ES 220 Lecture 1 Jack Ou 1.2 SI Units 1.4 Voltage and Current 1.6 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 K (0.01 Celsius or Farenheit) and partial vapor pressure of 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

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

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 reduced by 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

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 lose Energy. As positive charges move though a rise in energy, they gain energy. (power is delivered to the box,) i points into the + temrinal. (power is delivered to the box) (power is extracted from the box) i points into the – terminal. (power is extracted from the box)

Analogy Tank=battery The height of the water=voltage The volume of flow through the hole per second=current The smallness of the hole=resistance Water wheel hit by the flow from the hole=power

Thought Experiment #1 Fix the dimension of the hole Add 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 AppliancePower (Watts) Toaster712 Waffle maker1150 Projector192 Refrigerator130 Microwave1610 Hair dryer1532 Cable modem8 Wireless router9 Internet phone4 TV (25 inch, analog)70

Devices can consume power even when they are not used Bose speaker7 Watts Projector7 Watts Microwave6 Watts Rechargeable shaver3 Watts Toaster3 Watts TV3 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 Comparison Type: LED Brightness: 450 Lumens Life hours: 20,000 hours Power: 7.5 watts No Mercury Cost: Home Depot SKU: Type: CFL Brightness: 450 Lumens Life hours: 7,993 hours Power: 9 watts Contains Mercury Cost: Home Depot (per light bulb)

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