Chapter 23: Electric Current

Misconceptions about electric current

"current is propagated through the conducting wires by electrons bumping into one another" -NOT true: electrons that are free to move in a conductor are accelerated by the electric field impressed upon them -TRUE: they do bump into one another & other atoms, but this slows them down & offers resistance to their motions -electrons throughout the entire closed path of a circuit all react simultaneously to the electric field "Electrical outlets in the walls of the homes are a source of electrons" -NOT true: the outlets in homes are AC. Electrons make no net migration through a wire in an AC circuit -when you plug a lamp into an outlet, energy flows from the outlet into the lamp,not electrons. Energy is carried by the pulsating electric field & causes vibratory motion -Electrical utility companies sell energy. You provide the electrons

Parallel & Series Circuit Resistance

(picture in notebook)

Parallel Circuits

-Characteristics: 1.voltage is the same across each device 2.total current in the circuit divides among the parallel branches. The amount of current in each branch is inversely proportional to the resistance of the branch 3.total current in the circuit equals the sum of the currents in its parallel branches 4.as the number of parallel branches is increased the overall resistance of the circuit is decreased 5.break in one path does not interrupt the flow of charge in the other paths

Direct and Alternating Current (picture in powerpoint)

-Direct Current(DC): flows in one direction only, electrons always move from the - terminal toward the + terminal -Alternating Current(AC): electrons in the circuit are moved 1st in 1 direction & then in the opposite direction, alternating to & from about fixed positions -accomplished by alternating the polarity of voltage at the gener- ator or other voltage source -commercial electricity in North America: AC, 60 cycles per sec, voltage is 120V -power transmission is more efficient at higher voltages:Europe adopted 220V as its standard -U.S. continued w/ 120V b/c so much equipment was already installed -converting from AC to DC -household current is AC, but current in laptop is DC -converter uses a diode, a tiny electronic device that acts as a one-way valve to allow electron flow in one direction only a. when input to a diode is AC, output is pulsating DC b. slow charging & discharging of a capacitor provides continuous & smoother current c.pair of diodes is used, so there are no gaps in current output. The pair of diodes is used, so there are no gaps in current output. The pair of diodes reverses the polarity of alternate half-cycles instead of eliminating them

Light in Parallel

-Find resistance, current,voltage (each light) & power in each light (picture in notebook)

Light in Series

-Find resistance, current,voltage across each light & power in each lightbulb (picture in notebook)

Electric Current

-Flow of charged particles - In metal wires: -conduction electrons are charge carriers that are free to move throughout atomic lattice -protons are bound w/in the nuclei of atoms -In fluids: -positive ions & electrons constitute electric charge flow Rate of Electric Flow -measured in ampere (1 coulomb of charge per second) -speed of electrons (drift speed) through a wire is slow b/c of continuous bumping of electrons in wire -charge flows through a circuit; voltage is established across a circuit

Electric Circuits

-Parallel Circuits and Overlooking -homes are wired in parallel. As more & more devices are connected to a circuit, more current moves through the wires. There is an amount of current each devices can carry before it over-heats. When the current is excessive, over-heating can result in a fire -also,the addition of excess devices in a parallel circuit increases the amount of current moving through the wires,producing an overloading & overheating of the system, which can result in a fire

Light Emitting Diodes (LEDS)

-another light source even more long-lasting is the light-emitting diode (LEDS) -most primative being the little red lights that tell you whether you whether your stereo is on or off -btwn CFLS & LEDS common-use incandescent bulbs will soon be history

Series Circuit

-characteristics: 1. electric current through a single pathway 2.total resistance to current is the sum of individual resistances 3.current is equal to the voltage supplied by the source divided by the total resistance of the circuit 4. total voltage impressed across a series circuit divides among the individual electrical devices in the circuit so that the sum of the "voltage drops" across the resistance of each individual devices is equal to the total voltage supplied by the source 5.voltage drop across each device are proportional to its resistance 6.if one device fails, current in the entire circuit ceases

Measuring Electric Devices

-connect a multimeter in series to measure the current flow through lightbulb -connect a multimeter in parallel to measure the voltage drop across a lightbulb

Electric Resistance

-current in a circuit is dependent on: -voltage -electrical resistance in ohms -resistors: -circuit elements that regulate current inside electrical devices -Factors affecting electrical resistance: -inversely proportional to cross- sectional area (thin wires more resistance than thick wire) -directly proportional to length (doubling length, doubles resist- ance) -material (rubber- much more resistance than copper of the same size) -temp (higher the temp the more the resistance) -semiconductors:refers to materials that can alternate btwn being conductors & insulators EX: germanium, silicon -superconductors: materials w/ 0 electrical resistance to the flow of charge (flow of charge is w/o generation of heat) -High-temp superconductors: refers to ceramic materials that can carry much current at a low voltage

Compact Fluorescent Lamp (CFLS)

-incandecent bulb dissipate most of their energy in the form of heat, not light. so, they are not energy efficient -flourescent lamps emit much less heat, which is why you can touch them w/o burning yourself -CFLS are a type of fluorescent lamp that fits into a standard lightbulb socket -for the same wattage. CFLS emit much more light & much less heat than incandescent bulbs

Flow of Charge

-when the ends of an electrical conductor are at different electrical potentials- when there is a potential difference- charge flows from one end to the other end -analogous to water flowing from higher pressure to lower pressure -to attain a sustained flow of charge in a conductor, some arrangement must be provided to maintain a difference in potential while charge flows from one end to the other -a continuous flow is possible if the difference in water levels - hence the difference in water pressures- is maintained w/ use of pump

Speed and source of Electrons in a Circuit

-when we flip the light switch on a wall & the circuit an electric field is established inside the conductor -electrons continue their random motions while simultaneously being nudged by the electric field -current is established through the wires at nearly the speed of light -it is the electric field that can travel through a circuit at nearly the speed of light -if the voltage sources is DC, like the battery, electric field lines are maintained in 1 direction in the conductor -conduction electrons are acceler- ated by the field in a direction parrallel to the field lines -before they gain appreciable speed, they "bump into" the anchored metallic ions in their paths & transfer some of their KE to them

Saftey Fuses

-wires that melt when the given current is exceeded -connected in series along the supply line to prevent overloading in circuits -replaced by circuit breakers in modern buildings Circuit Breaker -atomic switch that turns off when the current is excessive

Electric Power

:rate at which electric energy is converted into another form P=l*V Power=current x voltage -units=watts EX:100-watt lamp draws 0.8ampere l=P/V=100W/120V= 0.8V

OHM's Law

:states that current in a circuit caries indirect proportion to the potential difference, or voltage, & inversely w/ the resistance -relationship btwn voltage, current, &resistance Current=voltage/resistance l=V/R EX: for a constant resistance,current will be twice as much for twice the voltage for twice the resistance & twice the voltage, current will be unchanged -Resistors:circuit elements that regulate current inside electrical devices (picture in powerpoint) -Electric Shock -damaging effects of shock result from current passing through the body -electric potential difference btwn 1 part of your body & another part depends on body condition & resistance, which can range from 100-50000ohms

Electric Current

Alternating Current -electrons oscillate to & fro around fixed positions -movement is produced by a generator or an alternator that switches the signs of charge periodically -commercial as circuit are used in most residential circuits throughout the world &can be stepped up to high voltage for transmission over great distances w/ small heat losses or stepped down where energy is consumed

Light Bulba

How much current does a 100W light bulb draw in a 120V outlet? l=P/V=100W/120V=o.833A Whats the resistance of 100W light bulb in a 120V outlet? R=V/I=120V/0.833A=144(Symbol in notebook)

Electric Circuits

circuits: any path along which electrons can flow from the negative terminal to the positive terminal -complete circuit allows contiuous flow of electrons w/ no opening or gap (picture in powerpoint) -connected in two common ways: -series:forms a single pathway for electron flow btwn the terminals of the battery, generator, or wall outlet -parrallel: forms branches, each of which is a seperate path for the flow of electrons

Voltage Sources (picture in powerpoint)

conductor: any material having free charged particles that easily flow through it when an electric force acts on them -electric potential difference -difference in potential btwn 2 points -charges in conductor flow from higher potential to lower potential -maintained for continuous flow by pumping device EX: water from a higher resevior to a lower one - flow continues until no difference -A battery or generator can maintain a steady flow of charge -work is done in pulling - charges apart from + ones -electromagnetic induction at the generator terminals provides the electrical pressure to move electrons through the circuit -in chemical batteries -work by chemical disingeneration of zinc or lead in acid -energy stored in chemical bonds is converted to electric PE