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Group Work 1. Light the bulb!

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Electric Current Effects of moving charges

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Objectives Relate current, potential and resistance using Ohm’s Law. Determine the power in a current flow.

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Charges Move in Circuits Charges travel in a circuit to maintain charge balance

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Light Bulb Anatomy

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Electric Current is Charge Flow Current = charge flux / time = qq tt Unit: C / s = ampere = A current

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What is the direction of the electric current? - - - - - - + + + + + + A. right B. left Poll Question

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Conductors and Insulators A conductor is a material through which current flows easily. An insulator is a material through which current does not flow. No material is a perfect insulator or conductor! –OK, superconductors do exist.

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Resistance Hinders Current Current does not flow unhindered Electrical resistance is akin to friction or drag Expressed as voltage needed to maintain a current, V/A

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Ohm’s Law I = V R I = current V = voltage = electric potential change R= resistance Unit: V / A = ohm ( )

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Voltage Causes Current Potential drop is the cause. Current is the effect. Resistance reduces the effect of voltage.

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Poll Question If you want to increase the current through a resistor, you need to A.Increase the resistance or voltage. B.Decrease the resistance or voltage. C.Increase the resistance or decrease the voltage. D.Decrease the resistance or increase the voltage.

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Ohm’s Law Rearranged I = V R I = current V = voltage R= resistance If you know two, you can find the third. R = V I V = I R

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Calculate the Current A 1.5-V battery powers a light bulb with a resistance of 9 . What is the current through the bulb? Ohm’s Law I = V / R V = 1.5 V; R = 9 I = (1.5 V ) / (9 V/A) = 1/6 A

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Group Work 2.A car headlight draws a current of 15 A when connected to a 12-V car battery. What is the resistance of the headlight?

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Electric Power Formula Voltage is work done per charge: V = E / q Current is charge per time: I = q / t So, (voltage times current) = (work per time) = power Power = VI

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Group Work 3.What is the power dissipated by the car headlight in problem 2?

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Another Formula If you know current I and resistance R Ohm’s law tells you V = I R Substitute I R for V in power formula V I = I R I = I 2 R Power = V I Power = I 2 R

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Yet Another Formula If you know voltage V and resistance R Ohm’s law tells you I = V /R Substitute V/R for I in power formula V I = V(V/R) = V 2 /R Power = V I Power = V 2 / R

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Verify the Formulas For the car headlight: –V I –I 2 R –V 2 /R

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Circuit Diagrams resistor Voltage source

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Kirchoff’s Laws 1.Current into any node equals current out of the node I 1 = I 2 + I 3 I1I1 I2I2 I3I3

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Kirchoff’s Laws 2.Potential drop around any closed loop is zero – V 1 – V 2 = 0 V 1 + V 2 = V1V1 V2V2

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Reading for Next Time Electric circuits Main ideas –Parallel and series pathways –Combining resistances Magnetic fields Main ideas –How magnetic forces act –How magnetic fields are created

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Resistance and resistivity. Current Current is sort of a vector Direction is constrained by conductor Restricted to forward or backward (+ or –)

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