Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student...
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Transcript of Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student...
![Page 1: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/1.jpg)
Chapter 18
Electric Currents
![Page 2: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/2.jpg)
Objectives
After studying the material of this chapter, the student should be able to:
Use the equations for electric power to determine the power and energy dissipated in a resistor and calculate the cost of this energy to the consumer.
![Page 3: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/3.jpg)
Remember:
P= W / t (Power=Work/time)
• Remember: W= VQ and I = Q/t
So: P= I V
![Page 4: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/4.jpg)
18.5 Electric Power
Power, as in kinematics, is the energy transformed by a device per unit time:
(18-5)
Hewitt – Electric Power
![Page 5: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/5.jpg)
Electric Power, P= I V
Known as Joule’s Law
P: is the power consumed by a resistor, R.
Unit: Joule/s= Watt
![Page 6: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/6.jpg)
Derive the following using P=IV and Ohm’s Law:
P=V2/R
P=I2R
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18.5 Electric Power
The unit of power is the watt, W.
For ohmic devices, we can make the substitutions:
(18-6a)
(18-6b)
![Page 8: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/8.jpg)
Problem 1
• The current through a car motor is 150A. The battery used is 12V. How much energy is supplied by the battery in 5s?
![Page 9: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/9.jpg)
Problem 1
• The current through a car motor is 150A. The battery used is 12V. How much energy is supplied by the battery in 5s?
Answer: 9,000J
![Page 10: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/10.jpg)
kWh
kiloWatt hour
What does the kWh measure, a) Energy or b) Power ?
![Page 11: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/11.jpg)
18.5 Electric Power
What you pay for on your electric bill is not power, but energy – the power consumption multiplied by the time.
We have been measuring energy in joules, but the electric company measures it in kilowatt-hours, kWh.
![Page 12: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/12.jpg)
Problem 2A small desktop radio has a resistance of 8,000 Ω. The voltage is 120V.
a)How much current does it draw?
b)How much power does it use?
c)How much does it cost to run the radio for 12 hours, if 1kWh costs $0.15?
![Page 13: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/13.jpg)
Problem 2A small desktop radio has a resistance of 8,000 Ω. The voltage is 120V.
a)How much current does it draw?
b)How much power does it use?
c)How much does it cost to run the radio for 12 hours, if 1kWh costs $0.15?
Answers:
a)0.015A, b)1.8W, c) $0.00324
![Page 14: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/14.jpg)
18.6 Power in Household Circuits
The wires used in homes to carry electricity have very low resistance. However, if the current is high enough, the power will increase and the wires can become hot enough to start a fire.
To avoid this, we use fuses or circuit breakers, which disconnect when the current goes above a predetermined value.
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Household CircuitsHousehold circuits are:
parallel circuits Typically designed to carry 15 Amperes of
current - maximum.
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Overloading CircuitsSince adding resistance to a parallel
circuit causes current to increase, adding appliances to a household circuit causes current in the house wiring to increase.
This means wires must dissipate more heat energy (P = I2R).
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Short CircuitsA short circuit is a very low resistance
path in a circuit.A short circuit will also cause large
amounts of current to flow in a circuit and overload the circuit.
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Fuses & Circuit BreakersFuses and circuit breakers act as
automatic switches in a circuit to prevent dangerous currents from flowing.
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18.6 Power in Household CircuitsFuses are one-use items – if they blow, the fuse is destroyed and must be replaced.
When the currentexceeds a certainvalue, the metallicribbon melts and the circuit opens.then the fuse mustbe replaced.
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18.6 Power in Household Circuits
Circuit breakers, which are now much more common in homes than they once were, are switches that will open if the current is too high; they can then be reset.
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18-6 Power in Household Circuits
Example 18-11: Will a fuse blow?
Determine the total current drawn by all the devices in the circuit shown.
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Example 18-11: Will a fuse blow?
Determine the total current drawn by all the devices in the circuit shown.
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20-A fuse or 30-A fuseIf the circuit is designed for a 20-A fuse, what willhappen?Hopefully the fuse will blow. What if it does not?It can overload the wires, which will get hot and canstart a fire.
What do you do to prevent this from happening?Move one of the devices to another circuit.
If the circuit is designed for a 30-A fuse, what will happen?
If it has a heavier wire and a 30-A fuse, the fuse should not blow.If it does, a short could be the problem, which could be in cord.
![Page 24: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/24.jpg)
18-6 Power in Household Circuits
Conceptual Example E: A dangerous extension cord.
Your 1800-W portable electric heater is too far from your desk to warm your feet. Its cord is too short, so you plug it into an extension cord rated at 11 A. Why is this dangerous?
![Page 25: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/25.jpg)
18-6 Power in Household Circuits
Conceptual Example E: A dangerous extension cord.
Your 1800-W portable electric heater is too far from your desk to warm your feet. Its cord is too short, so you plug it into an extension cord rated at 11 A. Why is this dangerous?
Solution: An 1800-W heater operating at 120 V draws 15 A of current, which exceeds the rating of the extension cord. This creates a risk of overheating and fire.
![Page 26: Chapter 18 Electric Currents. Objectives After studying the material of this chapter, the student should be able to: Use the equations for electric power.](https://reader036.fdocuments.us/reader036/viewer/2022062408/56649ea45503460f94ba8a2a/html5/thumbnails/26.jpg)
Homework
Chapter 18 Problems: 26, 28, 29, 31, 33
Inside a Flashlight (optional)
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Closure
Kahoot 18-5 and 18-6