1. Potentiometers Ohm’s Law Continued Power & Energy
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 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

4. 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)

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): A
B/C R

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

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
yields or
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 =
P = .2^2 * 1000 = .04 *1000 = 40 W

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

17. 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

18. 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.