Lecture%2021B

# Lecture%2021B - Physics 1B Lecture 21B Power Recall from...

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Physics 1B Lecture 21B

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Power Recall from mechanics that power was a measure of the work done in a given amount of time: The SI unit of power is still the Watt (Joule/sec). Although you should also be aware of kW and horsepower. Work is equal to the change in the potential energy (assuming KE is constant).
Power In a circuit, you will get power from a small amount of charge (Δq) moving across a potential difference (ΔV). This is the power that is transferred by electrical energy. Sometimes we write as:

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Power If you would like to find the power released (dissipated) in a device due to its resistance, use Ohms’ Law (ΔV = IR): These two equations are used to calculate power dissipation (the energy lost due to resistive effects). This power will never be recovered (it becomes heat, light, or some other form of energy).
Power When you deliver power over a time period to an element you are essentially transferring energy to the element (light bulb, heater, computer…). The unit of energy used by the electric company is the kiloWatt-hour. Where 1kWh = 1,000W x 3,600s = 3.60x10 6 J. This energy could go into lifting a mass: ΔPE = mgΔy or heating water. .. or anything Q = ΔE = mcΔT

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Why do electric companies make power lines with very high voltages to deliver electricity to your house? Let’s make some estimations: R line = 10Ω, V = 100,000V to deliver 100kW of power. First, don’t make a common mistake: This equation can only be used with a potential difference (ΔV) across a resistor. The potential difference across the power line is not given and is only a fraction of 100,000 V. <-no!
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## This note was uploaded on 01/30/2012 for the course PHYSICS 1B 1B taught by Professor Grosmain during the Winter '10 term at UCSD.

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Lecture%2021B - Physics 1B Lecture 21B Power Recall from...

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