Today’s Agenda

1. Today’s Agenda Potentiometers Ohm’s Law Continued Power & Energy

2. 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?

3. Potentiometers • A potentiometer is a variable resistor • The total resistance is fixed between terminals A and B • A portion of the resistanceis between A and C • The remainder is betweenB and C • C can be physically moved between A and B A C B

4. 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) A B/C

5. 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): R A B/C

6. 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?

7. + + _ _ 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

8. In-Class Activities • What is the effective resistance of each potentiometer in these circuits? • If R1 and R2 actually were the same potentiometer set to different values and R2 corresponds to C adjusted all the way to the B end (i.e. total resistance value), what percentage of the total resistance is R1? + + A A 10 V 5 V R1 C R2 C _ _ B B

9. Energy • Think of a battery like sand in an hour glass • Sand = charge • Voltage is the force that moves charge • Think of being 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 1/6th the Earth’s

10. 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).

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

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

13. 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.

14. Determining Power

15. Other Power Equations In this example,P =

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

17. In-Class Activity for Power and Ohms Law • In pairs, complete the following chart

18. In-Class ActivityPractice Problem 3.11 (p 86) • 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.