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

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?

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 C B

2 Basic Ways to Use Potentiometers B/C 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)

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): A B/C R

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?

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 + _

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? + _ R2 R1 5 V 10 V A B C

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

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).

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

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

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.

Determining Power

Other Power Equations In this example, P = P = .2^2 * 1000 = .04 *1000 = 40 W

Other Power Equations (continued) In this example, P = P = 12^2 / 10000 = 144/10000 = .144 W or 144mW

In-Class Activity for Power and Ohms Law In pairs, complete the following chart ITEM # CURRENT VOLTAGE RESISTANCE POWER 1 10 mA 4 W 2 32 V 16 mW 3 3.3 kΩ 231 mW 4 15 mA 45 V 5 24 mA 1.2 kΩ

In-Class Activity Practice 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.