10-18-18.docx - ● Power ○ Current flows in a direction...

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Power Current flows in a direction, from high voltage to low voltage But what is really happening is that electrons are flowing from low voltage to high voltage Using Electric Power The bulb filament has a large “resistance” ( R ) It is not a very good conductor Charge lose energy when flowing through resistance voltage decreases Resistance The bulb filament has a large “resistance” ( R ) Charges lose energy when flowing through resistance voltage decreases Current and resistance Larger the resistance, smaller the current Larger the voltage, larger the current Ohm’s law: I = V/R V = IR Resistance Resistance is measured in “Ohm”s, ( Ω) Resistances are often temperature-dependent: metals have larger resistance at higher temperatures Ohm Law Example Flashlight has a bulb with a resistance of 6 Ω . It is powered by two 1.5 V batteries, for a total of 3V What is the current flowing through the bulb? Answer: V = IR I=V/R = 3V/6 Ω = 0.5A What is the current flowing through the batteries Answer: Also 0.5A! It’s the same anywhere in the same circuit! LEDs An LED does not obey Ohm’s law! (Ohm’s law is not a “law of physics” - it’s an approximate rule that works for some (most) simple electric parts, like wires.) More about LED’s in chapter 13
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No current below voltage threshold (quantum mechanics!) Series Circuits Series link things end-to-end… Voltages add up Current is the same everywhere in the circuit Parallel Circuits Parallel circuits run two paths in “parallel” Voltages are the same (as the original voltage) from start to end of both paths! Current in each path depends on the resistance in the path! (I=VR) What is the current in one of the strips if the battery voltage is 12V and the strip resistance is 3 Ω ? Answer: There is 12V across each strip, since the strips are parallel circuits I = V/R = 12V/3 Ω = 4A Question 1:Why does a flashlight need batteries and a bulb?
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A: Batteries power the bulb, which then emits light Batteries: chemical energy electrostatic energy Bulb: electrostatic energy light energy Since this energy transfer is ongoing, think power Power is energy per unit of time The SI unit of power: 1 watt is 1 joule/ second Battery A battery is a chemically powered pump for charge It pumps charge from - end to + end It develops a voltage rise from - end to + end
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  • Spring '14
  • haas
  • Electron, Magnetic Field, Electric charge

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