Download current and circuits

Current and Circuits
Current
Physics I Honors
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Electric Current
• Voltage (which indicates the presence of an electric field)
causes charges to move in conductors provided there is a
complete path in which the charges can move.
Resistor
Battery
Current
Physics I Honors
Current
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Electric Current
• The rate at which electric charges move
through a wire from higher potential to lower
potential.
• This works just like gravity.
– For water flowing down a hill, the greater the
elevation change (greater change in gravitational
potential) the swifter the water flows.
– For electrical current, the greater the electrical
potential difference (or voltage) the greater the
current.
Physics I Honors
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Electric Current
• Electric potential (V), or voltage, is a
measurement of the energy available at a
location
– higher voltage means more energy per electron
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Electric Current
• I = Q/t
• 1 Ampere = 1 Coulomb/sec
– In wires, moving electrons create the current.
(Electrons moving in one direction are
mathematically equivalent to protons moving in
the other direction.)
– Conventional current says positive charges move.
• In some materials (liquids, gases, semi-conductors)
positive or negative changes can move.
Physics I Honors
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CURRENT AND CIRCUITS
TYPES OF CURRENT
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Conventional Current vs. Electron Flow
Conventional current: Direction a positive charge would flow
This is opposite the direction of electron flow in a conductor!
current
electron flow
+
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Alternating Current vs Direct Current
direct current (DC) – electrons flow in one
direction only
i.e. batteries
alternating current (AC) – the direction of
flow changes (60 cycle in United States)
i.e. power station
Although net electron flow over one cycle
is zero, can still do useful work!
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CURRENT AND CIRCUITS
Resistance
Current
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Resistance ()
Opposition to the flow of electrons
Resistance depends on:
–
–
–
–
Length (L)
Cross-sectional area (A) {diameter}
Conductivity of the material
Temperature (T)
• Ohm’s Law is not always valid!!
• Increased temperature increases resistance
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Less Resistance
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More Resistance
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Resistance ()
• Resistance = Resistivity * Length / Area
– R = L/A
– Resistivity depends upon the material.
Current
A = Area
Physics I Honors
L = Length
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Resistance ()
• R = V/I (Resistance = Voltage/Current)
– or V = I * R
It takes one volt to push one amp through one
ohm
1  = 1 V/A
V
Physics I Honors
V
V
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Resistance ()
R = 50 
V = 9 V
R = 100 
V=9 V
V = IR  I = V/R
V = IR  I = V/R
I = 9 V /50 ohms = .18 A
I = 9 V /100 ohms = .09 A
Physics I Honors
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Current (I)
• Current is the flow of charge.
• Charges flow when there is a potential
difference (voltage) and there is a
complete path for the charges to flow.
• To maintain a constant flow of charge
(current), a continuous potential
difference must be maintained.
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Teen dementia test
• 4 electrons pass a point in a circuit every
second. What is the current?
• A resistor experiences a current of 2 amps
and a voltage of 4 volts. What is the size of
the resistor?
Physics I Honors
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Current and Circuits
Circuits
Schematics
Use of Meters
Safety
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Circuit
• A path through which
electrons flow.
• The path is from a power
source’s negative
terminal, through the
various components and
on to the positive
terminal
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Circuit
• Open circuit –
– a break prevents the flow of electricity
• Closed circuit –
– electricity can flow
• Short circuit –
– the electricity completes a circuit without
going through the load
• Load –
– item using electricity
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Schematics - Symbols
wire or conductor
resistor or other load
bulb
V
voltmeter
battery
A
ammeter
switch
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generator
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Schematics – Simple Circuit
+
circuit diagram
A
+
-
A
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Schematics - Complex Circuit
+
Construct the
circuit diagram
A
+
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-
A
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Meters - Ammeter
• Measures current.
• It is a Galvanometer wired in
parallel to a resister.
• Ammeters are connected in
series to the circuit.
• Have low resistance
Physics I Honors
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Meters - Voltmeter
• Measures electric potential…voltage.
• It is a Galvanometer wired in series
with a resistor.
• Voltmeters are connected in parallel
to the load
• Have high resistance
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Fuses and Circuit Breakers
If an electric circuit gets “overloaded” (too
much current!) fuses or circuit breakers
interrupt the flow of current.
Circuit Breaker
Fuses
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Current and Circuits
Series Circuits
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Series Circuit
• Only ONE path for the electricity.
– Christmas lights
– Batteries in a series (more current storage)
• Total current is constant and voltage
decreases with additional resistors (i.e
dimmer lights)
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Series - Computing Current
• The current reading will be the same
anywhere in the circuit.
I1 = I2 = I3 …
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Series - Computing Resistance
• The total resistance is the sum of the
resistances of all of the loads.
RT = R1 + R2 + R3 …
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Series - Computing Voltage
• The total electricity (energy) used by the
loads is the sum of the usages of all of
the loads.
VT = V1 + V2 + V3 …
• All the energy is used before the current
returns to the energy source
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Sample Problem Series Circuit
Three 20  resistors are connected in series across
a 120 V generator.
What is the total resistance of the circuit?
RT = R1 + R2 + R3
= 20 + 20  + 20  = 60 
What is the current in the circuit?
I = V/RT
= 120V / 60  = 2 A
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Current and Circuits
Parallel Circuits
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Parallel Circuits
• two or more paths for the electricity.
• Total current increases and volts
remain constant with additional
resistors
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Parallel Bulbs
Voltage dropped is the same at each resistance
Current is different
But: total current out of battery = sum of the
currents through each bulb
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Parallel - Computing Current
• The sum of the currents through each
branch equals the total current
IT = I1 + I2 + I3 …
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Parallel - Computing Resistance
• Req (total resistance) is smaller than any
individual resistor;
1/Req = 1/R1 + 1/R2 + 1/R3…
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Parallel - Computing Voltage
• The voltage across each resistor is the same.
V1 = V 2 = V 3 …
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Sample Problem – Parallel Circuit
Three 20  resistors are connected in parallel and
placed across a 120 V potential difference.
What is the equivalent resistance?
1/Req = 1/20 + 1/20  + 1/20 
= 3/20  = 1/6.67 
Req = 6.67W
20 
20 
20 
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Sample Problem – Parallel Circuit
What is the current through the circuit?
I = V / Req
= 120 V / 6.67 
20 
the total current
I = 18A
20 
Physics I Honors
20 
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Sample Problem – Parallel Circuit
What is the current through each branch of the
circuit?
For each branch,
In = V / Rn
= 120V/20  = 6A
20 
20 
20 
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Current and Circuits
Combination Circuits
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Combination
• circuit with both series and parallel
paths for the electricity
• The parallel section of the circuit is like
a sub-circuit and actually is part of an
over-all series circuit.
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Combination - Series/Parallel
• This circuit is composed of two resistance
segments:
– R1 alone
– R2 and R3 in parallel
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Combination - Series/Parallel
• Resistors R2 and R3 can be combined to
form a single equivalent resistance: R2,3
• This leave a simple series circuit
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Using the series-parallel schematic
• What is the equivalent resistance of the
circuit?
• first treat the parallel section
– R1 = 3
– R2 = 4 
– R3 = 6 
– VT = 12 V
• 1/Req = 1/R2 + 1/R3
= 1/4 + 1/6
= 3/12 + 2/12 = 5/12 = 1/2.4 
• Req = 2.4  for the parallel portion
Physics I Honors
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Using the series-parallel schematic
• Then, add all the resistances as if they were in series
RT = R1 + R2,3
= 3  + 2.4  = 5.4 
Physics I Honors
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Using the series-parallel schematic
• What is the current in the circuit?
I = V/R = 12 V / 5.4  = 2.22 A
Physics I Honors
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Using the series-parallel schematic
• What is the current through each
resistance?
• I=V/R
• I2 = V2 / R2
= 5.33 V / 4  = 1.33 A
• I3 = V3 / R3
= 5.33 V / 6  = 0.89 A
• I2,3 = I2 + I3
= 1.33 A + 0.89 A = 2.22 A
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Using the series-parallel schematic
• How much voltage is lost at each
resistor?
• I = V / R so
• V1 = IR1
= 2.22 A * 3  = 6.67 V
• V2,3 = IR2,3
= 2.22 A * 2.4  = 5.33 V
• VT = V1 + V2,3
= 6.67 V + 5.33 V = 12 V
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Parallel/Series
• This circuit is composed of
two resistance segments:
– R1 and R2 in series
– R3 in parallel
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Current and Circuits
POWER
TRANSMISSION OF ELECTRICAL ENERGY
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POWER
Power is calculated by multiplying current
times voltage
P = IV
R
Resistors use energy.
Rate = Power = I2R
V
I
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POWER
• Energy per unit of time converted into
another form of energy.
• Thermal energy is produced at a rate
given by P = I2R
• Power is lost during transmission due to
production of heat ( conversion of
electrical energy to thermal energy)
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Transmission of Power
• In transmission of energy, it is important
to maintain the power and reduce the
loss of energy.
• Using P = I2R, we must reduce either
the resistance or the current.
• Reducing resistance
– using wire that is highly conductive and of
large diameter.
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Transmission of Power
• Reducing the current.
– Looking at P = IV,
– to reduce current, increase the voltage to
maintain the same power delivered.
– Long distance lines may carry voltages of
more than 500,000 V (that’s half a million
volts)
Physics I Honors
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Paying for Electricity
• You pay for electric energy, not power.
• Electric energy is paid for by the kilowatt
hour (kWh).
• 1 kWh = (1000 J/s)(3600 s)
= 3.6 x 106 J
• Light bulbs are often sold in wattage. A
100 Watt bulb uses 100 Watts of electric
energy in 1 hour.
Physics I Honors
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Electric Power
A microwave oven is rated at 120 V and 840 W.
a) What is the current drawn?
P = IV I = P/V = 7 A
b) What is the resistance of microwave oven?
P = I2R R = P/I2 = 17.14 
c) How much energy is used in 5 minutes?
E = Pt = 840 W * 5 min * 60 s = 2.52 x 105 J
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Injuries through Electricity
Current
1 mA
5 mA
10 mA
20 mA
100 mA
1000 mA
Physics I Honors
Effect
mild shock
painful
paralysis of
motor muscles
breathing stops
heart stops
serious burns
Fatal?
no
no
no
minutes
seconds
instantly
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