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Simple Circuits

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Challenge Questions 1. Why can a bird be perched on a high voltage wire? No potential difference between bird’s feet, therefore no current. http://www.youtube.com/watch?v=GLW6MEZ9Dcs 2. If a parachutist grabs onto a wire, what happens? What if it breaks? Why should the parachutist let go as it falls to the ground? No potential difference in the first situation, so no current. If they hold on and their feet touch the ground, there will be a current due to potential difference between the wire and the ground. http://www.youtube.com/watch?v=jleAxuFGknk http://www.youtube.com/watch?v=BtQtRGI0F2Q&feature=related

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Battery and Light Bulb Consider the diagram of the circuit you created to light the light bulb.

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Light Bulb How does a light bulb make a full conducting path?

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Electric Circuits Electric Circuit A set of electrical components connected so that they provide one or more complete paths for the movement of charges Ex. Light Bulb Filament is a resistor. When wire connects battery to the bulb, charges built up on one terminal of battery have a path to reach the opposite charges on the other terminal. Charges move creating a current. Current causes filament to heat and glow.

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Electric Circuits Circuit The path where electrons flow. Current The rate at which the charge flows past a point. Voltage The amount of “push” behind electrons. Resistance Equal to potential difference divided by current. EMF The energy per unit charge supplied by a source of electric current Load – any element in a circuit that dissipates energy (ex. Bulb) Closed circuit – a complete path from one battery terminal to another. Open circuit – no complete path, therefore no current

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Wire Resistor Bulb Plug Battery Switch Capacitor Schematic Diagram Symbols

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Think of Christmas lights. What happens when one light burns out? The circuit is no longer closed and all the bulbs go dark. So why use this? - It decreases the current needed. Several lesser resistances can add up to a single greater resistance. Important to have no current if something fails (ex. Burglar alarm)

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Resistors in Parallel Parallel Describes two or more components in a circuit that are connected across common points or junctions, providing separate conducting paths for the current. Ex. Christmas Lights In series, if a single burns out, they all go dark. In parallel, they have an alternative path. Current varies, potential difference remains the same.

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Resistors in Series R eq = R 1 + R 2 + R 3 … Equivalent resistance equals the total of individual resistances in series. I = ΔV/R eq ΔV = IR 1 and ΔV = IR 2 V T = V 1 + V 2 + V 3 … Resistors in Parallel 1/R eq = 1/R 1 + 1/R 2 + 1/R 3 … Equivalent resistance of resistors in parallel can be calculated using a reciprocal relationship. I T = I 1 + I 2 + I 3 … I = ΔV/R eq ΔV =IR eq

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Series and Parallel Resistors SeriesParallel Currentsame as totaladd to find total Potential Differenceadd to find totalsame as total

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Each Resistor =10 Ω Req = 10+10+10 Req = 30 Ω I = V/R I = 12V / 30 Ω I = 0.4A 12V V = IR V = (0.4A)(10 Ω) V = 4 volts V T = 4V + 4V + 4V = 12V I = V/R I = 4V / 10 Ω I = 0.4A P = IV P = (0.4A)(4V) P = 1.6 W

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Each Resistor =10 Ω 1/ Req = (1/10)+(1/10)+(1/10) = 3/10 Req = 3.33 Ω I = V/R I = 12V / 3.33 Ω I = 3.6A V = 12V I = V/R I = 12V / 10 Ω I = 1.2A I T = 1.2A + 1.2A + 1.2A = 3.6A P = IV P = (1.2A)(12V) P = 14.4W 12V

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