ES 220Lecture 1Jack Ou

1.2 SI Units1.4 Voltage and Current1.6 Power and Energy

SI Units

Sanity Check

The triple point of water, i.e. the pressure and temperature at which liquid water, solid ice and water vapor can coexist at a stable equilibrium occurs at 273.16 K (0.01 Celsius or 32.018 Farenheit) and partial vapor pressure of 611.73 Pascal.

Derived Units in SI

Chapter 1

Chapter 9

Chapter 6

Standardized Prefixes

The Gooey Monster May Not Pick Five Apples

Resistance Example

Current

Learning by discovery

Experience Electricity

• The sudden sharp tingle that you feel is caused by electricity from one terminal of the battery, through the moisture on and in your tongue, to the other terminal.

• Because the skin of your tongue is very thin and the nerves are close to the surface, you can feel the electricity easily.

• Warnings: Do not do this experiment at home

Movement of Electrons

• The sharp tingle you feel is caused by an electric current.

• An electric current is caused by the movement of electrons.

• The movement of electrons is caused by the battery.

Attraction/Repulsion

Intuition about Current

Motion of free electronic when a voltage is applied.

Mathematical Definition of Current

• Current = the rate of charge flow through a surface

Direction of Current

Voltage

• Two like charges will repel• If you want to push to charges

together, you need to put in work.• The amount of work you put in per

unit charge is defined as voltage.

Definition of Voltage

• Voltage is the energy per unit charge created by the separation

𝑉=𝑑𝑊𝑑𝑄

Thought Experiment #1: Hold Q constant

Positive Charge(Stationary)

Negative Charge(Stationary)

Positive Test Charge

Positive Charge(Stationary)

Negative Charge(Stationary)

Thought Experiment #2: Hold V constant

Positive Charge(Stationary)

Negative Charge(Stationary)

Positive Test Charge

Positive Charge(Stationary)

Negative Charge(Stationary)

If positive test charge is reducedby a factor of 2,work is reduced by a factor of 2

Intuition

• Thought Experiments:–Assume V is fixed. Hold 2 X of Q in your hand. Spend 2X of energy to move 2Q from –Q to Q.

–Assume Q is fixed. Increase V by 2X. Spend 2X of energy to move Q from –Q to Q.

Water Flow Analogy

Definition of Energy

• Energy (W)is the ability to do work.–The unit of energy is Joules (J).

Example

Definition of Power

• Power is the rate at which energy is used

• Unit of Power: Watts

Example of Power

• 360 J of energy is required to operate an MP3 player for an hour. What is the average power consumption?–1 hour=3600 seconds–360 J/3600 seconds=0.1 W or 100 mW

Polarity for Power

As positive charges move through a drop in voltage, they loseEnergy.

As positive charges move though a rise in energy, they gain energy.

(power is deliveredto the box,)

i points into the + temrinal.

(power is deliveredto the box)(power is extracted

from the box)

i points into the – terminal.

(power is extractedfrom the box)

Analogy

Tank=batteryThe height of the water=voltageThe volume of flow through the hole per second=currentThe smallness of the hole=resistanceWater wheel hit by the flow from the hole=power

Thought Experiment #1

Fix the dimension of the holeAdd more water →V↑→I↑→P↑

Thought Experiment #2

Fix the height of water in the tank (Voltage is fixed)Increase the hole dimension →I ↑→P↑.

How do you measure power?

Power consumed by typical House appliances

Appliance Power (Watts)

Toaster 712

Waffle maker 1150

Projector 192

Refrigerator 130

Microwave 1610

Hair dryer 1532

Cable modem 8

Wireless router 9

Internet phone 4

TV (25 inch, analog)

70

Devices can consume power even when they are not used

Bose speaker 7 Watts

Projector 7 Watts

Microwave 6 Watts

Rechargeable shaver 3 Watts

Toaster 3 Watts

TV 3 Watts

Point of comparison: MP3 player consumes 0.1 Watts

Power Consumed by Lights

14 lights on each floor.Each light bulb consumes about 20 Watts.There are 8 floors.20 Watts x 14 x 8=2240 Watts !!

Caution!

• Power is not the same as energy.• Power is the rate at which energy is

being used.• Example

– TV consumes 70 Watts or 70 Joules per second.

– How much energy is consumed by 1 hour of TV vs 40 hours of TV?

kWh

• There is another way to express energy.

• Since P=W/t, Energy (W) can be expressed as W=Pt

• Utility companies charged their customers by kWh

Example 4-3

• Determine the number of kilowatt-hours (kWh) for each of the following energy consumption:–1400 W for 1 hour–2500 W for 2 hours–100,000 W for 5 hours

Energy Consumed by Typical Household Appliances

Wireless router and cable modem:1.Consumes little power.2.Consume significant amount of energy.

Energy Consumed by OFF Devices

A toaster can consume more energy when it is off!!

Light Bulb ComparisonType: LEDBrightness: 450 LumensLife hours: 20,000 hoursPower: 7.5 wattsNo MercuryCost: 10.97 @ Home DepotSKU: 420232

Type: CFLBrightness: 450 LumensLife hours: 7,993 hoursPower: 9 wattsContains MercuryCost: 5.77 @ Home Depot (per light bulb)

Cost of Owning

number of

light bulbs

light bulb

purchase

d Duration

Power

(Watts)

Energy

(KWH)

Cost PER

KWH Cost

Cost of light

bulbs Total Cost

LED 1 1 20000 7.5 150 0.35 52.5 10.97 63.47

CFL 2.01 2 20000 9 180 0.35 63 11.54 74.54

Period: 20,000 hours or 18.26 years (assume 3 hours of usage per day)Questions:1. How often do you replace CFL?2. Are numbers realistic?