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ES 220 Lecture 1 Jack Ou 1.2 SI Units 1.4 Voltage and Current 1.6 Power and Energy.
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Transcript of ES 220 Lecture 1 Jack Ou 1.2 SI Units 1.4 Voltage and Current 1.6 Power and Energy.
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.
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.
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.
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.
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 #2
Fix the height of water in the tank (Voltage is fixed)Increase the hole dimension →I ↑→P↑.
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.
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?