Current Electricity and Circuits Reference Chapters 34 and 35

Flow of Charge When ends of a conductor are at different potentials, a charge will flow from high potential to low potential until both ends reach a common potential. When there is no difference, flow ceases. To sustain flow, a method must be present that maintains a difference much as a water system maintains flow using a pump.

Electric Current Flow of charge Carried by electrons in the wire Carried by anions and cations in electrolytes Measured in Amperes, coulombs per second Causes no net charge in a wire

Voltage Sources Provides a potential difference in a circuit Can be batteries(chemical to electrical) or generators(mechanical to electrical) Can be AC or DC Electrical potential energy per coulomb is voltage(or electromotive potential or emf) Unit is volt Household voltage is 120V AC Voltage causes current.

Electric Resistance Charge depends on voltage and resistance to flow Resistance depends on resistivity, length, cross-sectional area of conductor and temperature Resistors are color coded to tell their resistance.

Ohm’s Law Current = Voltage/Resistance For a given amount of current, voltage and resistance are proportional. The damaging effects of shock are caused by current. The body resistance ranges from 100 when wet to 500,000 when dry. 0.001 A can be felt; 0.070 A can be fatal.

Direct Current and Alternating Current Refers to direction of flow of electrons DC only one way, from negative to positive AC changes directions many times per second depending upon the frequency AC can be transmitted over great distances with little loss Converters change AC to DC using a transformer( to lower voltage), a diode(a one way “valve”), and a capacitor

The Speed of Electrons in a Circuit Appear to flow at infinite speed Really flow at an average of a few million kilometers per hour Motion is random so not all that productive Electric field must be present to induce a flow Field lines are maintained in one direction in a conductor Electrons bump into metallic ions and transfer some kinetic energy to them causing the wire to become hot

The Speed of Electrons in a Circuit In AC circuit, electrons oscillate about relatively fixed positions. Nearly speed of light. Electrons do not flow out of the wall into an appliance; energy does. Energy is carried by the field and causes vibrations.

Electric Power Rate at which electrical energy is converted into other forms of energy Power = current x voltage P = IV P = I x (IR) = I2R P = (V/R) x V = V2/ R Unit is watt or kilowatt

Electric Circuits Path along which electrons can flow from negative to positive Series circuits – only one path Parallel circuits – more than one path

Series Circuit

Series Circuits Current stays the same through all resistors. I1 = I2 = I3….. Voltage is used by each resistor individually and is said to “drop” as it passes through the resistor. VT = V1 + V2 + V3….. Resistance is unique to the resistor whether it is a lamp, an iron, a toaster or some other device. Req = R1 + R2 + R3…..

Series Circuits Schematics

Parallel Circuit

Parallel Circuits Current splits among all branches. IT = I1 + I2 + I3 +….. Voltage is the same through all branches. V1 = V2 = V3 =….. Resistances combine into an equivalent resistance. 1/RT = 1/R1 + 1/R2 + 1/R3 +…..

Parallel Circuits Schematics