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Today’s Agenda

Potentiometers

Ohm’s Law Continued

Power & Energy

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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?

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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

B

C

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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)

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In-Class Activity

A B/C

R

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):

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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?

+

_

+

_

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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

+

_

+

_

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In-Class Activities1. What is the effective resistance of each

potentiometer in these circuits?

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?

+

_

+

_

R2R15 V 10 V

A A

B B

C C

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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 1/6th the Earth’s

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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).

JP Watt W

s

t

EP

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Power & Energy

t

EP

tPE The change in energy can be written as:

We often assume initial energy is zero

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Power in terms of Voltage and Current

Previously you learned that

or

Charge

EnergyVoltage

Using this and

yields or

Since then P VIt

QI

tPE

Pt

QV

tPQV

Q

EV

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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.

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Determining Power

VIP

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Other Power Equations

2

P VI

V IR

P I R

In this example,

P =

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Other Power Equations (continued)

2

P VI

VI

RV

PR

In this example,

P =

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In-Class Activity for Power and Ohms Law

• In pairs, complete the following chartITEM # 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

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In-Class ActivityPractice Problem 3.11 (p 88)

• 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.

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In-Class Activity continued

• If I turn the AC off after 1 hour, how many kWh do I use?

• If, instead, I told you that the dishwasher goes through 4 20-minute cycles in the 2 hours (prewash, wash, rinse, dry), each of which draws 1500W and virtually no power is used between cycles; how many kWh do I use?