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The Science Behind Direct Current & Alternating Current

The Science Behind Direct Current & Alternating Current. Static Electricity. Static electricity = an imbalance of positively and negatively charged atoms. Electrons then jump from atom to atom, releasing energy. Examples of static electricity Lightning

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The Science Behind Direct Current & Alternating Current

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  1. The Science Behind Direct Current & Alternating Current

  2. Static Electricity • Static electricity = an imbalance of positively and negatively charged atoms. Electrons then jump from atom to atom, releasing energy. • Examples of static electricity • Lightning • Shocks from scuffing your feet across the carpet on a dry day and then touching a metal surface such as a doorknob

  3. Current Electricity • Current = a flow of electrons along a pathway • Direct current (DC) means that electrons flow in one direction. Batteries provide DC. • Batteries consist of two or more cells filled with chemicals. As the chemicals react, electrons are removed, leaving behind positively charged ions. The separation between the electrons and the ions creates voltage.

  4. Batteries • The voltage produced depends on the chemicals used. A “D” cell flashlight battery uses an alkaline reaction. A car battery uses a lead-acid reaction. • The voltage of a cell drops over time as the chemical reaction slows down and the battery dies.

  5. Batteries

  6. Vocabulary • 1 Coulomb = 6.25 x 1018 electrons; symbol is C • 1 Ampere of current = the movement of1 Coulomb of electrons past one point, in one second; symbol is A • Load: device or customer that receives power from the electric system. Load should not be confused with demand, which is the measure of power that a load requires

  7. Voltage • Voltage is the “push” behind the movement of electrons (current flow) • Unit is the Volt • Symbols DC Voltage Source AC Voltage Source

  8. Resistors • Resistance is the opposition to current flow • Unit is Ohm (Ω) • resistor : A component used to limit current flow in a circuit or to provide a voltage drop • Symbol : R

  9. Circuits • A simple DC Circuit consists of a source, a load, control and conductors connecting the three parts.

  10. Series DC Circuit • A series circuit has a single path for current to flow. (When the switch is open, the circuit is incomplete and the electrons cannot flow.)

  11. Parallel DC Circuit • A Parallel circuit has multiple paths for current to flow.

  12. Vocabulary • Capacitor: a device with the ability to store electric charge (Ex: condenser in a car) and release it; used in electronic circuits for blocking DC while allowing AC to pass • Transformer: transfers electricity from one circuit to another with an increase or decrease in voltage • Inductor: a coil of wire that can store energy in its magnetic field and resists any change in the amount of current flowing through it; when the current flowing through an inductor changes, it will create an opposing or reverse voltage

  13. Current Electricity • Alternating current (AC) means that electrons flow in two directions. Power plants provide AC. • AC current is generated when a conductor—often a coil of wire—is rotated in a magnetic field. • The magnetic field has polarity (north and south poles). As the coil rotates between the opposing poles, the change in polarity causes a change in direction of the current flow. • The directional change happens at regular intervals. In the U.S., 60 full 360º rotations through the field (cycles) happen every second.

  14. AC Circuit

  15. The War of Currents: General Electric (DC) vs. Westinghouse (AC) • Thomas Edison’s power plants provided DC to customers but could only send it about 1 mile before it lost power. Edison hired Nikola Tesla to find a solution and Tesla delivered, but then Edison allegedly refused to pay him. • Tesla quit and investors helped him establish a competing industrial laboratory where he manufactured the prototype for today’s AC generation and transmission system.

  16. AC electricity is more efficient and more cost effective than DC • DC was difficult to convert from higher to lower voltages and required separate transmission lines for different voltages; AC could be “stepped down” easily through the use of transformers • Separate transmission lines meant great cost and more danger to the public from overhead lines • DC required power plants to be located close to the end user (electric load); AC could transmit high voltage over long distances requiring fewer plants • AC motors are simpler in design than DC motors

  17. An AC power system benefits from: • Economies of scale in generation, when cost of production falls because output has increased • Load diversity, when the peak demands of a variety of electric customers occur at different times

  18. For More About… • Batteries and power systems http://www.eng.cam.ac.uk/DesignOffice/mdp/electric_web/DC/DC_11.html • Electrical components and systemshttp://science.howstuffworks.com/electricity-channel.htm • Inventor Nikola Teslahttp://www.pbs.org/tesla/

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