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

• 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?

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

Vaccuum 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 120 725 6

Flat Screen TV 120 120 1

Vaccuum 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?

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/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