I

the voltage in your house is 120 Volts (that's a standard in America). If you have a 100 Watt light bulb it will draw 100/120 = .83 amps of electricity.

If you leave this 100 watt light bulb on for 10 hours you have used 1 KWH hour of electricity. Alternatively you have used 8.3 amp-hours of electrons. Your utility company had to sell you this many electrons to kepp your bulb lit for 10 hours and that's what you paid for.

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