Chapter 6, Section 5

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Chapter 6, Section 5 . Electric Power: Load Limit. April 18, 2011 HW: 6.5 PTG, pg. 641, 1-13, Due Wed. LO Explain why fuses and circuit breakers are important safety devices . SC Define power, insulator, and conductor Use the equation P=IV

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## Chapter 6, Section 5

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### Chapter 6, Section 5

Electric Power: Load Limit

April 18, 2011HW: 6.5 PTG, pg. 641, 1-13, Due Wed
• LO
• Explain why fuses and circuit breakers are important safety devices.
• SC
• Define power, insulator, and conductor
• Use the equation P=IV
• Calcuate the power limit of a 120-V household circuit
• Differentiate between a fuse and circuit breaker
• Identify the need for the circuit breakers and fuses in a home
• Do Now:
• 6.4 Quiz
• Write LO and SC on new left side page
• WDYS/WDYT pg. 631
• Agenda:
• Do Now
• Investigate
• Physics Talk
Investigate
• #1-complete set up as shown. Answer #1a in your notebook
Investigate
• #2-6 Teacher Demo
Investigate #2
• Why do you think the fuse blew?
• Why did the circuit require multiple appliances to blow the fuse?
• Can you develop a model that explains why the fuse behaves the way it does?
Remember:
• Voltage is the #Joules per Coulomb (# pretzels per person)
• Current is the # Coulombs per second (people per second)
• Power is the #Joules per second (# Pretzels per second)
• P=VI
• Power=Voltage x Current

P

I

V

Investigate #3-6
• Complete table in your notebook
Investigate #3-5
• Create table in your log
Investigate 3-6
• Based on your calculations, if I have a 30A circuit breaker, what combinations of appliances would be needed to pop the circuit breaker?
• List at least 3 combinations with 3 or more appliances
• What is the total current and total power that cause the circuit to break?
April 19, 2011HW: 6.5 PTG, pg. 641, 1-13, Due Wed
• LO
• Explain why fuses and circuit breakers are important safety devices.
• SC
• Define power, insulator, and conductor
• Use the equation P=IV
• Calcuate the power limit of a 120-V household circuit
• Differentiate between a fuse and circuit breaker
• Identify the need for the circuit breakers and fuses in a home
• Do Now:
• Explain how a circuit breaker works
• Agenda:
• Do Now
• Physics Talk
• Summary
6.5 Physics Talk
• What is Power?
• The rate at which energy is transmitted, or the energy used in a given amount of time
• Joules (pretzels) per second
• Determines the brightness of a bulb
6.5 Physics Talk
• Examples
• 1 Volt battery, 1 Amp
• 1 Joule per Coulomb
• Turned on for 30 Seconds…
• 30 Joules left the battery
• 2 Volt battery, 2 Amp current
• 2 Joules per Coulomb
• Turned on for 30 seconds
• 120 Joules left the battery
6.5 Physics Talk
• What happens to power as you increase Voltage and Current?
• What is the equation for power?
• Power will increase
• Brighter bulb!
• P=VI
• Units Watt (W) or J/s

P

I

V

6.5 Physics Talk
• What is a fuse?
• Device placed in an electrical circuit that melts when too much current (amps) flow through it. Breaks the circuit and doesn’t allow the parts of the circuit to get damaged.
• Fuses must be replaced in order to restore the circuit to normal operation
6.5 Physics Talk
• What is a circuit breaker?
• Devices place in an electrical circuit that operates like an automatic switch to open the circuit when too much current flows through.
• Can be reset when the current is reduced.
6.5 Physics Talk
• What are conductors?
• A material that current can move through easily
• Metals are good conductors
• Copper is a good conductor and used for electrical wires
• Outer electrons are loosely bonded and can be easily shared
• Very low resistance
6.5 Physics Talk
• What is an insulator?
• Do not allow electric current to flow easily
• Outer electrons are tightly bonded
• Examples: air, glass, plastic, rubber, wood
• Used to provide protection from danger amounts of current. (rubber coating on electrical wires)
• Very high resistance!
6.5 Physics Talk
• How can you blow a fuse or trip a circuit breaker?
• Devices that generate lots of heat also use lots of energy
• Devices with high power ratings (require a lot of power) are responsible for blowing a fuse or tripping a circuit breaker.
• Examples: Hair dryer, toaster, things that heat up.
6.5 Physics Talk
• What happened in our experiment?
• As we added more appliances, you increased current (amps) until the fuse burned out.
• This opened the circuit and current flow stops.
6.5 Physics Talk
• Why do we need fuses and circuit breakers?
• Safety devices
• Usually 15 A or 20 A, depending on the wires
• Electrical current generates heat
• Prevents wires from melting/starting fires
6.5 Physics Talk
• How does all this work in our houses?
• Home circuits provide 120V
• You can find the Power Rating (Watts) on most appliances
• You can then calculate Current (amps)

How does this all work in our houses ?(con’t)

• 100 W bulb is brighter than a 60 W bulb because the 100 W bulb will pull more current
• 20 W compact florescent bulb produces the same amount of light as a 100 W conventional bulb but uses less power! This saves energy and money!

What are power limits?

• Voltage of the circuits in your house are 120 V
• Fuse/circuit breaker is 15A
• That means the power limit is 120V x 15A=1800 W
• If a toaster is 1200 W and a hair dryer is 1000W, they cannot operate on the same line since they total 2200 W. They would trip the circuit

What are current limits?

• Voltage of the circuits in your house are 120 V
• Fuse/circuit breaker is 15A
• Appliances are wired in parallel, total current of the circuit will be the sum off all individual currents
• If the total current in the circuit is greater than 15, the fuse will blow/circuit will trip.
Practice Problems
• Pg. 637-638
• http://webs.rps205.com/curriculum/science/files/A3874CAB2CB74F2B9EEEF914DB22E2EA.pdf
Reflection
• Essential Questions: Pg. 640
• What does it mean?
• How do you know?
• Why should you care?
• LO and SC reflection