Ch
20-23 Electricity
Updated 5/26/03
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OBJECTIVE:
IDENTIFY STATIC AND CURRENT ELECTRICITY.
ELECTRIC
FIELD--THE AREA AROUND AN ELECTRIC CHARGE WHERE ITS ELECTRIC FORCE CAN BE FELT
BY OTHER CHARGES.
COULOMB--THE
UNIT OF ELECTRIC CHARGE = 6.28 1018 electrons
POTENTIAL
DIFFERENCE--THE DIFFERENCE BETWEEN THE POTENTIAL ENERGY OF TWO POINTS IN AN
ELECTRIC FIELD. THE GROUND WILL
ABSORB ANY CHARGE AND HAS A POTENTIAL ENERGY OF ZERO.
VOLTAGE--THE
DIFFERENCE IN POTENTIAL ENERGY THAT CAUSES A CURRENT TO FLOW.
conductor:
substances which permit electrons to pass through it.
ex: copper, aluminum, silver (best)
insulators:
substances that do not permit electrons to pass through them.
ex: glass, plastic, rubber,
wood.
electric
charges
an electric charge occurs when an object gains or looses electrons.
ATOMS CAN LOSE ELECTRONS BY FRICTION.
if an object gains electrons it has a negative charge.
if it loses electrons it has a positive charge.
LIKE CHARGES REPEL; UNLIKE CHARGES ATTRACT.
there
are two types of electricity
static - and - current
NOT CONDUCTED - AND - CONDUCTED
NON-MOVING - AND - MOVING
static
[NON-MOVING] electricity:
WITHOUT A CONDUCTOR THE CHARGE, CREATED by friction, WILL NOT MOVE BUT WILL
builD up in one place.
static discharge occurs when a charged object TOUCHES ANOTHER OBJECT AT LOWER
POTENTIAL ENERGY.
IF THE POTENTIAL DIFFERENCE BETWEEN THE TWO OBJECTS IS LARGE SPARKS MAY JUMP THE
GAP. ex: lightning rod.
OBJTV:
LIST THE PROPERTIES OF DIRECT AND ALTERNATING CURRENT ELECTRICITY.
current
electricity
electric current is the movement of electrons through a conductor.
THERE ARE 2 TYPES OF CURRENT ELECTRICITY--DIRECT AND ALTERNATING CURRENT.
1 direct CURRENT
(dc): electrons flow in one
direction ONLY
POWER
SOURCE--BATTERY
2 alternating
current (ac): electron FLOW
REVERSES DIRECTION--60
TIMES
[cycles] per second.
POWER
SOURCE--GENERATOR
CIRCUITS
Updated 5/26/03
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OBJECTIVE:
DIAGRAM PARALLEL AND SERIES CIRCUITS; PREDICT WHICH BULBS WILL GO OUT IF
THE CIRCUIT IS BROKEN IN DIFFERENT PLACES.
circuit--a
complete path through which an electric current can pass.[DIAGRAM]
to
diagram a circuit use the following symbols for: resistance, light, switch, battery, generator, ground,
connection, no connection.
3
types of circuits are series, parallel, & complex.
series:
one LOOP--[DIAGRAM]
break the circuit in one place and no electricity flows in the whole branch.
voltages and resistances connected in series add, current does not change
ex: 3 voltages + 3 resistances
total volts =, total resistance = ?
parallel:
two or more LOOPS--[DIAGRAM]
break the circuit in one place and electricity flows through the other branch[s]
THE VOLTAGE DROP IS THE SAME IN BOTH BRANCHES;
THE TOTAL RESISTANCE IS LESS THAN THE LARGEST RESISTANCE;
THE CURRENT DIVIDES IN EACH BRANCH;
TOTAL CURRENT = CURRENT FOR THE WHOLE CIRCUIT
COMPLEX:
SERIES & PARALLEL CIRCUITS CONNECTED. [DIAGRAM]
EACH SECTION IS ANALYZED INDIVIDUALLY.
grounding
is done to provide a path for overload CURRENT. THIS protectS against electrocution and fires.
birds
can perch safely on power lines because no part of them offers the current a
path to the ground
in
a short circuit, the current flows over an unintended path that has very low
resistance--usually uninsulated or exposed wire. a large current flows. the
result is rapid overheating and possibly fire.
when this happens a fuse or circuit breaker should act to shut off the
current.
stu
dent needs a 12 v battery to start his car.
stu should connect the d batteries in (series or parallel).
he should connect ___ d batteries to get 12 v.
Ohm's
Law
Updated 5/26/03
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ohm's
law:
1 resistance
(ohms - r): how difficult it is to
move electrons
through a
conductor.
2 voltage
(volts - v): pressure pushING the
current through a wire
3 current
(ampere - I): the volume or number
of electrons flowing in
a conductor
per second
voltage
V
VOLTS
resistance = --------
R = ---
OHMS = -----
current I
AMPS
Ohm's Law
water
electric current
volume water flow
electron flow
LITERS/min
amperes (6.28 1018 electrons/SEC)
resistance length
&
length & diameter of wire
diameter
ohm: one ampere of current
of pipe
to flow at one volt
pressure NEWTONS/CM2
volts: pressure needed to
force a current of 1 amp
through a resistance of 1 ohm
Chemical
reactions occur faster in liquids than in solids.
Thus wet cell batteries can deliver more electricity per
minute than dry cells.
That is you can draw greater current or amperage.
A
fuse is a thin wire that will break if it gets too hot.
AAA,
AA, A, C, and D batteries all have 1.5 volt difference between their terminals.
their seize difference indicates a difference in the total energy
available
Conductor Size
Updated 5/26/03
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The
size of the conductor to be used to carry a given current may be determined by
any one of the following 3 factors:
1]
Permissible temp rise
2]
Permissible voltage drop in the conductor.
3] An
economic balance between the value of the power [I2R] loss in the conductor and
the capital charges against the conductor.
1]
V = IR = CONSTANT===> R = V/I
2] R = k/A2
3] FOR A TUNGSTEN FILAMENT THE
REQUIRED TEMP AND THEREFORE HEAT, IS A CONSTANT:
H = I2R ===> R = H/I2
Eq #1 & #2: V/I = H/I2 ===>
V = H/I===>
4] I = H/V
Eq #1 & #4: H/V = V/R===>
5] R = V2/H
Eq 2 & 5: k/A2 = V2/H ===>
A = sqrt(kH)/V
piD2/4 = sqrt(kH)/V===>
D = 2 sqrt[sqrt(kH)/Vpi]
A
10% voltage drop in a tungsten resistor will change the amount of light emitted
drastically. IF A TUNGSTEN FILAMENT
IS TOO SMALL IT BURNS OUT AND IF IT IS TOO LARGE IT DOESN'T GLOW.
A BALANCE MUST BE STRUCK TO FIND THE OPTIMAL DIAMETER.
EXAMPLE:
60 WATT BULB IN A 120 V CIRCUIT
1] V = IR = CONSTANT===> I = V/R
2] FOR A TUNGSTEN FILAMENT THE
REQUIRED TEMP AND THEREFORE HEATING RATE, IS A CONSTANT. SINCE THE HEATING RATE AND THE POWER ARE EQUAL:
P = VI ===> I = P/V
SETTING THESE 2 EQUATIONS EQUAL:
I = V/R = P/V ====> R = V2/P = 120*120/60 = 240 OHMS
3] SUBSTITUTING k/A2 FOR R: k/A2 =
V2/P ===> A = SQRT(kP)/V
piD2/4 = sqrt(kP)/V===>
D = 2 sqrt[sqrt(kP)/Vpi] = 2 sqrt[sqrt(k60)/120*3.14]
D
= 0.287(k)1/4
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