Transcript CHpt. 18 - 19 - Conroe High School

Electric Current:
Charges in Motion
• Lightning was found to be an electric
discharge- a gigantic spark but until
1800 not much usefulness
• Allesandro Volta (1745 – 1827)
invented the electric battery- from
this discovery a new era opened,
Today's electrical technology is
based on electric current
Volta disproves Galvani’s animal
electricity and invents the 1st
battery
• Scientists recognized that batteries
produce electricity by transforming
chemical energy into electrical
• electrodes- 2 dissimilar metals (one
can be carbon). These are immersed
in a solution known as the…
• Electrolyte: today’s dry cell – this is a
powdery paste
• Battery refers to multiple cells
Continued…
• Terminals: two electrodes that
stick out of the electrolyte
• A chemical reaction occurs
that causes electrons to flow
from the negative terminal to
the positive. When the
chemical reaction stops, we
say the battery is “dead”
Electric Current
• A circuit is produced when a
continuous path connects the
terminals of a battery
+
-
SIGN FOR
BATTERY
Image 18-6
Pg. 530
Formula
I = ΔQ
Δt
Current (amperes)= Charge(coulombs)
Time (seconds)
Sample Problems
• A steady current of 2.5 amps
flows in a wire for 4 minutes.
a) How much charge passed
through a point in the
circuit?
b) How many electrons would
this be?
Answer!
A) ΔQ = I Δt
2.5C/s (240sec) =
600C
B) Charge of 1 electron is 1.6x10-19C
so 600C would be…
600/ 1.6 x 10-19C
= 3.8 x 1021 electrons
Current Flow
• Electrons actually flow from
negative terminal to positive;
when we are speaking of
conventional current, that is
from positive to negative. In
liquids and gasses, both
positive and negative
charges (ions) can move.
OHMS LAW
• Current is proportional to voltage
• water analogy:
– 2 pipes
Which one will
let the most
water flow??
OHMS LAW
• Resistance is opposite to the
flow of electricity
• R OHMS (Ω)
• I = V/R
• Current (amps)= Voltage (volts)
Resistance (ohms)
• V = IR
Sample Problem:
• A small flashlight bulb
draws 300mA from its 1.5
volt battery
a) What is the resistance of
the bulb?
b)If the voltage dropped to
1.2V, how much would the
current change?
ANSWER!
A) R = V/I
1.5V/ .3A = 5Ω
B) If resistance stays constant
I = V/R
1.2V/ 5Ω
.24A
a drop of 60mA
All Appliances Have
Resistance
• Hair dryer, TV’s, Lightbulbs,
electric motors, ect.
• In many electronic devices
resistors are used to control
current. They can be less than 1
ohm to millions of ohms
• Symbol for resistance:
Pg. 534
Pg. 534
Pg. 534
Resistivity
• Resistance can change for a
substance
Low
High
Thick Cross Section
Thin
Short Length
Long
Chart pg. 535
R = P(L/A)
R = resistance (Ω)
L = length (m)
A = cross section area (m2)
P = Resistivity (coefficient for
substance)
Temperature Affects
Resistivity
• Resistance increases
with temperature
• At super low
temperatures,
resistance becomes
zero- super conducting
Electric Power
• Electric energy is useful
because it can easily be
converted into other forms of
energy.
• Motors transform electricity
into mechanical work, heaters,
stoves, toasters; all transform it
to thermal energy, light bulbs
convert it to heat and light!
Formulas
P = QV/t
OR
P = IV
Power (watts)=Current (Voltage)
BY SUBSTITUTION
P= IV = I (IR) = I2R
=(V/R)V = V2/R
Sample
• Calculate the resistance of a
40W headlight designed for
a 12V automobile
=
R=
2
V /P
2
(12V) /
40W
= 3.6Ω
Sample
• An electric heater draws 15A on a
120V line. How much power does it
use and how much does it cost per
month (30 days) if it operates 3 hours
per day and the electric company
charges 10.5 cents per kWh?
P = IV = 15A(120V) = 1800Watts
1.8 kW
30 days (3hr/ day) = 90hrs
1.8 kW (90hrs)($.105)= $17
Sample
• A typical lighting bolt can transfer
109J of energy across a potential
difference of 5x107 Volts during a
time interval .2 seconds. Calculate
charge transferred, current,
average power over the time
9J
interval.
9
I
=
Q/t
P
=
10
Q = 10 J
5x107V
= 20C
20C
.2sec
=100A
.2sec
= 5x109W
5 Giga Watts
Power in Household
Circuits
• SAFETY:
– Thickness in wires
– Fuses
– Circuit Breakers
– Shorts
– Fires
– GFCI outlet near a water source
in your home
Will a Fuse Blow?
Find the total current drawn by all the
devices
Sum of Power =
100 + 1800+ 350 + 1200
Voltage = 120V
I = P/V 3450W/ 120=
28.8A
this would blow a 20 amp fuse!
AC DC
• Batteries move electrons in one
direction DC
• Electric generators at power
plants produce alternating
current AC
• This type changes the direction of
electron flow many times per
second (In US 60Hz – In Europe
50Hz)
• The AC voltage is sinusoidal
Electrons Moving in a
Wire
• When we flip a light switch- there's
light!
• Do electrons move fast in a wire?
• - NO it only appears that way
Add
Water
Over
Flow
Water
Electron drift speed is .05mm/sec or 5½ hrs to
go 1m
CHAPT. 19
DC Circuits
• Electric circuits are
everywhere- cars,
homes, schools,
businesses, radios, TVs
and computers
Basic Symbols
Battery
Capacitor
Resistor
Wire with negligible resistance
Switch
Resistance in Series
and Parallel
Series: one path
Parallel: multiple
paths
Series Circuit
• Rtot=R1+R2+R3+…
• The Current through each
resistance is the same: I tot=I1=I2=…
• The Voltage across each resistance
splits up proportionally among the
R’s. Vtot=V1+V2+V3…
Parallel Circuit
• Current from the source splits
into separate branches
Wiring in businesses and houses are like
this- that way if one bulb burns out, the
rest stay on!
Parallel Circuit
Current splits up among the resistance so
they add…
Itot = I1 + I2 + I3
• Voltage is the same for each resistor
Vtot=V1=V2=V3
• But I = V/R
• SO resistors in parallel add this way:
1/Rtot = 1/R1 + 1/R2 + 1/R3
Example
• Two 8Ω resistors in
parallel and have a
Rtot of 4Ω
1/Rtot = 1/8Ω + 1/8Ω = 2/8Ω = 1/4Ω
Rtot = 4Ω
Water analogy Dual
water pipe
Water pipes in parallel- analogy to
electric currents in parallel
Example 19-1
EMF
• Electro Motive Force (not a true
force- not measured in Newtons)
• This is the potential difference
across the terminals of a battery
(generator) when NO current flows
into an external circuit.
• Battery Voltage isn't constant
(Start your car, headlights dim)
Battery can’t supply current fast
enough to maintain the full EMF
EMF
• Batteries have their own
internal resistance
designated r
• Terminal Voltage is what we
measure across the
terminals of a battery when
no circuit flows
Terminal Voltage = Vab = EMF - Ir
Kirchoff’s Rules
• Deals with complicated circuits
• Junction Rule (conservation of
charge): At any junction point, the
sum of all currents entering the
junction must equal the sum of all
currents leaving the junction.
• Loop Rule (conservation of energy):
The sum of the changes in potential
around any closed path of a circuit
must be zero.
Electric Hazards
• The severity of electric shock
depends on the magnitude of
the current. Also how long it
acts and through what part of
the body it passes.
• Heart and Brain are most critical
• Electric current heats up tissue
and causes burns
Electric Hazards
• The severity of shock depends on the
effective resistance of a body. Living
tissue has quite low resistance (cells
contain ions) and is a good conductor.
However, the outer layer of skin, when dry,
offers a high resistance 104 to 106 ohms.
• When the skin is wet, resistance can be
103 Ohms or less, this allows for more
current to flow and a more severe shock
Grounding
A) An electric oven operating normally with 2
prong plug
B) Short to the case with ungrounded case:
shock!
C) Short to the case with the case grounded
with Third prong.
Circuit Breakers
Birds and squirrels don’t
get shocked?