Chapter 6, Section 5

Electric Power: Load Limit

April 18, 2011HW: Grade Report Signed, 6.5 PTG, pg. 641, 1-13, Due Wed

Honors meeting Wed at lunch!

• LO – Explain why fuses and circuit

breakers are important safety devices.

• SC– Define power, insulator, and

conductor– Use the equation P=IV– Calculate 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

• http://its-about-time.com/physics/videos.html

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?

April 19, 2011HW: 6.5 PTG, pg. 641, 1-13, Due Wed

Honors meeting Wed at lunch!

• 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 fuse

works. Why do you think this is important?

• Agenda:– Do Now– Investigate– Physics Talk– Summary

Expectations Review• What are the expectations

for:– Being on task?– Music/Ipods?– Supply Management and

use?– Language?– Work Habits?– Group talk/Class

discussions

• You should be working on task at all times. NO Excuses!

• You may listen to music when I say it is OK during individual work. NEVER during notes or group work.

• Supplies are to be used properly and not stolen or used for any purpose other than lab. (Tape, buckets, etc.)

• Cursing and non-academic language is unacceptable. You will be given one warning and then a phone call or referral.

• You are to be working at all times. Off topic conversations are wasting our time.

• On topic, SLANT, quiet and respectful

Why?

• To be more successful• To make your life and my life easier while you

are in class• To be respectful of each other and our

equipment• So I’m not “cranky” and “mean”• So we can have more fun.

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

VI

Investigate #3-6

• Complete table in your notebookAppliance Voltage Power (Watts) Current

(Amps)

Dishwasher 120 20

Hair Dryer 120 1875

Laptop 120 50

Refrigerator 120 6

Flat Screen TV 120 1

Vacuum 120 1440

Investigate #3-5

• Create table in your logAppliance Voltage Power (Watts) Current

(Amps)

Dishwasher 120 2400 20

Hair Dryer 120 1875 15.6

Laptop 120 50 0.42

Refrigerator 120 720 6

Flat Screen TV 120 120 1

Vacuum 120 1440 12

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?

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/fi

les/A3874CAB2CB74F2B9EEEF914DB22E2EA.pdf

Reflection

• Essential Questions: Pg. 640– What does it mean?– How do you know?– Why should you care?

• LO and SC reflection