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 3 – CURRENT AND RESISTANCE Learning Goal: You will understand how electric current is used to do work and how objects that electric current do work on provide resistance. Success Criteria: You will know you have met the learning goal when you can truthfully say: 1.I can calculate the current in a wire. 2.I can describe the differences between alternating current and direct current. 3.I can relate voltage, current, and resistance to each other. Image(s) from Bing Images

Success Criteria 1: I can calculate the current in a wire. The reason why electricity has been able to transform our world, being able to power our devices from hundreds of miles from power plants, is because an electric field can be transmitted through a wire by an electric current.

Success Criteria 1: I can calculate the current in a wire. When a voltage is set up across two ends of a wire, the resulting field creates an electromotive force, causing electrons to move through the wire. This electric current, which is given the variable I, is measured in amperes, abbreviated amp or A. One amp is equal to one coulomb of charge flowing through a cross section of wire in one second. I = Q/t Common misconception: Electrons don’t actually travel that fast through wires. It would take about 68 minutes for an electron to travel 1.0 meter (2.46x10 -4 m/s).

Success Criteria 1: I can calculate the current in a wire. Task (6 points): Answer the questions. a)CCC: Scale, proportion, and quantity. Go to the “orders of magnitude (current)” Wikipedia page and look up the current required to power the following devices: Portable hearing aid, common LED, 120 V incandescent bulb, 12 V car headlight, 120 V toaster, the Z machine. b)What would the current be in a wire where.025 C pass through it every 4.0 seconds? c)If 15mC of charge pass through a wire in 3.2 s, what is the current? d)How much charge passes through a section of wire if a 170mA current flows for 45 s? e)How long would it take a 2.5 A current to transfer C of charge? f)Compared to water flowing through a pipe, would the amperage be more like the pressure or the amount of water flowing? Explain.

Success Criteria 2: I can describe the differences between alternating current and direct current. Batteries provide a constant voltage source which creates a current in one direction. This is called direct current. Electrical generators, on the other had, create a voltage source that rapidly changes direction. This is called alternating current. In the U.S., AC voltage changes directions 60 times per second (60 Hz) and produces a 120 V potential. While most devices require DC in order to operate, AC can be transmitted at high voltage over longer distances. It is then converted into DC for use in our homes.

Success Criteria 2: I can describe the differences between alternating current and direct current. Task (2 points): Answer the questions. a)How does the voltage that creates direct current compare to the voltage that creates alternating current? b)Even though most electronics run on direct current, why is alternating current used to carry electrical energy from power plants to our homes?

Success Criteria 3: I can relate voltage, current, and resistance to each other. Just like doing work on an object requires pushing the object against an opposing force (such as lifting a box against gravity or pushing a table against friction), a device that is powered by an electric current (such as a light bulb or a computer) resists the flow of the current. This resistance is measured in ohms ( Ω ) and is related to current and voltage according to Ohm’s Law: V = IR

Success Criteria 3: I can relate voltage, current, and resistance to each other. Before we get into using Ohm’s Law, let’s talk about factors that affect resistance. FactorExamples that decrease resistance MaterialA conducting material has less resistance then an insulator. Width of pathA thicker wire has less resistance because there is more room for electrons to flow. Length of pathThe shorter the wire, the less overall resistance the wire provides. TemperatureA colder wire’s slower-atoms get in the way less, so resistance decreases. Ion concentration (for solutions) Adding salts to water makes it conduct electricity better because you are adding charge carriers (ions) to the solution.

Success Criteria 3: I can relate voltage, current, and resistance to each other. Using V = IR and the factors affecting the resistance, you can see what alterations you could make to control the current flowing through an electrical system. Controlling the current is essential for making sure electronic components get enough power, but not so much that they get overloaded or damaged.

Success Criteria 3: I can relate voltage, current, and resistance to each other. Task (6 points): Answer the questions. a)What is the resistance of a wire that allows a 0.15 A current to flow through it with a voltage of 9.0 V? b)How many ohms of resistance is a lightbulb providing if a 3.30 V potential causes a 62 mA current to flow? c)If you increase the voltage, what happens to the current through a wire if the resistance stays the same? d)How can you decrease the current in a wire if you have a voltage source that doesn’t vary? e)CCC: Structure and Function. Powerlines carry electricity at a higher voltage than we use in our homes. Use V=IR to explain why. f)Write true or false in each blank. _____ A thick wire has more resistance than a thin wire. _____ A cold wire has more resistance than a warm wire. _____ Salty water has less resistance than pure water. _____ An insulator has less resistance than a conductor. _____ A long wire has less resistance than a short wire.

Success Criteria 3: I can relate voltage, current, and resistance to each other. Task (7 points): Use the “Circuit Construction Kit (DC Only)” at phet.Colorado.edu to complete the data table and answer the questions. SetupVoltageCurrentResistance a) A lightbulb connected to a battery b) Same setup as a), but with the resistance of the battery increased from 10 ohms to 20 ohms. c) Two light bulbs connected in series to a batteryV bulb 1 = V bulb 2 = V total = d) Two batteries connected in series to one light bulb SEC: Analyzing and interpreting data. e) Does changing the resistance change the current or the voltage? f) What effect does adding light bulbs have on the resistance and current? g) Looking at part c), if you add together the voltages across each light bulb, does it equal the total voltage supplied by the battery?

Task (4 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.