ELECTRIC CURRENT AND CIRCUITS TOPIC 1 – ELECTRICAL ENERGY AND VOLTAGE TOPIC 2 – CAPACITANCE TOPIC 3 – CURRENT AND RESISTANCE TOPIC 4 – ELECTRIC POWER TOPIC 5 – EQUIVALENT RESISTANCE AND CIRCUITS

TOPIC 4 – ELECTRIC POWER Learning Goal: You will understand how devices are powered using electricity and what factors affect electric power. Success Criteria: You will know you have met the learning goal when you can truthfully say: 1.I can calculate electric power as a function of current, resistance, and voltage. 2.I can determine the amount of electrical energy used. Image(s) from Bing Images

Success Criteria 1: I can calculate electric power as a function of current, resistance, and voltage. We’ve learned that power is defined as the amount of word done in a given time: P = Wd/t = Fd/t = Fv While this definition holds true for electrical power as well, it is not at useful to work with. Instead, we describe power in terms of electric current, resistance, and voltage: P = IV = I 2 R = V 2 /R In the next unit when we talk about magnetism and its relationship to electricity, we will talk about how a wider variety of electrical devices are powered. In this unit, we will focus mainly on how current, resistance, and voltage interact to output a certain amount of power. Remember that power is measured in watts (W) with one watt equaling 1 joule/sec. The resistance caused by electrons moving through the tungsten filament in this light bulb cause it to radiate heat and light.

Success Criteria 1: I can calculate electric power as a function of current, resistance, and voltage. Task (9 points): Complete the tables (Remember from the electrostatics unit: q = (P – E)1.60x ). PowerVoltageCurrentResistance 65 W1.5 V 120 V3.2 A 12 kW13 A 68 mV 25 m Ω 1.78 µW 174 p Ω 380 mA 0.42 Ω PowerVoltageCurrentResistanceChargeTimeElectrons 602 W120 V862 C 724 mA 12 Ω 3600 s 8.9 V 9520 µ Ω 8.51x10 20

Success Criteria 1: I can calculate electric power as a function of current, resistance, and voltage. Task (7 points): Answer the questions. a)CCC: Cause and effect. Out of P, I, V, and R, which variables would you consider “manipulated” variables and which would you consider “responding” variables. Explain. b)A 12.0 V potential induces a 3.2 A current through a wire. What is the power? c)If an 82 Ω light bulb allows 1.2 A of current to flow through it, what is the power. d)How many volts would it take to output 50.0 W of power through a 160 m Ω resistor? e)If an 8.2 kW motor allows 9.2 A of current to flow through it, what is the voltage? f)Some light bulbs have resistances of around 140 ohms. What voltage would it take to power a W light bulb with this resistance? g)A 3.5 W LED light bulb with a resistance of 1882 m Ω would have what current flowing through it?

Success Criteria 1: I can calculate electric power as a function of current, resistance, and voltage. Engineers have to know the effects of making changes to a system. Changing the voltage or resistance in a circuit can have a variety of consequences. This set of questions will help you begin to think through what those changes might be. Task (7 points): Answer the questions. a)Increasing the voltage will _________ the power, increasing the current will _________ the power, and increasing the resistance will _________ the power. b)Doubling the voltage and keeping the resistance the same has what effect on the current? c)If you double the current by doubling the voltage, what happens to the power? d)If you triple the resistance, what would you have to do to the voltage to keep the power the same? e)CCC: Cause and effect. The brightness of an incandescent lightbulb is a function of its power. If a second lightbulb is added in series after the first, the brightness of both bulbs decreases. Show why this is using your understanding of power, voltage, current, and resistance. f)SEP: Engaging in argument from evidence. Would a thicker tungsten filament in an incandescent light bulb cause it to shine brighter? Why or why not? g)SEP: Constructing Explanations. Analogizing the flow of water and the flow of electricity, why does it seem logical that increasing the voltage and increasing the current both increase the power?

Success Criteria 2: I can determine the amount of electrical energy used. When we pay our electric bills, we pay for an amount of energy we employ powering the devices in our homes. While the joule is the SI unit of energy, the electrical energy we buy from the power company (such as Puget Sound Energy) is measured in kilowatt hours (kWh). 1 kWh = 3.6x10 6 J (3.6 megajoules) Since power is work/time, and work is a form of energy, we can use the electric power equations multiplied by time to get the amount of energy we use: W electric = Pt = IVt = VQ Electricity bills are calculated based on the amount of energy your home uses in a month.

Success Criteria 2: I can determine the amount of electrical energy used. Multiplied by the cost per kWh, you can find the amount of money it costs to power a device for a given amount of time. Cost = Rate x W electric Currently, PSE charges about $0.102 per kWh, so a 100 W light bulb left on for 8.0 hours would cost you $0.082 (about 8 cents). This can be determined most easily using unit cancelling, keeping in mind that 1 watt = 1 joule/sec: (100 J/s) x (3600 s/hr) x (8 hr) x (1 kWh/3,600,000 J) x ($0.102/kWh) = $0.082

Success Criteria 2: I can determine the amount of electrical energy used. Task (3 points): Answer the questions. a)How much energy would you use if your 820 W television were left on for 93 hours in one month? b)What would it cost you ifyou were to leave your 230 W computer on all day every day for a month if electricity were $0.114 per kWh? c)If your electricity bill were $ for January and the cost of electricity were $0.094 per kWh, how many joules of energy would your house have used during January?

Task (8 points): Write at least 8 things you learned in this topic (1/2 point each). If you do this in your notebook, please do it in list form, rather than paragraph form.