Transcript Topic 7 - A Grace Martin

Topic 7: Electricity in the Home
Science 9 with Mrs. M
Please take off your hoods and hats
Bring a calculator to class next week
Topic 7
Power Grid
Topic 7
Power Grid
• The grid is a complex network that links
generating stations to electric energy users
(homes, factories, etc).
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Power Grid
Topic 7
• Transformers reduce or increase voltage of an
alternating current
• “Step up” voltage for long-distance
transmission
• “Step down” voltage at destination
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Transformers
“step up” or “step down”
voltage of AC current
Topic 7: From the Grid to Your Home
Substation:
Step up
Transformer
Distribution
Station
Residential
Step down
Transformer
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Distribution lines -> Power meter ->
Circuit breaker box
Career connection: electrician, power line worker, home renovations
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Circuit breaker/ Main breaker
• Acts as a switch and safety device to cut
power to the home if current is too large.
• Older homes have a fuse box. A fuse melts
when overheated by excessive current.
• Prevents possible fires from overheated wires.
Main breaker
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Branch circuits
• Additional circuit breakers or fuses for each
branch circuit in your home
• Each branch supplies power
to a room or electrical plug
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Home circuitry
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Wires
• Two live wires = neutral wire (white insulated)
and hot wire (black insulated)
• Ground wire (bare copper or green insulation)
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• Digital devices: machines that process
numerically coded information (number codes)
• Binary code: on/off, 0/1 (example: switches)
• Transistor: electronic switch in modern devices
Microprocessors contain millions of transistors
https://www.youtube.com/watch?v=IcrBqCFLHIY
Topic 7
Measuring Electric Power
• Power = energy per unit of time
• Electric power = amount of electric energy
converted to other forms (heat, light, sound,
motion, etc) every second
• Power (in Watts) = Energy (in Joules)
Time (in seconds)
• P=E/t
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Power
• A light bulb converts 60 J of electricity to light
every second. What is the power of the bulb?
• P=E/t
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Power
• What is the energy produced every second in
a 100 Watt light bulb?
• P=E/t
Pxt=E
E=Pxt
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We can use voltage and current to
calculate Power
• Power (in Watts) = Current (in Amps) x Voltage
(in Volts)
• Power = Current x Voltage
P=IxV
• Current = Power
Voltage
• Voltage = Power
Current
I=P/V
V=P/I
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1.36 A current passes through an electric heater
plugged into a 110 V outlet. What is the
power of the heater?
• P=IxV
Topic 7
P=IxV
• Practice Problems Page 324
1. What is the power (in watts and kilowatts) of
a hair dryer that requires 10 A of current to
operate on a 120 V circuit?
2. The maximum current that a 68.5 cm
television can withstand is 2 A. If the
television is connected to a 120V circuit, how
much power is the television using?
Topic 7
P=IxV
• Practice Problems Page 324
3. A 900W microwave oven requires 7.5A of
current to run. What is the voltage of the
circuit to which the microwave is connected?
4. A flashlight using two 1.5V D-cells contains a
bulb that can withstand up to 0.5A of current.
What would be the maximum power of the
bulb?
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Paying for Power
• Your electric company charges by kilowatt
hours
• One kilowatt hour (kWh) is the total energy
supplied to a 1000 W load during one hour of
operation.
• Example: a hair dryer running for 1 hour that
turns 1000 Joules of electrical energy to heat
energy is using 1kW per hour.
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Cost Problem:
• A family uses 3000 kWh of electric energy in a
two month period. If the energy costs 11
cents, what is the electric bill for that period?
• E = 3000 kWh
• Cost = 0.11$ per kWh
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Practice Problems Page 325
• 1a)
You need to know conversions:
1 day = 24 hours
1 hour = 3600 seconds
• 30 days x 24 hours x 3600 seconds
1 day
1 hour
Calculate: 30 x 24 x 3600
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Practice Problems Page 325
1. a) If a refrigerator requires 700W of power to
function, how many kilowatt hours of power
will it require in a 30 day period?
b) If electricity costs 11 cents per kilowatt
hour, how much would the refrigerator cost
to operate in that period?
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Practice Problems Page 325
2. A home-owner finds that she has a total of 42
light bulbs (100 W) in use in her home.
– If all of the bulbs are on for an average of 5 hours
per day, how many kilowatt hours of electricity
will be consumed in a 30 day period?
– At 11 cents per kilowatt hour, how much will
operating these lights cost the home-owner
during that period?
– How much money would the home-owner save if
she switched all of the bulbs to energy-saving 52
W light bulbs?
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Practice Problems Page 325
3. Bob has a stereo that operates at 120 V, using
2.5 A.
– How much power does Bob’s stereo need to
operate? (Hint: Think back to the previous powercalculation)
– If Bob plays his stereo for an average of 5 hours
each day, how much electricity will he use in a 30
day period?
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Power Rating
• You can buy 60 W, 100 W, or 120 W light bulbs
• The power is stamped on the product
• “EnerGuide” labels help consumers decide
what to buy
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Electrical Devices and Efficiency
• No device is 100% efficient
• Efficiency = useful energy output x 100%
total energy input
• Recall P = E/t
• E=Pxt
• Energy is the power multiplied by time the
device is on
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Light Bulbs
• Incandescent bulbs 5% efficient
most of the energy is converted to heat
• Halogen bulbs are 15% efficient
waste heat can be a fire hazard
halogens can last 2 to 6x longer than incandescent
• Fluorescent tubes 20% efficient
last 13x longer than incandescent
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What is the Efficiency?
• 1000 W electric kettle takes 4 min to boil. The
energy used is 1.96x10^5 J (196000 J).
P = 1000 W
t = 4min x 60sec/min = 2400seconds
E=Pxt
• Efficiency = useful energy output x 100%
total energy input
Practice Problems Page 329
• Find the efficiency of a 23W fluorescent tube that is
used 4.0 hours per day and in that time produces
6.624x10^4 J of useful light energy.
• A 100W incandescent bulb also produces about
6.624x10^4 J over a 4.0 hour period. What is the
efficiency of this bulb?
• Based on your answers to questions 1 and 2, how
much money would you save in a 30-day month if you
replaced 25 of the 100W incandescent bulbs with 23 W
fluorescent bulbs? Assume that the bulbs operate 4
hours a day, and that electricity costs 11 cents per
kilowatt hour.
Topic 7
Home Electrical Safety
• Electric shock = current flowing through the
body
• Short circuit = when electricity transfers on
unintended path. It happens when bare wires
touch directly and current flows between
them
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Home Electrical Safety
• Don’t overload power outlets
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Home Electrical Safety
• Never change a light-bulb or clean devices
that are still plugged in
• Water + electricity = bad news
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Electric Safety Outdoors
• Don’t go swimming in a lightning storm
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Electric Safety Outdoors
• Don’t touch live wires or power lines
• Don’t dig up underground utility wires
REVIEW
• P = I x V is the formula on your PAT
• Be able to manipulate it for:
– I = P/V
– V = P/I