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Topic 7: Electricity in the Home Science 9 with Mrs. M Please take off your hoods and hats Bring a calculator to class next week

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Power Grid Topic 7

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Power Grid The grid is a complex network that links generating stations to electric energy users (homes, factories, etc). Topic 7

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Power Grid Topic 7

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Transformers reduce or increase voltage of an alternating current “Step up” voltage for long-distance transmission “Step down” voltage at destination Topic 7

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Transformers “step up” or “step down” voltage of AC current

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Topic 7: From the Grid to Your Home Substation: Step up Transformer Residential Step down Transformer Distribution Station

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Distribution lines -> Power meter -> Circuit breaker box Career connection: electrician, power line worker, home renovations Topic 7

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Circuit breaker/ Main breaker Acts as a switch and safety device to cut power to the home if current is too large. Older homes have a fuse box. A fuse melts when overheated by excessive current. Prevents possible fires from overheated wires. Main breaker Topic 7

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Branch circuits Additional circuit breakers or fuses for each branch circuit in your home Each branch supplies power to a room or electrical plug Topic 7

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Home circuitry Topic 7

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Wires Two live wires = neutral wire (white insulated) and hot wire (black insulated) Ground wire (bare copper or green insulation) Topic 7

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Digital devices: machines that process numerically coded information (number codes) Binary code: on/off, 0/1 (example: switches) Transistor: electronic switch in modern devices Topic 7

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Microprocessors contain millions of transistors https://www.youtube.com/watch?v=IcrBqCFLHIY https://www.youtube.com/watch?v=IcrBqCFLHIY

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Measuring Electric Power Power = energy per unit of time Electric power = amount of electric energy converted to other forms (heat, light, sound, motion, etc) every second Power (in Watts) = Energy (in Joules) Time (in seconds) P = E / t Topic 7

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Power A light bulb converts 60 J of electricity to light every second. What is the power of the bulb? P = E / t Topic 7

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Power What is the energy produced every second in a 100 Watt light bulb? P = E / t P x t = E E = P x t Topic 7

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We can use voltage and current to calculate Power Power (in Watts) = Current (in Amps) x Voltage (in Volts) Power = Current x Voltage P = I x V Current = Power I = P / V Voltage Voltage = PowerV = P / I Current Topic 7

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1.36 A current passes through an electric heater plugged into a 110 V outlet. What is the power of the heater? P = I x V Topic 7

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P = I x V Practice Problems Page 324 1. What is the power (in watts and kilowatts) of a hair dryer that requires 10 A of current to operate on a 120 V circuit? 2. The maximum current that a 68.5 cm television can withstand is 2 A. If the television is connected to a 120V circuit, how much power is the television using? Topic 7

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P = I x V Practice Problems Page 324 3. A 900W microwave oven requires 7.5A of current to run. What is the voltage of the circuit to which the microwave is connected? 4. A flashlight using two 1.5V D-cells contains a bulb that can withstand up to 0.5A of current. What would be the maximum power of the bulb? Topic 7

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Paying for Power Your electric company charges by kilowatt hours One kilowatt hour (kWh) is the total energy supplied to a 1000 W load during one hour of operation. Example: a hair dryer running for 1 hour that turns 1000 Joules of electrical energy to heat energy is using 1kW per hour. Topic 7

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Cost Problem: A family uses 3000 kWh of electric energy in a two month period. If the energy costs 11 cents, what is the electric bill for that period? E = 3000 kWh Cost = 0.11$ per kWh Topic 7

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Practice Problems Page 325 1a) You need to know conversions: 1 day = 24 hours 1 hour = 3600 seconds 30 days x 24 hours x 3600 seconds 1 day1 hour Calculate: 30 x 24 x 3600 Topic 7

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Practice Problems Page 325 1.a) If a refrigerator requires 700W of power to function, how many kilowatt hours of power will it require in a 30 day period? b) If electricity costs 11 cents per kilowatt hour, how much would the refrigerator cost to operate in that period? Topic 7

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Practice Problems Page 325 2. A home-owner finds that she has a total of 42 light bulbs (100 W) in use in her home. – If all of the bulbs are on for an average of 5 hours per day, how many kilowatt hours of electricity will be consumed in a 30 day period? – At 11 cents per kilowatt hour, how much will operating these lights cost the home-owner during that period? – How much money would the home-owner save if she switched all of the bulbs to energy-saving 52 W light bulbs? Topic 7

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Practice Problems Page 325 3. Bob has a stereo that operates at 120 V, using 2.5 A. – How much power does Bob’s stereo need to operate? (Hint: Think back to the previous power- calculation) – If Bob plays his stereo for an average of 5 hours each day, how much electricity will he use in a 30 day period? Topic 7

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Power Rating You can buy 60 W, 100 W, or 120 W light bulbs The power is stamped on the product “EnerGuide” labels help consumers decide what to buy Topic 7

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Electrical Devices and Efficiency No device is 100% efficient Efficiency = useful energy output x 100% total energy input Recall P = E/t E = P x t Energy is the power multiplied by time the device is on Topic 7

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Light Bulbs Incandescent bulbs 5% efficient most of the energy is converted to heat Halogen bulbs are 15% efficient waste heat can be a fire hazard halogens can last 2 to 6x longer than incandescent Fluorescent tubes 20% efficient last 13x longer than incandescent Topic 7

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What is the Efficiency? 1000 W electric kettle takes 4 min to boil. The energy used is 1.96x10^5 J (196000 J). P = 1000 W t = 4min x 60sec/min = 2400seconds E = P x t Efficiency = useful energy output x 100% total energy input Topic 7

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Practice Problems Page 329 Find the efficiency of a 23W fluorescent tube that is used 4.0 hours per day and in that time produces 6.624x10^4 J of useful light energy. A 100W incandescent bulb also produces about 6.624x10^4 J over a 4.0 hour period. What is the efficiency of this bulb? Based on your answers to questions 1 and 2, how much money would you save in a 30-day month if you replaced 25 of the 100W incandescent bulbs with 23 W fluorescent bulbs? Assume that the bulbs operate 4 hours a day, and that electricity costs 11 cents per kilowatt hour.

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Home Electrical Safety Electric shock = current flowing through the body Short circuit = when electricity transfers on unintended path. It happens when bare wires touch directly and current flows between them Topic 7

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Home Electrical Safety Don’t overload power outlets Topic 7

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Home Electrical Safety Never change a light-bulb or clean devices that are still plugged in Water + electricity = bad news Topic 7

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Electric Safety Outdoors Don’t go swimming in a lightning storm Topic 7

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Electric Safety Outdoors Don’t touch live wires or power lines Don’t dig up underground utility wires Topic 7

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REVIEW P = I x V is the formula on your PAT Be able to manipulate it for: – I = P/V – V = P/I

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