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Chapter 28

Chapter 28. Direct Current Circuits. R connections in series and in parallel Define DC (direct current), AC (alternating current) Model of a battery. Review. Concepts: Charge: positive, negative, conserve, induction. potential Electric: field flux

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Chapter 28

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  1. Chapter 28 Direct Current Circuits • R connections in series and in parallel • Define DC (direct current), AC (alternating current) • Model of a battery

  2. Review Concepts: Charge: positive, negative, conserve, induction. potential Electric: field flux Electrostatic equilibrium: no moving charge. Current: moving charge Capacitance: charge over potential Resistance: potential over current Resistance and Resistivity (conductivity) and temperature:

  3. Laws: Coulomb's: force and charge. Gauss’s: electric flux and charge. Ohm’s: electric potential and current. Circuits and components: Symbols: wire, battery, Capacitor, Switch, Resistor Power: current times potential difference: Capacitor: in parallel - in series Resistor: discuss today

  4. In series. Condition: In parallel. Condition: Resistor connections

  5. Resistor connections In series, : voltage sharing power sharing In parallel, : current sharing power sharing

  6. Resistors connections, summary • In series • In parallel

  7. Combinations of Resistors • The 8.0-W and 4.0-W resistors are in series and can be replaced with their equivalent, 12.0 W • The 6.0-W and 3.0-W resistors are in parallel and can be replaced with their equivalent, 2.0 W • These equivalent resistances are in series and can be replaced with their equivalent resistance, 14.0 W

  8. More examples

  9. Direct Current and Alternating Current • When the current direction (not magnitude) in a circuit does not change with time, the current is called direct current • Most of the circuits analyzed will be assumed to be in steady state: with constant magnitude and direction, like the one powered through a battery. • When the current direction (often also the magnitude) in a circuit changes with time, the current is call alternating current. • The current from your car’s alternator is AC.

  10. battery load Model of a battery • Two parameters, electromotive force (emf), e, and the internal resistance r, are used to model a battery. • When a battery is connected in a circuit, the electric potential measured at its + and – terminals are called The terminal voltage DV, with DV = e – Ir • If the internal resistance is zero, the terminal voltage equals the emf e. • The internal resistance, r, does not change with external load resistance R.

  11. battery load Battery power figure The power the battery generates (through chemical reactions): The power the battery delivers to the load, hence efficiency: The maximum power the battery can deliver to a load We have and From Where the emf is a constant once the battery is given. From We get to be the condition for maximum , or power delivered to the load.

  12. battery load Battery power figure One can also obtain this result from the plot of Where when reaches the maximum value The efficiency of the battery at this point is 50% because

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