1. Electric Power: Load Limit
Chapter 6, Section 5
Electric Power: Load Limit

2. April 18, 2011 HW: 6.5 PTG, pg. 641, 1-13, Due WedLO
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

3. Investigate
#1-complete set up as shown. Answer #1a in your notebook

4. Investigate #2-6 Teacher Demo

5. 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?

6. 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

7. Investigate #3-6 Complete table in your notebook Appliance Voltage
Power (Watts)
Current (Amps)
Dishwasher
120 20
Hair Dryer
1875
Laptop 50
Refrigerator 6
Flat Screen TV 1
Vaccuum
1440

8. Investigate #3-5 Create table in your log Appliance Voltage
Power (Watts)
Current (Amps)
Dishwasher
120
2400 20
Hair Dryer
1875
15.6
Laptop 50
0.42
Refrigerator
725 6
Flat Screen TV 1
Vaccuum
1440 12

9. 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?

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

11. 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

12. 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

13. 6.5 Physics Talk Power will increase Brighter bulb! P=VI
Units Watt (W) or J/s
What happens to power as you increase Voltage and Current?
What is the equation for power? P I V

14. 6.5 Physics Talk
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
What is a fuse?

15. 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.

16. 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

17. 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!

18. 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.

19. 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.

20. 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

21. 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)

22. 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!

23. 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

24. 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.

25. Practice Problems Pg

26. Reflection Essential Questions: Pg. 640 LO and SC reflection
What does it mean?
How do you know?
Why should you care?
LO and SC reflection