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 • A portion of the resistance is between A and C C • The remainder is between B and C B • C can be physically moved between A and B 2 Basic Ways to Use Potentiometers • As a variable resistor: A – The center tap (C) is connected to one end (B) B/C – 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): R A B/C 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 1. What is the effective resistance of each potentiometer in these circuits? + + A A 5V C R1 10 V C R2 _ _ B B 2. 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? 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 E is a measure of how quickly P the hourglass is emptying t • Units: [E] = Joules (J). [t] = seconds (s). J P Watt W s Power & Energy E P t The change in energy can be written as: E P t We often assume initial energy is zero Power in terms of Voltage and Current Energy Previously you learned that Voltage Charge E or V Using this and E P t Q Q yields V Q P t or V P t Q Since I then P VI t 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 P VI Other Power Equations P VI V IR In this example, P I R2 P= Other Power Equations (continued) P V I V I R In this example, V 2 P P= R In-Class Activity for Power and Ohms Law • In pairs, complete the following chart ITEM # CURRENT VOLTAGE RESISTANCE POWER 1 10 mA 4W 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.