Download Unit 7 - Electrical Circuits and Systems

Inside the Atom
• Protons
Unit 7: Electrical Circuits and Systems
– Positive charge
– Found in nucleus
– Little freedom to move
• Neutrons
•How is electricity measured and described?
•What is the relationship between current and voltage in a circuit?
•What are the different types of circuits?
•How much energy is carried by electricity?
– No charge
– Found in nucleus
Inside the Atom
Electric Current
• Electric Current (I)
• Electrons
– Negative charge
– Found in electron
cloud outside
nucleus
– Freedom to move
– The flow of electric
charge
• Usually electrons
– Able to do work
Electric Circuits
• If you touch a light bulb to a
battery will it light up?
– No, it needs a complete
circuit
• Electric Circuit
– Complete path through which
electricity travels
– Requires a source of energy
– Ex: Battery or outlet
Electric Circuits
• Closed Circuit
– Complete loop of electricity
– Current flows through
• Open Circuit
– Circuit with a break in it
– Current does not flow through
– Ex: Light bulb burns out
• Switch
– Used to open or close a circuit
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Conductors and Insulators
Conductors and Insulators
• Semiconductors
• Conductors
– Devices that allow
current to flow
– Ex: Metals
• Insulators
– Do not easily allow
current to flow
– Ex: Rubber, glass and
wool
– Carry a medium amount of
current
– Conductivity is usually variable
– Ex: computer chips
• Wires have insulators outside and
conductors inside
Batteries
Batteries
• Cathode
• Batteries
– Use chemical energy to
create voltage
difference
– Adds potential energy
to charges and moves
them through the
circuit
– Voltage adds from all
batteries in circuit
Resistors
• Resistors
– Convert electrical energy
into other energies
– Ex: Light bulb, Heater,
Speaker, Motor
• Potentiometer (Pot)
– Charge flows out
– Usually the negative
side
• Anode
– Charge is collected
– Usually the positive
side
Electric Circuits
• Circuit Diagram
– Uses symbols to
show components of
circuit
– Shorthand for
describing a working
circuit
– Variable resistor
– Ex: dimmer switch, volume
control
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Electric Circuits
Voltage
• Voltage (V)
• Current flows from positive to
negative
• In diagrams, long line is
positive, short line is negative
• Arrow shows direction of
movement
– Electric potential energy
– Measured in Volts (V)
– 1 V = 1 J /(A ‧ s) = 1 W/A
– Circuit must have a
voltage difference across
it for energy to flow
Current
Resistance
• Current (I)
• Resistance (R)
– Measured in
Amperes (A)
– Current in equals
current out
– Flows from positive
end to negative end
Andre-Marie Ampere
– Opposition to flow of
current
– High resistance means
current flows slowly
– Circuit wires usually have
little to no resistance
– Measured in Ohms (Ω)
– 1 Ω = 1 V/A
Resistance
• Human skin usually has a resistance of
~100,000 Ω or more
• Wet skin – less resistance – more dangerous
• You feel current of 0.0005 A or more
• A 9 V battery will cause 0.00009 A, so you
won’t feel it
• A wall socket will cause 0.0012 A, which is
dangerous
Ohm’s Law
• Ohm’s Law
– Used to calculate the
current in a system
V = IR
– V – Voltage (V)
– I – Current (A)
– R – Resistance (Ω)
Georg Ohm
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Current and Resistance
• The amount of resistance determines the current
• More resistance means less current
Series Circuits
• Series Circuits
– Only one path for current
flow
– Current is same at all
points
– Break in circuit means
current stops everywhere
in the circuit
– Ex: Old Christmas Lights
Series Circuits
• Series Circuits
– Resistance is sum of
all resistances
– Use Ohm’s Law to
find current
Voltage Drop
• Kirchhoff’s Voltage Law
– Voltage drops must add up
to battery voltage
– Energy in circuit is
conserved
Voltage Drop
• Voltage Drop
– Voltage difference
across a resistor
– Ex: 3 light bulbs with
3 volts
Voltage Drop
• Kirchhoff’s Voltage Law
– If resistors are identical:
• Divide the total voltage by number of devices
– Ex: A series circuit contains a 9-volt battery and
three identical bulbs. What is the voltage drop
across each bulb?
• 3V
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Voltage Drop
Parallel Circuits
• Parallel Circuits
• Kirchhoff’s Voltage Law
– If resistors are not identical:
1. Calculate the total resistance
2. Use Ohm’s Law to find the current
3. Use Ohm’s Law at each resistor to find the
voltage drop
Parallel Circuits
• Parallel Circuits
– Voltage is always the
same across each
branch
• Even when branches
have different
resistances
– Each branch has voltage
drop equal to total
voltage
Parallel Circuits
• Each branch has the
same voltage, but a
different current
• Use Ohm’s law to find
the current in each
branch (I = V/R)
• Add up all branches to
find total current
– Multiple current paths
– Each path is a Branch
– Branch Current is the amount of current through
each branch
Parallel Circuits
• Kirchhoff’s Current Law
– All current in each branch
must come out again
– Total current is equal to sum
of branch currents
Parallel Circuits
• More resistors lower the
resistance in parallel
circuits
• To find the total
resistance:
• Draw the diagram
• Add all the branch
currents
• Use Ohm’s law to find the
total resistance ( R = V/I)
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Parallel Circuits
• Are houses wired in
series or parallel?
• Parallel, so you can turn
lights on and off
without affecting other
rooms
• Also so you can use the
blender and microwave
at the same time with
full power
Electrical Power
Electrical Power
• Electrical Power (P)
– Measure of the rate of
work done by a circuit
– Rate at which electricity
is converted to sound,
heat, light, etc.
Buying Electricity
• Electrical Power (P)
– Measured in Watts (W)
P = IV
• Kilowatt-hour (kWh)
– Unit of energy
• 1,000 W of power
used for 1 hour
• 3,600,000 J
– Used to bill customers
from power company
Buying Electricity
AC vs. DC
• Kilowatt-hour (kWh)
– Calculations
• Convert W to kW (1,000 W = 1 kW)
• Multiply kW by time in hours
• kWh to $ using dimensional analysis
• Direct Current
– Always from positive to negative
– Produced by batteries
• Alternating Current
– Switches constantly
– In US, 60 hz or 60 times/second
– You need adapters for other
countries
– Provided by power lines
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AC vs. DC
Winner of War of Currents
Nikola Tesla – Westinghouse
AC Current Advocate
Circuit Safety
• Wires
Thomas Edison – GE
DC Current Advocate
– Can handle different
amounts of power
– Can absorb current
• Usually make it into heat
– Larger diameter can handle
more current
– Longer wires have more
resistance
Circuit Safety
• Short Circuits
– Occur when there is
little or no resistance in
a circuit
– Usually if two wires at
different points on the
circuit touch
– Produces heat and can
start a fire
Circuit Safety
• Outlets
– 3 connections
– Hot wire – 120 V
– Neutral Wire – 0 V
– Ground wire – 0 V
• Sends power to
ground in short
circuit
Circuit Safety
• Circuit Breaker
– Automatic switch
– Can be reset
– Opens when current builds up too
high
• Fuse
– Low-resistance resistor
– Burns up when current is too high
– Must be replaced after
Circuit Safety
• Ground Fault Interrupt (GFI)
– Breaks circuit if current in does
not match current out
– Required near water or outdoors
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Circuit Safety
• Transformers
– Change the voltage from the
power plant to home
• Higher voltage through
wires because of
increased resistance
– Charging devices change AC
to DC to power batteries
• Ex: cell phone charger
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