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Resistance

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Definition Opposition to the flow of electrons When electrons pass through materials with high resistance, they collide with its molecules more often and in doing so transfer some of their energy into heat Examples: heating coils light bulb filaments fuses – metal that will heat up and melt and break if too much current passes through it

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Relationships Resistance is measured by comparing the potential difference with the current Plot of V vs I, V = slope(I), slope = V/I

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Ohm’s Law For any given conductor, the V/I ratio is constant Ohms law – the potential difference between any two points in a conductor varies directly with the current between them The constant slope is call the resistance ( R) R = V/I Units V/amp = 1 ohm (Ω) Some non-metals are conductors that don’t obey ohm’s law (called non-ohmic)

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Examples Find the resistance of an electric light if there is a current of.80A and the potential drops 120V as it goes through the bulb. What is the potential difference across a toaster which has a resistance of 13.7 Ω when the current is 8.75A? What is the current through a heater with a resistance of 38 Ω and a potential difference of 240V across it?

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A small flashlight bulb draws 300mA from a 1.5V battery. A) what is the bulb’s resistance? B) if the voltage drops to 1.2V, how would the current change?

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Resistors Devices used to control the amount of current in a circuit Symbol of resistor on diagrams Two types Wire round resistors – coil of thin wire Composition resistors – made of some material that offers resistance (semi-conductor) Resistors are rated by color code banding

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Resistivity Thicker wires have more area for electrons to pass through – less resistance Longer wires have more obstacle for electrons – higher resistance Resistance ( R) of material directly proportional to L and inversely proportional to A R = ρ (L/A) Resistivity – ρ – constant that depends on the type of material

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Example Suppose you want to connect your stereo to some speakers. A) If each wire must by 20m long, what diameter of copper wire should you use to keep the R less than.10 Ω? B) If the current to each speaker is 4.0A, what is the voltage across each wire?

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Temperature and Resistance Resistivity of a material depends on the temperature Faster moving atoms interfere with electrons more – higher temp, higher R R = R 0 [ 1 + α T ] α – temperature coefficient (pg. 508)

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Example Variation of the resistance of metals with temperature can be used as a way of measuring temperature. A piece of platinum at 20°C is found to have a resistance of 164.2 Ω. When placed into a solution its resistance is measured at 187.4 Ω. What is the solution’s temperature?

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19.3 Resistance and resistivity

19.3 Resistance and resistivity

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