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  1. 1. Circuits March 30th, 2009
  2. 2. Electric Circuits <ul><li>A circuit is simply a closed loop through which charges can continuously move. </li></ul><ul><li>The word circuit is from </li></ul><ul><li>the same base as the </li></ul><ul><li>word circle, which </li></ul><ul><li>describes how the electrons move </li></ul>
  3. 3. Requirements of a Circuit <ul><li>There must be an energy supply capable doing work on charge to move it from a low potential energy to a high potential energy location. This establishes a potential difference (voltage) across the two ends of the external circuit. </li></ul><ul><li>There must be a closed </li></ul><ul><li>conducting loop in the circuit </li></ul><ul><li>which stretches from the </li></ul><ul><li>high potential, positive terminal </li></ul><ul><li>to the low potential, negative terminal. </li></ul>
  4. 4. An analogy to help you… <ul><li>A circuit is like a rollercoaster </li></ul><ul><ul><li>It is a complete loop </li></ul></ul><ul><ul><li>There is a place where energy is put in to increase potential energy (a battery in a circuit and the chain pulling up the hill in the rollercoaster) </li></ul></ul><ul><ul><li>The rollercoaster naturally flows downhill just like charge in a circuit naturally flows from high potential to low potential </li></ul></ul>
  5. 5. <ul><li>The diagram at the right shows a light bulb connected to a 12-V car battery. The + and - terminals are shown. </li></ul><ul><ul><li>As a + charge moves through the battery </li></ul></ul><ul><ul><li>from D to A, it ________ (gains, loses) electric potential. </li></ul></ul><ul><ul><li>As a + charge moves through the external </li></ul></ul><ul><ul><li>circuit from A to D, ________ (gains, loses) </li></ul></ul><ul><ul><li>electric potential. </li></ul></ul><ul><li>Use >, <, and = signs to compare the electric potential (V) at the four points of the circuit. </li></ul><ul><ul><li>VA ___ VB ____ VC ____ VD </li></ul></ul>
  6. 6. Decide if each statement is true or false: <ul><li>When an electrochemical cell no longer works, it is out of charge and must be recharged before it can be used again. </li></ul><ul><li>2. An electrochemical cell can be a source of charge in a circuit. The charge which flows through the circuit originates in the cell. </li></ul><ul><li>3. Charge becomes used up as it flows through a circuit. The amount of charge which exits a light bulb is less than the amount which enters the light bulb. </li></ul><ul><li>4. The local electrical utility company supplies millions and millions of electrons to our homes everyday. </li></ul>
  7. 7. All statements were FALSE!! <ul><li>When a battery is “dead” it is out of energy required to push a charge from one terminal to another </li></ul><ul><ul><li>This energy can be restored in some batteries with a reversible chemical reaction…so it is more like reenergizing than recharging your batteries </li></ul></ul><ul><li>Batteries are the source of energy in a circuit, charges carry the energy </li></ul><ul><li>Charge is not used up in a circuit, energy is </li></ul><ul><li>Electrons are already in your home, the power company supplies the energy to these electrons </li></ul>
  8. 8. Circuits <ul><li>A circuit is a set of electrical components connected so they provide one or more complete paths for the movement of charges </li></ul><ul><li>A load is any component or group of components that dissipates energy </li></ul><ul><li>A simple circuit consists of a source of potential difference (electrical energy) and a load </li></ul>
  9. 9. Drawing Circuits with Schematic Diagrams <ul><li>Graphic representation </li></ul><ul><li>of an electric circuit </li></ul><ul><li>Uses standardized </li></ul><ul><li>symbols representing </li></ul><ul><li>circuit components </li></ul><ul><li>COPY DOWN </li></ul><ul><li>SYMBOLS FROM </li></ul><ul><li>OVERHEAD </li></ul>
  10. 10. Light Bulbs <ul><li>A light bulb contains </li></ul><ul><li>a complete conducting path </li></ul><ul><ul><li>See Overhead </li></ul></ul><ul><ul><ul><li>(regular and decorative lights) </li></ul></ul></ul><ul><li>A light bulb is considered </li></ul><ul><li>a load because it dissipates </li></ul><ul><li>electrical energy (by </li></ul><ul><li>converting it to light energy) </li></ul>
  11. 11. EMF <ul><li>Electromotive Force </li></ul><ul><ul><li>The energy per unit charge supplied by a source of electric current </li></ul></ul><ul><ul><li>Any device that increases the potential difference and energy of charges circulating in a circuit </li></ul></ul><ul><ul><li>Current flow shows </li></ul></ul><ul><ul><li>the direction of + </li></ul></ul><ul><ul><li>charges </li></ul></ul><ul><ul><li>EMF shows the flow </li></ul></ul><ul><ul><li>of electrons </li></ul></ul>
  12. 12. Types of Circuits <ul><li>Closed circuit </li></ul><ul><ul><li>Path from one terminal to the other is complete. A potential difference exists and electrons move from one terminal to the other. </li></ul></ul><ul><li>Open Circuit </li></ul><ul><ul><li>No complete path. No charge flow and no current </li></ul></ul><ul><li>Short Circuits </li></ul><ul><ul><li>Without a load the circuit contains little resistance to the movement of charges. </li></ul></ul><ul><ul><li>This causes high current which can damage circuit components. </li></ul></ul>
  13. 13. Types of Circuits <ul><li>Series Circuit </li></ul><ul><ul><li>A circuit or portion of a circuit </li></ul></ul><ul><ul><li>that provides a single conducting </li></ul></ul><ul><ul><li>path without junctions </li></ul></ul><ul><li>Parallel Circuit </li></ul><ul><ul><li>A circuit or portion of a circuit in which two or more components are connected across a common point or junction, providing multiple conducting paths for the current </li></ul></ul>
  14. 14. Resistors in Series <ul><li>Resistors in series all have the same current </li></ul><ul><li>The total (or equivalent) resistance can by found by finding the sum of all resistors </li></ul><ul><li>R EQ = R 1 + R 2 + …. </li></ul>
  15. 15. Practice <ul><li>A 9.0 V battery is connected in series to four resistors with values of 2.0  , 4.0  , 5.0  , and 7.0  . Find the equivalents resistance for the circuit and the current in the circuit. </li></ul><ul><li>Hint: Draw a diagram! </li></ul><ul><li>Answer: R= 18  and I=0.50A </li></ul>
  16. 16. Resistors in Parallel <ul><li>Resistors in parallel have the same potential difference (voltage) across them. </li></ul><ul><li>The sum of the currents in parallel resistors equals the total current </li></ul><ul><li>Equivalent resistance is the sum of the reciprocals </li></ul><ul><li>1/R EQ = 1/R 1 + 1/R 2 + …. </li></ul>
  17. 17. Practice <ul><li>A 9.0 V battery is connected to four resistors in parallel with values of 2.0  , 4.0  , 5.0  , and 7.0  . Find the equivalents resistance for the circuit and the current in the circuit. </li></ul><ul><li>Hint: Draw a diagram! </li></ul><ul><li>Answer: R=0.917  and I =9.8 A </li></ul>