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Electromotive Force Series and Parallel Circuits

Learn about EMF, terminal voltage, power output, Kirchhoff's rules, series and parallel circuits, safety notes, and electrical measuring devices in this comprehensive AP Physics lesson.

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Electromotive Force Series and Parallel Circuits

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  1. Electromotive ForceSeries and Parallel Circuits AP Physics C Mrs. Coyle

  2. Positive charges are “pumped” by the battery from low to high potential. When transversing a resistor with the current there is a decrease in potential. Conventional Current

  3. Electromotive Force (emf) E,of a Battery (Load Resistance) Terminal Voltage

  4. Electromotive Force of a Battery • The electromotive force (emf), e, is the maximum possible voltage that the battery can provide between its terminals • The emf supplies energy, it does not apply a force • The power delivered by the battery to a load resistor is maximum when the load is equal to the internal resistance of the battery.

  5. Symbol E Voltage of the open circuit Unit : Volts This is how a battery is labeled Symbol V Voltage of closed circuit depends on the current (DV = e – Ir) Slightly lower than E Unit Volts If the internal resistance is zero, the terminal voltage equals the emf Electromotive Force (emf) Terminal Voltage Unless otherwise stated assume that E=V

  6. Power • Total power output of the battery: P = IDV = I E Total power= + P = I E = (I 2r) + (I 2R) Power delivered to internal resistor (I 2r) Power delivered to external resistor (I 2R)

  7. Kirchhoff’s Rules • 1st Rule: (Junction Theorem-Conservation of Charge): At a junction (node), current in= current out Iin =Iout • 2nd Rule: (Loop Theorem-Conservation of Energy): In a closed loop the sum of the voltages is zero. ΣV=0

  8. Sign Conventions When the battery is traversed from - to +, the potential diff V is + When the resistor is traversed in the direction of the current, the potential across the resistor is– IR

  9. Series Parallel Batteries in Series and in Parallel

  10. Series Circuits-One current path I = I1 = I2 = I3 V = V1 + V2 + V3 IReq = IR1 + IR2 + IR3 Equivalent resistance Req = R1 + R2 + R3

  11. Parallel Circuits-common voltage, current is branched I = I1 + I2 + I3 V/Req= V/R1 +V/R2 + V/R3 V=V1=V2=V3 Equivalent Resistance 1/Req= 1/R1 +1/R2 + 1/R3

  12. Note • When resistors are added in parallel the equivalent resistance is smaller than the smallest one. • A note on safety: can you think of why you should not plug in too many appliances in the same outlet?

  13. Electric current detector. It has a coil in a magnetic field. When a current is passed through the coil, the coil experiences a torque proportional to the current. Galvanometer

  14. Measures electric current. It must be placed in series with the measured branch. It must have very low resistance to avoid significant alteration of the current it is measuring. Ammeter

  15. Measures the change in voltage between two points in an electric circuit. It must be connected in parallel with the portion of the circuit on which the measurement is made. Must have a very high resistance so that it does not have an appreciable affect on the current or voltage associated with the measured circuit. Voltmeter

  16. Used to measure an unknown resistance, Rx. Rs is varied until the G reads zero (no current in the G branch). Since there is no current in G, Vb=Vc and therefore Vab=Vac, and Vbd=Vcd .. Rx=( R2/R1) Rs Wheatstone Bridge

  17. Example 1 Given: V = 6 Volts Find the current and the voltage across each resistor.

  18. Example 2 Find the current in each resistor.

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