s1-3-18 explain parallel circuits, components, and safety of house wiring. s1-3-21 develop a formula...
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ELECTRICITY @ HOME
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S1-3-18 Explain parallel circuits, components, and safety of house wiring.
S1-3-21 Develop a formula for power consumption and solve related problems.
S1-3-22 Analyze the electrical energy consumption of a household appliance.
KEY WORDSOverload Circuit breaker FusePower Efficiency
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House circuits are different than the circuits made with batteries:
1. Uses alternating current (AC) instead of direct current (DC)
2. Operates at a higher voltage - 120 V or 240 V3. Electricity controlled and moved through a
main panel (fuse box)4. All branches off the main line are in parallel
Common household circuits are 120 volts and 15 amps
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Remember this?
Generators produce A/C current for the home
DC: electrons travel in one direction – (-) to (+)AC: e- move back and forth many times a second
DC is like rolling balls down a hill – need to keep adding more to the top (or run out)
AC is like balls rolling along a teeter totter – don’t need more since they will just move back and forth
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There are 3 wires that bring electricity into a home:• Neutral wire (white)• Two “Hot” wires (black) – carrying 120 V each
There is also a safety “ground” wire - green or bare
copper - removes stray current safely
Normal use:1 hot connected to neutral creates 120 V potential
Larger appliances: Both hot wires connect to neutral creates 240 V
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• each branch delivers typical 120 volts• total current of all branches equals 15 A• break any branch without affecting any other
All house branch lines link from the main panel.
Outlets and switches are all connected in parallel
Remember what parallel circuits offer…
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Safety issues:
Short circuit: current bypasses the appliance. • less resistance – increases branch current• Branch heats up – risk of fire
There are two components that
safeguard against an increase in current
Overload: too many things plugged into one outlet. • increased branching – increases total current• Main heats up – risk of fire
A A
AA
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main
Circuit Breakers: • contain either magnet / metal / semiconductor• all are altered by excess current - breaks circuit
(kill switch)
Fuses: • uses a strip of metal that easily melts• excess current heats metal – breaks circuit
Breakers can be reset
Fuses “burn out” and need to be replaced
A A
AAmain
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Power and Energy Consumption
Power is a measure of energy output over time:• running the same distance faster – more power• lifting heavier weight in the same time – more power
1 Watt is the power of completing 1 joule of work in 1 second
• E is energy/work done in joules• t is the time in seconds
P =tE
Power – measured in watts – is the energy used or work done every second
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An iPhone charger moves 72 000 joules of energy in the 4 hours to fully charge a drained battery. What is the power of the charger?
P =tE Formula must be in seconds to work properly
– time must be converted
4 h · 60 min · 60 sec = 1 h 1 min
14 400 sec
P =14400 seconds
72000 joules= 5 watts (W)
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Rarely do we know energy values, but we can find voltage and current requirements - sometimes they
are written on the component.
P =tE
Remember: Voltage = energy per coulomb of electrons
Current = coulombs of electrons per second
P = V · I
tQ
QE
·P = V · I
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A 10 amp car heater is plugged in to a standard house outlet. What is the power of the heater?
Large power measurements are converted to kilowatts
P = V · I Standard home outlet is 120V
P = 120 V · 10 A P = 1200 W
1200 W · 1 kW = 1000 W
1.2 kW
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Energy companies charge a fee for total energy consumption (usually) per month – “hydro” bill
Total energy is calculated by multiplying power by time used
Purchasing Electrical Energy
• P is the power in kilowatts (kW)• t is the time in hoursE = P · tP =
t
E
Notice the units: Time is in hours – seconds are not useful for householdsPower is in kilowatts – watts are too small to be useful
tt
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You make smoothies every day in September (30 days) using a 900 W blender for 15 min. How much energy did the blender use that month?
E = P · t900 W · 1 kW =
1000 W 0.9 kW
15 min · 1 h · 30 (days) = 60 min
7.5 hours
E = 0.9 kW · 7.5 h
E = 6.75 kWh
Energy usage is measured in kilowatt-
hours (kWh) instead of joules (too small)
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Reading a Hydro Meter
Hydro meter uses 5 dials to measure energy usage:• far right dial is “ones” - far left dial is “ten thousands”• a reading is taken every 1-2 months
Reading
23 930 (kWh)
20 769 (kWh)
3161 (kWh) energy used this month
Hydro charges about $0.08 for every kilowatt-hour (kWh) used = $253 this month
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This household uses natural gas for heat and has a second bill for the total amount of gas used.
Sample Hydro bill:
kWh usedTime
rate
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Efficiency measures how well a device converts energy into work:• every device wastes some energy • lost as heat, sound, leakage…
If it is more efficient it does the same work with
less input energy
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EnerGuide labels give consumers an idea of the efficiency of an appliance:• gives energy usage in regular use over time (month/year) • allows consumer to estimate cost of using the device• efficient devices will have a lower energy usage
Energy star: government-regulated symbol used to identify most efficient devices (based on tests).
This appliance will use 125 kWh of electricity per month of normal use
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CAN YOU ANSWER THESE QUESTIONS?
S1-3-18:How is a household wired to ensure safety?
S1-3-21:What is power consumption and how does it affect your electricity bill?
S1-3-18:How can you lower electrical energy use of a household?
KEY WORDSOverload Circuit breaker FusePower Efficiency