Circuits
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Circuits

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    Circuits Circuits Presentation Transcript

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