EGR 1011 Today’s Agenda Potentiometers Ohm’s Law Continued Power & Energy

EGR 1012 Review from Last Week How is voltage related to charge and energy? What is the formula for resistance? What is Ohm’s Law? What does it mean?

EGR 1013 Potentiometers A potentiometer is a variable resistor The total resistance is fixed between terminals A and B A portion of the resistance is between A and C The remainder is between B and C C can be physically moved between A and B A B C

EGR 1014 A B/C 2 Basic Ways to Use Potentiometers As a variable resistor: –The center tap (C) is connected to one end (B) –The total resistance is only from A to C As a voltage divider (to be covered in a later lecture)

EGR 1015 In-Class Activity AB/C R If you have a 1k Ω potentiometer and the center tap, C, is set ¼ of the way between A and B (closer to A), What is the resistance between A and C and between B and C? What is the resistance R if the potentiometer is connected as below (assume C has not been moved):

EGR 1016 Relationship between Current and Voltage Current through a FIXED resistance –Increases when the voltage increases –Decreases when the voltage decreases The current changes as a result of the change in voltage! What is the value of the resistance? + _ + _

EGR 1017 Relationship between Current and Resistance For a FIXED voltage, –The current decreases proportionally to an increase in resistance –The current increases proportionally to a decrease in resistance The current changes as a result of the change in resistance + _ + _

EGR 1018 In-Class Activities 1.What is the effective resistance of each potentiometer in these circuits? 2.If R 1 and R 2 actually were the same potentiometer set to different values and R 2 corresponds to C adjusted all the way to the B end (i.e. total resistance value), what percentage of the total resistance is R 1 ? + _ + _ R2R2 R1R1 5 V 10 V AA B B C C

EGR 1019 Energy Think of a battery like sand in an hour glass –Sand = charge Voltage is the force that moves charge –Think of gravity on the moon vs the Earth Energy = V. Q –You use much more energy to move sand on Earth than on the moon where gravity is 1/6 th the Earth’s

EGR Power & Energy The Instantaneous Power, P, is the Change of Energy, E, per unit time. –In our sand analogy, power is a measure of how quickly the hourglass is emptying Units:[E] = Joules (J). [t] = seconds (s).

EGR Power & Energy The change in energy can be written as: We often assume initial energy is zero

EGR Power in terms of Voltage and Current Previously you learned that or Using this and yields or Since then

EGR Power -The amount of energy used per unit time -The battery shown below uses 1 J/s to generate current – it has used 1 W of power.

EGR Determining Power

EGR Other Power Equations In this example, P =

EGR Other Power Equations (continued) In this example, P =

EGR In-Class Activity for Power and Ohms Law In pairs, complete the following chart ITEM #CURRENTVOLTAGERESISTANCEPOWER 110 mA4 W 232 V16 mW 33.3 kΩ231 mW 415 mA45 V 524 mA1.2 kΩ

EGR In-Class Activity Practice Problem 3.11 (p 88) Calculate the total energy used by a 1500W dishwasher, a 3600W clothes dryer, and a 750W air conditioner that are all being used for 2 hours. Report your answer in J and Btu. Report your answer in kWh. Use the internet to find a recent cost per kWh and report the total cost for this problem.

EGR In-Class Activity continued If I turn the AC off after 1 hour, how many kWh do I use? If, instead, I told you that the dishwasher goes through 4 20-minute cycles in the 2 hours (prewash, wash, rinse, dry), each of which draws 1500W and virtually no power is used between cycles; how many kWh do I use?