Lecture 1

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

SI Units

Derived Units in SI

Standardized Prefixes

Exercise 1

How many hours of video will fit in a 32 GB memory? Assume the followings:• 1 GB=210 Bytes• 480 x 320 Pixels per Frame• 2 bytes per Pixel• Rate of display: 30 Frames per Second

Solution

• 480 x 320 [Pixels/Frame] x 2 [Bytes/Pixel] x 30 [Frames/Second]=9,216,000 Bytes/Second

• 9,216,000 Bytes/Second=30.89 GB/Hour• 32 GB/(30.89 GB/Hour)=1.035 Hour=62

minutes

Exercise 2

• If a signal can travel in a cable at 80% of the speed of light, what length of cable, in inches, represents 1 ns?–Assume the speed of light is 3 x 108

m/s

Solution

• 80 % of speed of light is 2.4 x 108 m/S• 2.4 x 108 m/S x 10-9 S=0.24 m=24 cm• 2.54 cm =1 inch• 24 cm/2.54 cm=9.45 in 1 nS

Voltage and 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

Definition of Voltage

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

𝑉=𝑑𝑊𝑑𝑄

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.

Definition of Current

• Current = the rate of charge flow

Intuition about Current

Direction of Current

Water Flow Analogy

Definition of Energy

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

Energy 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

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 712Waffle maker 1150Projector 192Refrigerator 130Microwave 1610Hair dryer 1532Cable modem 8Wireless router 9

Internet phone 4TV (25 inch, analog) 70

Devices can consume power even when they are not used

Bose speaker 7 WattsProjector 7 WattsMicrowave 6 WattsRechargeable shaver 3 WattsToaster 3 WattsTV 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 more energy than you think.

Energy Consumed by OFF Devices

My toaster consumes more energy when it is off!!

Cost

• Delivery charges: Cost of delivery electricity to homes. ($0.26 per kWh)

• Generation charges: cost of purchasing electricity. ($0.09 per kWh)

• Total cost per kWh. (0.35 per kWh)

Cost of typical household items

Even though it may cost just a few pennies a day, over the cost of 1 year,the cost can skyrocket pretty quickly.

Cost of Not Unplugging the OFF devices.

Each device may not consume much power in its off state.But over the course of one year it can be expensive.

Cost of Not turning Lights Off

Our condo can save as much as 2300 a year if we only turn them on at night.

Electric Car

• PG&E says that the E-9 rate is mandatory for those customers who plan on refueling an EV on their premises.

• Driving Tesla 40 miles a day would use 11.2 kW-h.– Electricity cost: between $0.56 and $3.18

depending on the time of day chosen for recharging.

• Driving an internal combustion engine car for 40 miles a day at 25 mpg would use 1.6 gallons of gas, and at a cost of 3 dollars per gallon would cost 4.80.