Transcript ELECTRIC CURRENT

ELECTRIC CURRENT
A FLOW OF CHARGE
FLOW OF CHARGE
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POTENTIAL DIFFERENCE
OR DIFFERENCE IN VOLTAGE
BETWEEN THE TWO ENDS OF A
WIRE.
EXAMPLE: WIRE – ONE END ON THE
GROUND AND THE OTHER ON THE
VAN DE GRAAFF GENERATOR THAT
HAS A HIGH POTENTIAL CHARGE.
WHAT WILL HAPPEN?
ELECTRIC CURRENT
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A SUSTAINED FLOW OF CHARGE
WILL ONLY HAPPEN IF A
DIFFERNECE IN POTENTIAL IS
MAINTAINED.
CURRENT FLOWS FROM AREAS OF
HIGH POTENTIAL TO AREAS OF LOW
POTENTIAL.
Electric current
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THE FLOW OF ELECTRIC CHARGE
CHARGE FLOWS THROUGH THE
CIRCUIT USING THE ELECTRONS IN
THE CONDUIT TO TRAVEL.
CONDUCTION ELECTRONS
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THE ELECTRONS THAT CHARGE
USES TO TRAVEL
IONS
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IN FLUIDS POSITIVE AND NEGATIVE
IONS MAY ADD IN THE TRANSFER OF
CHARGE.
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WHY?
Wire
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PROVIDES THE PATH FOR THE
CURRENT
HAS NO CHARGE ITSELF
UNITS
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MEASURED IN AMPERES
NAMED AFTER THE French physicist
Andre’-Marie Ampe’re
1 AMP = 1 COULOMB/SECOND
VOLTAGE SOURCES
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SOMETHING THAT PROVIDES A
SUSTAINED POTENTIAL DIFFERENCE
VOLTAGE SOURCES
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BATTERIES AND GENERATORS
SUPPLY ENERGY AND SLLOW
CHARGE TO MOVE
TYPES OF BATTERIES
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WET CELL
DRY CELL
• CHEMICAL ENERGY CONVERTS TO
ELECTRICAL ENERGY
GENERATORS
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CAR ALTERNATORS
DAMS
• CONVERT MECHANICAL ENERGY TO
ELECTRICAL ENERGY
ELECTROMOTIVE FORCE
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EMF
ξ = PE/Q
EMF IS THE VOLTAGE BETWEEN THE
TERMINALS OF A POWER SOURCE.
VOLTAGE TO HOMES
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U.S. – 120 VOLTS SERVICE
EUROPE – 240 VOLT SERVICE
CONFUSION
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CHARGE FLOWS THROUGH A CIRCUIT
LIKE WATER FLOWS THROUGH A PIPE.
VOLTAGE CAUSES THE CHARGE TO FLOW
BUT DOES NOT FLOW ITSELF LIKE
PRESSURE CAUSES WATER TO FLOW IN A
PIPE.
THE VOLTAGE DOES NOT FLOW JUST
LIKE THE PRESSURE DOES NOT FLOW.
ELECTRIC RESISTANCE
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DEPENDS UPON 4 THINGS
MATERIAL
•
HOW TIGHT IS THE ATOMIC STRUCTURE
TEMPERATURE
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R=0 OHMS AT LOW TEMP. IN SOME MATERIAL
LENGTH
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THE LONGER THE WIRE THE GREATER THE
RESISTANCE
CROSS-SECTIONAL AREA
•
THICK WIRES HAVE LESS RESISTANCE
SI UNIT FOR RESISTANCE
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MEASURED IN OHMS
NAMED FOR GEORG SIMON OHM
OHM’S LAW
• CURRENT = VOLTAGE/RESISTANCE
• I = V/R
• 1 AMP=1 VOLT/1 OHM
ELECTRIC SHOCK
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DAMAGING EFFECTS OF SHOCK
RESULT FROM CURRENT PASSING
THROUGH THE BODY
RESISTANCE OF THE BODY
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RESISTANCE OF THE BODY RANGES
FROM 500,000 OHMS WITH DRY SKIN
TO 100 OHMS WITH WET, SALTY
SKIN.
WHY SALTY?
WHAT HURTS?
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DRY FINGERS
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WET FINGERS
• 12 VOLTS – NO SENSATION
• 24 VOLTS – SMALL SENSATION
• 12 VOLTS – TINGLE
• 24 VOLTS - PAINFUL
EXAMPLE
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WHAT IS THE RESISTANCE OF AN
ELECTRIC FRYING PAN THAT DRAWS
12 AMPS OF CURRENT WHEN
CONNECTED TO A 120 V CIRCUIT?
WHY DOES ELECTRIC SHOCK
DAMAGE?
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OVERHEATS TISSUES
DISRUPTS NORMAL NERVE
FUNCTION
CAN IT BE HELPFUL?
PUZZLE: WHAT CAUSES ELECTRIC
SHOCK – CURRENT OR VOLTAGE?
DEATHS
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MANY PEOPLE ARE KILLED EACH
YEAR FROM 120 VOLT ELECTRIC
CIRCUITS
IF YOU TOUCH A FAULTY 120 V LIGHT
SOCKET THE POTENTIAL OF THE
SOCKET AND YOUR FEET ARE
DIFFERENT. THAT MAKES CURRENT
FLOW.
BIRDS
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BIRDS SURVIVE BECAUSE THEY
ONLY LAND ON ONE WIRE.
WHY?
WHAT WOULD HAPPEN IF THEY
LANDED ON TWO DIFFERENT
WIRES?
PUZZLE
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WHAT WOULD HAPPEN TO YOU IF
YOU FELL OF A BRIDGE AND
GRABBED A WIRE WITH 50,000
VOLTS ACTING ACROSS IT?
TWO TYPES OF CURRENT
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DC – DIRECT CURRENT
ALWAYS FLOWS IN ONE DIRECTION
BATTERIES PRODUCE DC
AC – ALTERNATING CURRENT
CURRENT CHANGES DIRECTION
PRODUCED BY GENERATORS
FREQUENCY OF AC CURRENT
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IN THE US MOST CURRENT IS AT 60
HERTZ OR 60 CYCLES PER SECOND
SOME PLACES HAVE 25,30 AND 50
HERTZ.
WHERE WAS IT POPULAR
FIRST?
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IN THE UNITED STATES
THAT IS WHY WE USE 120 VOLT
SERVICE
POPULAR IN EUROPE LATER.
THAT IS WHY THEY USE 240 VOLT
SERVICE.
SOME APPLIANCES NEED 240
SERVICEOVENS, STOVE,
DRYERS…ETC.
HOW DO WE GET 240 SERVICE
THEN?
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ELECTRIC SERVICE IS PROVIDED
WITH THREE WIRES.
COMBINE THE TWO 120 VOLT WIRES
TO GET 240.
120 V
SERVICE POWER SOURCE
GROUND
120 V
WHAT DOES THE GROUND WIRE
DO?
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ALL WIRES IN THE HOUSE ARE
ULTIMATELY CONNECTED TO A
GROUND TO PREVENT SHOCKS.
APPLIANCES ARE GROUNDED TO
PREVENT SHOCKS.
GROUND FAULT BREAKERS….???
PRIMARY USE OF ELECTRIC
CURRENT
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TRANSPORT OF ENERGY
• QUIETLY
• FLEXIBLY
• CONVENIENTLY
CONVERTING A.C. TO D.C.
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AC – DC CONVERTER
• HAS A DIODE THAT ONLY ALLOWS
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CURRENT TO FLOW IN ONE DIRECTION.
DIODE CONVERTS AC TO DC
CAPACITOR SMOOTHS THE CURRENT
HOW FAST ARE THE
ELECTRONS?
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DRIFT SPEED - SPEED OF
ELECTRONS IN A CIRCUIT
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DRIFT SPEED IS SLOW
ELECTRIC FIELD SPEED IS FAST
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DRIFT
ELECTRIC
FIELD
SOURCE OF ELECTRONS
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BATTERIES
GENERATORS
ENERGY FLOWS FROM POWER
SOURCE
CAUSES e- TO VIBRATE IN THE
FILAMENT
POWER COMPANIES SELL ENERGY
YOU SUPPLY THE ELECTRONS
POWER
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POWER = CURRENT X VOLTAGE
1 WATT = 1 AMPERE X 1 VOLT
P=IV
P=E/t
1000 WATTS = 1 KILOWATT
1 KILOWATT• HOUR = ENERGY
ELECTRIC ENERGY = Pt
CURRENT EQUATION
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I = Q/t
ELECTRIC CURRENT = CHARGE
PASSING THROUGH A GIVEN AREA
TIME INTERVAL
THE CURRENT IN A LIGHT BULB IS
0.835 A. HOW LONG DOES IT TAKE
FOR A TOTAL CHARGE OF 1.67 C TO
PASS A POINT IN THE WIRE.
capacitance
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The amount of energy a capacitor is able
to hold.
Capacitor – electrical storage unit
Energy is stored between the plates
Capacitance = charge/voltage
C = q/v
Measured in Farads
Usually μF = x 10-6 Farads