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Lesson 6 SERIES AND PARALLEL CIRCUITS Lesson 6: Equivalent Resistance of Circuits AP Physics B Objectives III.C.2. Steady State DC circuits with batteries and resistors 2. Steady-state direct current circuits with batteries and resistors only a) Students should understand the behavior of series and parallel combinations of resistors, so they can: (1) Identify on a circuit diagram whether resistors are in series or in parallel. (2) Determine the ratio of the voltages across resistors connected in series or the ratio of the currents through resistors connected in parallel. (3) Calculate the equivalent resistance of a network of resistors that can be broken down into series and parallel combinations. Student Objectives Students will be able to: 1. Determine the equivalent resistance of series and parallel combinations of resistors. 2. Determine the voltage drop and/or current through a series or parallel circuit. 3. Construct series and parallel combinations of resistors. Announcements Current HW #5 & 6 due TUESDAY Modern HW #5 due TUESDAY HW QUIZ on TUESDAY Lunch labs will be ONLINE this week Fluids & Thermo packet due Thurs, March 28 Electricity packet due Mon, April 8 Modern, Optics, Waves packet due Wed, April 10 Series arrangement of components Series components are put together so that all the current must go through each one I Three bulbs in series all have the same current. Parallel arrangement of components Parallel components are put together so that the current divides, and each component gets only a fraction of it. 1/3 I I 1/3 I 1/3 I 1/3 I I 1/3 I 1/3 I Three bulbs in parallel Voltmeter A voltmeter measures the drop in electric potential, or voltage, between any two points in a circuit. Voltmeters are always connected in parallel with the circuit. MiniLab #5 Draw a circuit containing two cells in series, two bulbs in series, and a switch. Wire these together. a) What changes in the circuit when you unscrew one of the bulbs? b) Measure the voltage across the battery and across each bulb. How does the voltage of the batteries compare to the voltage across each bulb? MiniLab #6 Draw a circuit containing two cells in series, two bulbs in parallel, and a switch. Wire this on your circuit board. a) What do you observe happens to the bulbs when you unscrew one bulb? b) Measure the voltage across the battery and across each bulb. What do you observe? Data Analysis Series Parallel Current Same through each bulb Splits at junction Voltage Divides Same through each Power Dimmer than parallel Brighter than series Remove a bulb All lamps go out Only the removed bulb goes out; others shine at same intensity Terminal Voltage and EMF A real battery is then modeled as if it were a perfect emf E in series with a resistor r. Terminal voltage Vab is measured by meter connected in parallel When no current is drawn from the battery, the terminal voltage equals the emf. When a current I flows from the battery, there is an internal drop in voltage equal to Ir, thus the terminal voltage (actual voltage delivered) is Vab = E - Ir