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Electric Power. Electric Power. Recall from 1 st Semester that power is defined as the rate at which work is done. Power = Work/Time The units of Power are Watts 1 Watt = 1 Joule / 1 Second The Power it takes to move a Charge across a Voltage in a certain time is:

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## Electric Power

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**Electric Power**• Recall from 1st Semester that power is defined as the rate at which work is done. • Power = Work/Time • The units of Power are Watts • 1 Watt = 1 Joule / 1 Second • The Power it takes to move a Charge across a Voltage in a certain time is: • P = (V∙q)/t = V∙(q/t) = I∙V • Recall Ohm’s Law: • I = V/R or V = I·R • Combine with P = I·V: • P = V2/R • P = I2·R • See next page for graphic organizer of equations relating P, I, V, and R**Example Problem #1**• The iPod Nano uses 5.0 Watts of power. • Determine how much energy it consumes during 10 hours of listening time. • Determine the resistance of the IPod if it uses a 5.2 Volt battery. • What quantity of charge passes through the IPod battery during this 10-hr time period?**Example Problem #2**• Calculate the energy used and current drawn by a: • 200 W window fan when plugged into a 110 V outlet and left on for an 8 hour time period. • 3400 W air conditioner that is wired to a 220 V circuit and left on for an 8 hour time period.**The Cost of Electricity**Cost per unit of Electrical Energy Electrical Energy Used Total Cost = x ($) (kWh) ($ per kW·h) • The electric company charges you for the amount of ENERGY they provide to you. • Energy provided is “Work” • Measured in strange unit: “KiloWatt-Hours” • $0.0776 per kWh in NoVA • Household Electric Sockets operate at 120 V in the U.S. • Heavy equipment outlets (welders, clothes dryers, ovens, etc) operate at 240 V • Other countries operate at different voltages • Cars operate at 12 V**Example Problem #3**• A 2000 W hand dryer in a public bathroom at a zoo runs for 30 seconds per cycle. • At a cost of 13 cents per kW•hr, determine the cost of running the dryer 200 times in a day. • What is the annual cost of the hand dryer assuming it is used on average 200 times per day? • Calculate the resistance and the current for the hand dryer if it is connected to a 220-V circuit.

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