This Hour Potentiometers Ohm’s Law Continued Power & Energy EGR 1011

2 Review 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 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 =