Physics Chapter 23

Light-emitting Diodes

-Another light source even more long-lasting is the light-emitting diode(LED) -The most primitive being the little red lights that tell you whether your stereo is on or off -Between CFLs and LEDs, common use incandescent bulbs will soon be history.

Electric Circuits

-Any path along which electron can flow from the negative terminal to the positive terminal -Complete circuit allows continuous flow of electrons with no openings or gaps

Safety Fuses

-Are wires that melt when the given current is exceeded -Are connected in series along the supply line to prevent overloading in circuits -Are replaced by circuit breakers in modern buildings

Circuit Breakers

-Automatic switch that turns off when the current is excessive

Alternating Current(ac)

-Electrons in the circuit are moved first in one direction and then in the opposite direction, alternation to and fro about fixed positions -This is accomplished by alternation the polarity of voltage at the generator or other voltage source

Direct Current(dc)

-Flows in one direction only -Electrons always move from the negative terminal towards the positive terminal

Parallel Circuits and Overloading

-Homes are wired parallel. As more and more devices are connected to a circuit, more current moves through the wires. There is an amount of current each device can carry before it overheats. When the current is excessive, overheating can result in a fire.

Compact Florescent Lamps(CFLs)

-Incandescent bulbs dissipate most of their energy in the form of heat, not light. SO they are not energy efficient. -Fluorescent lamps, on the other hand, emit much less heat, which is why you can touch them without burning yourself. -Compact fluorescent lamps are a type of fluorescent lamp that fits into a standard lightbulb socket. -For the same wattage, CFLs emit much more light and muss less heat than incandescent bulbs.

Superconductors

-Materials with zero electrical resistance to the flow of charge -flow of charge is without generation of heat. High temperature superconductors -Refers to ceramic materials that can carry much current at a low voltage

Electric Power

-Rate at which electric energy is converted into another form -In equation form: Power=current x voltage -In units: watts Example: 100-watt lamp draws 0.8 ampere

Electrical Resistance: Semiconductors

-Refers to material the can alternate between being conductors and insulators Examples: -germanium -silicon

Ohm's Law

-Relationship between voltage, current, and resistance -States that the current in a circuit varies in direct proportion to the potential difference, or voltage, and inversely with the resistance

Flow of charge

-To attain a sustained flow of charge in a conductor, some arrangement must be provided in water pressures- is maintained with the use of a pump. -Battery pumps up the voltage

Flow of Charge

-When the ends of an electrical conductor are at different electric potentials- when there is a potential difference- charge flows from one end to the other. -Analogous to water flowing from higher pressure to lower pressure

Electric potential difference

-Work is done in pulling negative charges apart from positive ones. -Electromagnetic induction at the generator terminals provide the electric pressure to move electrons through the circuit.

Parallel Circuits

4. As the number of parallel branches is increased, the overall resistance of the circuit is decreased 5. A break in one path does not interrupt the flow of charge in the other paths.

Electric potential difference

A battery or generator can maintain a steady flow of charge

Electric Current

Alternating current -electrons oscillate to and fro around fixed positions. -movement is produced by a generator or an alternator that switches the sign of charge periodically. -Commercial ac circuits are used in most residential circuits through that world and can be stepped up to high voltage for transmission over great distances with small heat losses, or stepped down where energy is consumed

Voltage Sources: Conductor

Any material having free charged particles that easily flow through it when an electric force acts on them.

Series Circuits

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 5. The voltage drop across each device are proportional to its resistance 6. If one device fails, current in the entire circuit ceases

Parallel Circuit

Characteristics: 1. Voltage is the same across each device. 2. The 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. The total current in the circuit equals the sum of the currents in its parallel branches.

Direct and Alternating Current

Commercial electricity in North America -Alternation current(ac) -60 cycles per second(60 seconds in a minute) -Voltage is 120 V Power transmission is more efficient at higher voltages -Europe adopted 220 V as its standard - U.S. continued with 120 V because so much equipment was already installed

Electrical Resistance

Current in a circuit is dependent on -voltage -electrical resistance in ohms Resistors -circuit elements that regulate current inside electrical devices

Electric Potential Difference

Difference in potential between two points -CHarges in conductor flow from higher potential to lower potential -Flow of charge persists until both ends of conductor reach the same potential -Maintained for continuous flow by pumping device

Ohm's Law

Electric Shock -Damaging effects of shock result from current passing through the body -Electrical potential difference between one part of your body and another part depends on body condition and resistance, which can range from 100 ohms to 500,00 ohms

Electric Potential difference

Example: Water from a higher reservoir to a lower one- flow continues until no difference -No flow of charges occurs when potential difference is zero.

Electrical Resistance

Factors affecting electrical resistance -Temperature: the higher the temperature, the more the resistance

Electric Resistance

Factors affecting electrical resistance: -Inversely proportional to cross-sectional area-thin wires, more resistance than thick wires -Directly proportional to length-doubling the length, doubles the resistance -Material-rubber, much more resistance than copper of the same size

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 within the nuclei of atoms. In fluids -Positive ions and electrons constitute electric charge flow.

Voltage Pressure

In chemical batteries -Work by chemical disintegration of zinc or lead in acid -Energy stored in chemical bonds is converted to electric potential energy

Speed and Source of Electrons in a Circuit: Misconception

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 and other atoms, but this slows them down and offers resistance to their motion. -Electrons throughout the entire closed path of a circuit all react simultaneously to the electric field.

Speed and Source of Electrons in a Circuit: Misconceptions

Misconceptions about electric current: "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 and causes vibratory motion. -Electrical utility companies sell energy. You provide the electrons.

Ohm's Law

Ohm's law (continued) In equation form: Example: Current=Voltage % Resistance -For a constant resistance, current will be twice as much for twice the voltage. -For twice the resistance and twice the voltage, current will be unchanged. Resistors -Circuit elements that regulate current inside electrical devices.

Electric Current

Rate of electric flow - flow of charge -measured in ampere (1 coulomb of charge per second) -speed of electrons(drift speed) through as wire is slow because of continuous bumping of electrons in a wire -charge flows through a circuit; voltage is established across a circuit -energy moves fasters then electrons

Speed and Source of Electrons in a Circuit

When we flip the light switch on a wall and the circuit, an electric field is established inside the conductor -The electrons continue their random motions while simultaneously being nudged be the electric field -Current is established through the wires at nearly the speed of light -It is not the electrons that move at this speed. -It is the electric field that can travel through a circuit at nearly the speed of light.

A. Current

When you double the voltage in a simple electric circuit, you double the A. current. B. resistance. C. Both A and B. D. Neither A nor B.

Series Circuit

forms a single pathway for electron flow between the terminals of the batter, generator or wall out let

Parallel Circuit

forms branches, each of which is a separate path for the flow of electrons