Transcript 150Lecture 3 Basic Circuits Lecture Notes Page

FOWLER CHAPTER 2
LECTURE 3 ELECTRICAL QUANTITIES
AND UNITS
CHAPTER 2 ELECTRICAL QUANTITIES AND UNITS
P-17
COULOMB: BASE UNIT OF CHARGE=
CHARGE SYMBOL =Q
ABBREVIATION
=C
P-22
AMPERE

6.25 1018 ELECTRONS
CURRENT= Q/t CHARGE/TIME
1A  1C
s
ONE COULOMB/SEC OR
6.25 1018 ELECTRONS/ SEC
ELECTRIC CURRENT (I): MOVEMENT OF CHARGED PARTICLES IN A SPECIFY DIRECTION
CAN BE +OR-
LIKE CHARGES REPEL,UNLIKE CHARGES ATTRACK.
+ CHARGE CAN BE + ION
- CHARGE IS AN ELECTRON
THESE CHARGES CAN MOVE THRU SOLIDS,LIQUIDS OR GASES. IN OUR
MOST COMMON CONDUCTOR COPPER, ONLY ELECTRONS CAN MOVE.
P-18
SYMBOL I WAS CHOSEN BECAUSE IT WAS THOUGH TO REPRESENT
THE INTENSITY OF ELECTRICITY IN A WIRE. CURRENT MOVES AT LESS
THAN THE SPEED OF LIGHT IN ANY WIRE.
WHEN A ELECTRIC FIELD IS PLACED ACROSS A WIRE. SAY FROM A BATTERY ,
VALENCE ELCTRONS MOVE FROM ATOM TO ATOM, CREATING + AND- IONS WHICH
UNDER THE INFLUENCE OF THE ELECTRIC FIELD ALLOWS CURRENT TO FLOW.
Basic Electricity - What is an amp?
http://www.youtube.com/watch?v=8gvJzrjwjds
CURRENT FLOWING IN THE SAME DIRECTION ALL
THE TIME IS KNOWN AS DIRECT CURRENT
+
VOLTAGE
TIME
0
_
P-19
CURRENTS IN GASES
IN GASES UNDER THE INFULENCE OF A STRONG ELECTRIC FIELD + AND –
CHARGES ARE FORMED i.e. + IONS AND ELECTRONS. THIS IS CALLED IONIZATION.
METAL PLATE
METAL PLATE
+
+
-
-
+
+
-
-
+
-
+
+
NEON FILLED TUBE IN A NEAR VACUUM. NEON ATOM
HAS 8 VALENCE ELECTRONS IN ITS OUTER RING
CURRENTS IN LIQUIDS
P-19
AGAIN + AND – CHARGES SERVE AS CURRENT CARRIES. UNLIKE
GASES,IN LIQUIDS BOTH + AND - CURRENT CARRIES ARE IONS.
- ARE Cl IONS
+ ARE Na IONS
METAL PLATES
+
+
+
-
+
+
+
-
-
NaCl SOLUTION
-
+
A LIQUID THAT CAN CARRY CURRENT IS CALLED AN ELECTROLYTE (e.g.
SEA WATER,SULFURIC ACID IN CAR BATTERIES.)
P-21
CURRENTS IN VACUUM
IN THIS CASE AN ELECTRON TUBE IS USED AS AN EXAMPLE.
+
CATHODES ARE COATED
WITH THORIUM,BARIUM OR
STRONTIUM
+
-
THERMIONIC EMISSION: EMITTION OF ELECTRONS FROM A HEATED PLATE. AS THE
PLATE IS HEATED IT EMITS ELECTRONS WHICH ARE DRAWN TO THE POSITIVE
PLATE BY THE ELECTRIC FIELD CREATED BY THE BATTERY. ON A SIMPLE SCALE,
THIS IS HOW A CRT WORKS.
ELECTRON MOVEMENT IN A TUBE FILAMENT
CLOSE UP VIEW OF A TUBE FILAMENT
P-23
VOLTAGE=EMF(ELCTROMOTIVE FORCE)=POTENTIAL DIFFERENCE
(All mean the same thing)
KINETIC VS. POTENTIAL ENERGY
KINETIC: ENERGY OF MOTION OF AN OBJECT.
POTENTIAL ENERGY: STORED ENERGY OF AN OBJECT AT REST.
STATIC ELECTRICITY IS POTENTIAL ENGERY (PE)
STATIC SPARK CONVERTS PE OF THIS SPARK INTO HEAT AND LIGHT
IN ELECTRICITY, THE PE IS DUE TO THE ELECTRIC FIELD
AND THE CHARGE SEPARATION BETWEEN TWO OBJECTS.
YOU TUBE:A cell phone and static electricity:
Bill Nye The Science Guy on Static Electricity:
www.youtube.com/watch?v=gct1BmKNvU0
www.youtube.com/watch?v=Z-77IzaXGcg
A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate
very high voltages on a hollow metal globe on the top of the stand. It was invented by American
physicist Robert J. Van de Graaff in 1929. The potential difference achieved in modern Van de
Graaff generators can reach 5 megavolts.
ELECTROSTATIC ATTRACTION BETWEEN 2 CHARGED SPHERE’S
electric flux animation
http://www.youtube.com/watch?feature=fvwp&v=THybl_QGssw&NR=1
http://phet.colorado.edu/en/simulation/balloons
TAKE ELECTROSTATIC PRECAUTIONS WHEN
WORKING ON ANY ELECTRONIC EQUIPTMENT
P-24
ELECTRIC FIELD LINES
SPHERE WITH EXCESS
POSITIVE CHARGES
-
SPHERE WITH EXCESS
NEGATIVE CHARGES
AS ELECTRON’S MOVE ACROSS THE ELECTRIC FIELD
WORK IS DONE, ENERGY IS LOSS MOVING FROM – TO +
THE PE IS DUE TO THE ELECTRIC FIELD AND ELECTRIC
CHARGE BETWEEN TWO OBJECTS. THIS FIELD
CREATES A POTENTIAL DIFFERENCE
SHOW MU29 1300:1630
http://www.youtube.com/watch?v=PeBPPudNpAw
P-25
UNIT OF VOLTAGE (V) VOLT
WE NEED A WAY TO INDICATE THE P.E. DIFFERENT BETWEEN THE TERMINALS
OF OUR BATTERY.
ENERGY IS TRANSFORMED BY CHARGES(ELECTRICAL) ACROSS THE BATTERY TERMINAL.
SINCE THE BASE UNIT OF ENERGY IS THE JOULE AND THE BASE
UNIT OF CHARGE IS THE COULOMB;
WE CAN DEFINE VOLTAGE AS JOULE/COULOMB
V=JOULE/COULOMB
V= ENERGY/CHARGE= W/Q
OR W=VQ
W(WORK OR ENERGY)= VQ
EXAMPLE: D CELL BATTERIES ARE RATED AT 1.5V
A FLASHLIGHT WITH 2D CELLS = 3 V
WHAT DOES THIS MEAN?
WHEN THE FLASHLIGHT IS ON , WITH A POTENTIAL DIFFERENT OF 3V
V = J/C 3 JOULES OF ENERGY IS CARRIED BY 1 COULOMB OF CHARGE,
WHICH ARE CONVERTED TO HEAT AND LIGHT.
BATTERY IS DISCHARGING
P-26
BATTERIES
BATTERY IS CHARGING
+
‫־‬
‫־‬
DIRECTION OF ELECTRON FLOW
+
ELCTRON FLOW IS REVERSED
THERE IS A P.E. DIFFERENT BETWEEN THE + AND – TERMINALS OF
THE BATTERY. WHEN CURRENT FLOWS WORK IS DONE. P. E. IS
CONVERTED INTO ANOTHER FORM OF ENERGY, IN THIS CASE AS
HEAT IN THE RESISTOR.
P-25
POLARITY: HELPS TO INDICATE WHICH DIRECTION
CURRENT IS FLOWING. CAN BE + OR -
WHEN TERMINALS ARE IDENTIFIED ON A ELECTRONIC DEVICE, IT IS SAID TO BE POLARIZED.
POLARITY MUST BE CORRECT FOR THE DEVICE TO WORK. IF THE DEVICE IS CONNECTED
BACKWARD, IT IS SAID TO BE IN A STATE OF REVERSE POLARITY AND NO CURRENT WILL
FLOW.
The Mechanical Universe - 31 – Voltage, Energy and Force 6:00-8:00,12:15-14:00
http://www.youtube.com/watch?v=jnWc_6PqJ6I
GEOTHERMAL
REMEMBER: ONE FORM OF ENERGY MUST
BE CONVERTED TO ANOTHER TO CREATE
VOLTAGE.EXAMPLE:POWER PLANT
GENERATORS
SOURCES OF VOLTAGE
COAL
CONVERT MECHANICAL
ENERGY INTO
ELECTRICAL ENERGY
NUCLEAR
SOLAR
HYDROELECTRIC
P-26
OTHER WAYS TO GENERATE ELECTRICITY
CRYSTALS (VIA THE PIEZOELECTRIC EFFECT)
BY VARYING PRESSURE APPLIED TO CRYSTALS CAN CREATE
ELECTRICITY. USES:PHONOGRAPH PICKUPS, MICROPHONES
IN MIRCOPHONES, SOUND ENERGY IS CONVERED TO MECHANICAL
ENERGY, WHICH IS APPIELD TO A CYRSTAL TO CREATE ELECTRICITY
THERMOCOUPLE(SEEBECK EFFECT): THE DIFFERENT IN TEMPERTURE
BETWEEN TWO MATERIALS(USUALLY METALS OR SEMICONDUCTORS)
CAN BE CONVERTED INTO ELECTRICITY.
YOU TUBE: Thermocouples
http://www.youtube.com/watch?v=gtO0kQ-PT_0
P-26
RESISTANCE: OPPOSITION TO CURRENT FLOW IN A MATERIAL.
ELECTRICAL ENERGY IS CONVERTED INTO HEAT.
CONDUCTORS: OFFER LITTLE RESISTANCE TO CURRENT FLOW. WHY? DO TO THE
EASE AS WHICH VALANCE ELECTRONS CAN BE RELEASED FROM THE OUTER SHELL
OF AN ATOM.
GOOD CONDUCTORS: SILVER,GOLD,ALUMINIUM,COPPER.
BAD CONDUCTORS: RUBBER,WOOD,CERAMIC MATERIAL, GLASS
INSULATORS HAVE HIGH RESISTANCE TO CURRENT FLOW.
A BAD CONDUCTOR IS A GOOD INSULATOR.
MOST INSULATORS ARE COMPOUNDS OF TWO OR MORE ELEMENTS. THEYSHARE ELECTRONS
BY COVALENT BONDING.(MAKES IT HARDER FOR ELECTRONS TO BE FREED.)
GLASS INSULATORS
METAL CONDUCTORS
UNIT OF RESISTANCE IS THE OHM Ω
1Ω

AMOUNT OF RESISTANCE IN A MATERIAL WITH 1
VOLT APPLIED WITH A CURRENT FLOW OF 1 AMP.
4 FACTORS THAT EFFECT RESISTANCE
1. TYPE OF MATERIAL
2. LENGTH OF OBJECT
3. CROSS SECTIONAL AREA
4. TEMPERATURE OF OBJECT
R  L
A
http://phet.colorado.edu/sims/resistance-in-a-wire/resistance-in-a-wire_en.html
AS TEMPERATURE. INCREASES SO DOES THE RESISTANCE
TEMP. COEFFICENT

THE CHANGE IN RESISTANCE OF A MATERIAL AS THE
TEMPERATURE INCREASES OR DECREASES.
MOST MATERIALS HAVE + TEMP. COEFF. i.e. AS TEMP ↑ SO
DOES THE RESISTANCE.
SOME HAVE – TEMP. COEFF. (EXAMPLE CARBON)
TEMP COEFFICENT IS EXPRESSED AS THE CHANGE OF RESISTANCE(IN OHMS) PER
PPM (PART PER MILLION) /C°
RESISTIVITY:THE RESISTANCE OF A MATERIAL AS DEFINED BY A CERTAIN SIZE OF THE
MATERIAL.
CAN BE 1cm³, 1m³, 1ft³
THE RESISTIVITY UNIT IS THE Ω·cm. SEE APPENDIX E P.479
THE LOWER THE RESISTIVITY THE BETTER CONDUCTOR IT IS.
R  L
A
RESISTIVITY x LENGTH/CROSS SECTIONAL AREA
RESISTORS: USED IN CIRCUITS TO CONTROL THE FLOW OF CURRENT.
RANGE<1Ω TO>1 MILLION Ω’S
RSISITANCE IN CIRCUIT WIRES IS SO SMALL THAT IT DOSEN’T AFFECT THE
PERFORMANCE OF THE CIRCUIT.
A heating element converts electricity into heat through the process of Joule heating.
Electric current through the element encounters resistance, resulting in heating of the
element.
How a Toaster Works
https://www.youtube.com/watch?v=WYcw_DcZsak
Most heating elements use Nichrome 80/20 (80% nickel, 20% chromium) wire, ribbon,
straight or coiled. Nichrome 80/20 is an ideal material, because it has relatively high
resistance and forms an adherent layer of chromium oxide when it is heated for the first
time. Material beneath this layer will not oxidize, preventing the wire from breaking or
burning out.
Used in common items such as toasters and hair dryers, furnaces for industrial heating,
floor heating, roof heating, pathway heating to melt snow, dryers, etc.
Knife into toaster FAIL
https://www.youtube.com/watch?v=So7I70Z0uCU
P-30
POWER/ENERGY: IS ABILITY TO DO WORK, A GAGE OF HOW ENERGY IS CONVERTED
FROM ONE FORM TO ANOTHER.
POWER CAN ALSO BE THOUGHT OF AS THE RATE OF USING ENERGY OR WORK.
HOW DO WE MEASURE THIS IN A PRACTICAL WAY?
LIKE THIS: HOW MUCH ENERGY IS BEGING USED IN A GIVEN TIME.
UNIT OF POWER IS THE WATT (W)
WATT = 1 JOULE/ SECOND = 1 J/S
POWER (P) =W/t = ENERGY/ TIME
WATTSECOND = 1 J/ S·S =1 J
JOULE IS A TINY AMOUNT OF ENERGY.
POWER COMPANIES USES KILOWATT-HOUR INSTEAD OF WATTSECOND
OLD ANALOG POWER METER
NEW DIGITAL POWER METER