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AND MR. SEREDA PRESENT…. Resisting the movement of Charge Copy Resistance Electrons that travel through a light bulb Filament experience Resistance Resistance is a property that hinders motion AND converts electrical energy to other forms of energy • E.g.: resistance of Tungsten wire is 400 times greater than copper • When a current flow through the wire it is Light converted into ________________ and Heat ________ Copy Describing Resistance How much energy does it take to push a 50 kg crate across the floor? Depends on resistance Pushing across a carpet would be harder than pushing across ice. Copy Resistance How much energy does it take to push a coulomb of charge across a filament? Defined as the ratio of POTENTIAL DIFFERENCE across the LOAD POTENTIAL DIFFERENCE = VOLTAGE (V) LOAD = CURRENT (I) V R= --I Voltage Resistance = ------Current Copy Units of Resistance Units of Resistance are called OHMS Named in honour of Georg Simon Ohm The symbol for Ohms is the greek letter OMEGA (Ω) Ohm was the first to publish results of various resistances on types of wires Copy Georg Ohm’s Findings Ohm took various types of conductors Applied a current across the wire No matter what the Potential Difference (Voltage) --- the Resistance was the same Called Ohm’s Law Copy Ohm’s Law V = IR Voltage = Current x Resistance • http://www.mhhe.com/physsci/physical/gia mbattista/circuits/circuits.html • http://www.riedon.com/riedon_calculator.ht m Copy Model Problem What is the resistance of the heating coil of an electric heater if a current of 12.5 A runs through it? I = 12.5 A V = 120 V R = V / I R = 120 / 12.5 R = 9.6 V / A R = 9.6 Ω Copy Wasting Energy Do resistors have to convert energy to a different source? Can be used to control the current or Potential difference at different points in the circuit. Measuring Resistance What is the Resistance? R = V / I 3 Amps V = 6 I = 3 R = 6 / 3 = 2 Ω Potential Difference =6 Copy Designing Circuit Materials Four factors Affect Resistance Length Resistance INCREASES with Length The longer the wire…the more resistance Width Resistance DECREASES with Width The wider the wire the less resistance Temperature Resistance INCREASES with temperature The hotter the wire the more resistance Material Some material allows electrons to move more freely Copy Conducting wires that carry large currents have large diameters to minimize resistance. This is why stove cord is bigger than the cord for a kettle Copper Core Insulating Material • Do Questions 1 – 6 on Page 342 of the Textbook • Worksheet: – http://www.kathimitchell.com/elect2.htm