Potential difference

: charge flows when there is a difference in voltage across the ends of a conductor

Electric current

is simply the flow of electric charge or electrons.

• • • A

voltage source

provides a sustained potential difference to allow electrons to flow.

Dry cells, wet cells, and generators are good voltage sources.

Dry cells and wet cells are the result of a chemical reaction.

Generators convert mechanical energy to electrical energy.

Voltage

– is the electric potential per charge moving between terminals.

This is like the electric pressure pushing the electrons.

Example: The potential difference between two slots in a household outlet is ~120 volts or 120 joules/coulomb of charge.

Voltage is the potential energy /unit of charge that pushes the electrons.

Electric current is measured in

amperes

.

5 amps = 5 coulombs of charge in one second or about 31.25 billion billion electrons each second.

The net charge of any wire is zero because just as many electrons leave the wire that enter it.

Electric resistance

flow of electrons.

can slow down the Resistance depends on the conductivity of the wire, and also the thickness and length of the wire.

Thick wires

have less resistance.

Short wires

have less resistance.

Resistance is measured in ohms (Ω)

Ohm’s law

: V = IR

Resistors

are used in electrical devices to control the resistance.

Resistance in the body can be from about 100 ohms if you’re soaked in salt water to about 500,000 ohms for dry skin.

Touching a 120 volt electric circuit normally would do no harm because of high resistance If you were standing in water, the resistance is lowered so that the current travels through faster.

Bibliographic Entry Result (w/surrounding text) Standardized Result Cutnell, John D., Johnson, Kenneth W. Physics. 4th ed. New York, NY: Wiley, 1998.

"Currents of approximately 0.2 A are potentially fatal, because they can make the heart fibrillate, or beat in an uncontrolled manner." 0.2 A Carr, Joseph J. Safety for electronic hobbyists Britannica.com.

. Popular Electronics. October 1997. as found in "In general, for limb-contact electrical shocks, accepted rules of thumb are: 1-5 mA is the level of perception; 10 mA is the level where pain is sensed; at 100 mA severe muscular contraction occurs, and at 100-300 mA electrocution occurs." 0.1–0.3 A "Electrical Injuries." The Merck Manual of Medical Information: Home Edition. Pennsylvania: Merck, 1997.

"At currents as low as 60 to 100 milliamperes, low-voltage (110-220 volts), 60-hertz alternating current traveling through the chest for a split second can cause life-threatening irregular heart rhythms. About 300-500 milliamperes of direct current is needed to have the same effect." 0.06–0.1 A (AC) 0.3–0.5 A (DC) Zitzewitz, Paul W., Neff, Robert F. Merrill Physics, Principles and Problems. New York: Glencoe McGraw-Hill, 1995.

"The damage caused by electric shock depends on the current flowing through the body - 1 mA can be felt; 5 mA is painful. Above 15 mA, a person loses muscle control, and 70 mA can be fatal." 0.07 A Watson, George. SCEN 103 Class 12 . University of Delaware. March 8, 1999.

"0.10 death due to fibrillation > 0.20 no fibrillation, but severe burning, no breathing" Miller, Rex. Industrial Electricity Handbook. Peoria, IL: Chas. A. Bennet, 1993.

0.1–0.2 A "Currents between 100 and 200 mA are lethal." 0.1–0.2 A

Your heart has an electrical component as well.

Birds standing on an electric wire are not harmed because there is no potential difference.

Direct current (dc)

current flows in only one direction. Ex: battery – is when the

Alternating current (ac)

current flows in one direction then the other. Ex: outlets – the

In the U.S. We alternate the electric current at about 60 cycles per second or 60 hertz.

Diodes

can be used to change ac to dc current. Diodes allow only half the electrons to flow through so that they are all flowing in the same direction.

When material is proportional to the electric current voltage across it, the material is said to be "ohmic", or to obey in a Ohm's law . A microscopic view suggests that this proportionality comes from the fact that an applied electric field superimposes a small drift velocity on the free electrons in a metal. For ordinary currents, this drift velocity is on the order of millimeters per second in contrast to the speeds of the electrons themselves which are on the order of a million meters per second. Even the electron speeds are themselves small compared to the speed of transmission of an electrical signal down a wire, which is on the order of the speed of light, 300 million meters per second .

At room temp. electrons have a speed of a few million kilometers per hour.

The electric field around the electrons moves at nearly the speed of light.

Inside wire, the electric field is directed along the wire.

In dc current the electrons drift about .01cm/s In ac current the electrons move a fraction of a cm in one direction then move back the same amount equaling no net movement.

Electrons themselves

do not

flow.

Electrons already in a material vibrate causing energy to flow by the electric field.

Therefore power plants do not sell electrons, they sell energy and you supply the electrons.

Electric power

is the rate in which electricity is changed to mechanical energy, heat, or light.

Electric power = current x voltage Or 1 watt (J/S)= 1 amp (C/S) x 1 volt (J/C) P = I x V Use Ohm’s Law to derive other expressions for power

A

kilowatt

is 1000 volts.

A

kilowatt hour

is the amount of energy consumed in one hour at the rate of 1 kilowatt.