1. Today’s special HW check The Cosmos Test results Notes HW I due next
CERN sign-ups

2. A little electrical history…

3. 17 Electrical Energy and Current
Pd.2: Use diagrams to illustrate an electric field (including point charges and electric field lines).
Pd.3: Summarize current, potential difference, and resistance in terms of electrons.
Pd.6:Differentiate between alternating current (AC) and direct current (DC) in electrical circuits.
Pd.11:Predict the cost of operating an electrical device by determining the amount of electrical power and electrical energy in the circuit.

4. Be true to your work, your word, and your friend.
Henry David Thoreau
Fact: Stewardesses is the longest word typed with only the left hand.

5. Preview Section 1 Electric Potential Section 2 Capacitance
Section 3 Current and Resistance
Section 4 Electric Power

6. Electricity timeline
Click here.

7. What do you think?
You may have purchased batteries for radios, watches, CD players, and other electronic devices. Batteries come in a variety of different sizes and voltages. You probably have 1.5 volt, 3 volt, and 12 volt batteries in your home.
What do volts measure?
Is the number of volts related to the size of the battery?
How is a 3 volt battery different from a 1.5 volt battery?
When asking students to express their ideas, you might try one of the following methods.
(1) You could ask them to write their answers in their notebook and then discuss them.
(2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups.
The most important aspect of eliciting student’s ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally.
It is likely that the students do not have clear ideas about the meaning of volts. After some discussion, it might be worthwhile to point out that AAA, AA, C, and D cell batteries are all 1.5 volt if it has not already been mentioned. This may or may not help them clarify their understanding of the term “volt”.

8. Electrical Potential Energy
A uniform electric field exerts a force on a charged particle moving it from A to B.
Will the particle shown gain or lose PEelectric as it moves to the right?
Lose energy (because it is moving with the force, not against it)
Similar to a falling object losing PEg
PEelectric = Wdone = Fd = -qED
In order to help with the sign of the PEelectric,, make a comparison to the gravitational PE. Lifting an object against gravity gives it a positive change in PE. Similarly, moving a positive particle to the left (against the field) would give it a positive change in PE. Moving a negative particle to the right (against the force) would give it a positive change in PE. When a particle moves in the direction of the force, the change in PE is negative. This issue is covered again on the next slide. Electrical PE is a little more difficult than gravitational PE because of the two different types of charge.
Remind students this derivation is true only if the field is uniform, so that the force remains constant. This would be typical of the E field between two charged metal plates.

9. Electrical Potential Energy
PEelectric is positive if the charge is negative and moves with the field.
PEelectric is positive if the charge is positive and moves against the field.

10. Classroom Practice Problem
A uniform electric field strength of 1.0 x 106 N/C exists between a cloud at a height of 1.5 km and the ground. A lightning bolt transfers 25 C of charge to the ground. What is the change in PEelectric for this lightning bolt?
q= 25 C
E=1.0 x 10 6 N/C
d= 1500 m
PE electric = -(25 C)(1.0 x 10 6 N/C)(1500 m) =
-3.8 x 1010 J of energy
For problems, it is a good idea to go through the steps on the overhead projector or board so students can see the process instead of just seeing the solution. Allow students some time to work on problems and then show them the proper solutions. Do not rush through the solutions. Discuss the importance of units at every step. Problem solving is a developed skill and good examples are very helpful.
For this problem, change in PE = -qEd The charge that is transferred to the ground loses a large amount of energy that is changed into heat and light and sound during the transfer from the cloud to the ground.

11. Gravitational Potential Difference
Suppose a mass of 2.00 kg is moved from point A straight up to point B a distance of 3.00 m. Find the PEg for the mass if g = 9.81 m/s2. Repeat for a mass of 5.00 kg.
Answer: 58.9 J and 147 J
What is the PEg per kg for each?
Answer: 29.4 J/kg for both
The change per kg does not depend on the mass. It depends only on points A and B and the field strength.
There is an analogous concept for electrical potential energy, as shown on the next slide.
The calculations on this slide should help students understand the concept of potential difference and how it differs from electrical potential energy.
We never defined gravitational potential difference, but the concept is the same. With gravitational energy, it is the energy per kg, and with electrical energy, it is the energy per coulomb.
See the next slide.

12. Potential Difference
Potential difference (V) is the change in electrical potential energy per coulomb of charge between two points.
Depends on the electric field and on the initial and final positions
Does not depend on the amount of charge
SI unit: joules/coulomb (J/C) or Volts (V)

13. Potential Difference
The potential difference is calculated between two points, A and B.
The field must be uniform.
You may wish to have students derive this equation by substituting the equation for PEelectric (slide 3) into the equation for potential difference (slide 7).

14. Potential Difference Near a Point Charge
The above V determines the potential energy per coulomb at a point compared to a very distant point where V would equal zero.
Potentials are scalars (+ or -) so the total potential at a point is the sum of the potentials from each charge.

15. Batteries
A battery maintains a constant potential difference between the terminals.
1.5 V (AAA, AA, C and D cell) or 9.0 V or 12 V (car)
In 1.5 V batteries, the electrons use chemical energy to move from the positive to the negative terminal.
They gain 1.5 joules of energy per coulomb of charge
When connected to a flashlight, the electrons move through the bulb and lose 1.5 joules of energy per coulomb of charge.

16. Now what do you think?
You may have purchased batteries for radios, watches, CD players, and other electronic devices. Batteries come in a variety of different sizes and voltages. You probably have 1.5 volt, 3 volt, and 12 volt batteries in your home.
What do volts measure?
Is the number of volts related to the size of the battery?
How is a 3 volt battery different from a 1.5 volt battery?
Volts are not related to the size of a battery. Volts measure the amount of PE in each coulomb of charge (6.25 x 1018 electrons). A 3 volt battery can deliver 3 J of energy for each coulomb of charge leaving the negative side and traveling to the positive side.
The size of the battery is related to how long the battery will last before it can no longer deliver the PE to the electrons through chemical reactions.

17. What do you think?
The battery shown has a potential difference of 6.0 volts. It has just been connected to two metal plates separated by an air gap. There is no electrical connection between the two plates and air is a very poor conductor.
If a light bulb replaced the two metal plates and the battery was connected, electrons would flow out of the negative and into the positive terminal. Will this also occur with the two metal plates?
If not, why not?
If so, is the flow similar or different from that with the light bulb? Explain.
When asking students to express their ideas, you might try one of the following methods.
(1) You could ask them to write their answers in their notebook and then discuss them.
(2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups.
The most important aspect of eliciting student’s ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally.
It is likely that students’ experience with capacitors is limited, so these questions may be confusing for them. The term capacitor is not introduced yet to reduce confusion. Try getting out a light bulb and connecting it to a battery. Discuss the flow of electrons from high PE (negative) to low PE (positive). Now replace the light bulb with parallel plates (or at least have students imagine the situation), and then move on to the second bullet point.

18. Capacitors
The two metal plates are electrically neutral before the switch is closed. What will happen when the switch is closed if the left plate is connected to the negative terminal of the battery?
Electrons will flow toward lower PE.
From the battery to the left plate
From the right plate to the battery
Electrons are repelled from the negative terminal. As a result, they have high PE and flow toward the neutral plate. Eventually, as electrons build up on the plate, the PE of electrons on the plate equals that of the battery, and the flow stops. Similarly, electrons flow from the neutral right plate to a lower PE, the + terminal of the battery. Another explanation is that repulsion builds up between electrons on the left and right plates. This repulsion drives the electrons on the right plate to the battery terminal.

19. Parallel Plate Capacitors
Electrons build up on the left plate, giving it a net negative charge. The right plate has a net positive charge.
Capacitors can store charge or electrical PE.

20. Leyden Jar Demonstration

21. Capacitance Capacitance measures the ability to store charge.
SI unit: coulombs/volt (C/V) or farads (F)
In what way(s) is a capacitor like a battery?
In what way(s) is it different?
Capacitors and batteries both have one terminal with excess electrons and another with a deficiency of electrons. They both store electrical PE. In both cases, electrons will flow from high PE to low PE when the two terminals are connected with a conductor.
Capacitors, however, do not have the ability to maintain the PE difference or voltage between the terminals. Batteries use chemical energy to maintain the difference in charge (or energy) between the two terminals.

22. Capacitance
How would capacitance change if the metal plates had more surface area?
Capacitance would increase.
How would it change if they were closer together?
This equation is discussed further on the next slide.

23. Capacitance
 is a constant that is determined by the material between the plates (0 refers to a vacuum).
Combining the two equations for C yields:

24. Dielectrics The space between the plates is filled with a dielectric.
Rubber, waxed paper, air
The dielectric increases the capacitance.
The induced charge on the dielectric allows more charge to build up on the plates.
The better dielectrics are those that have charges induced on their surface more easily. This charge reduces the electric field between the plates, so more electrons can build up. The induced positive charge negates some of the repulsive force for the incoming electrons toward the top plate, so more electrons can build up on the top plate.

25. Capacitor Applications
Connecting the two plates of a charged capacitor will discharge it.
Flash attachments on cameras use a charged capacitor to produce a rapid flow of charge.
Some computer keyboards use capacitors under the keys to sense the pressure.
Pushing down on the key changes the capacitance, and circuits sense the change.

26. Energy and Capacitors
As the charge builds, it requires more and more work to add electrons to the plate due to the electrical repulsion.
The average work or PE stored in the capacitor is (1/2)QV.
Derive equivalent equations for PEelectric by substituting:
Q = CV and V = Q/C
The equivalent equations for PEelectric are:
PEelectric = 1/2C(V)2
PEelectric = Q2/2C

27. Classroom Practice Problem
A 225 F is capacitor connected to a 6.00 V battery and charged. How much charge is stored on the capacitor? How much electrical potential energy is stored on the capacitor?
Answers: 1.35 x 10-3 C , x 10-3 J
For problems, it is a good idea to go through the steps on the overhead projector or board so students can see the process instead of just seeing the solution. Allow students some time to work on problems and then show them the proper solutions. Do not rush through the solutions. Discuss the importance of units at every step. Problem solving is a developed skill and good examples are very helpful.
These solutions are straightforward applications of the equations. Some of the units are new and unfamiliar to students, and this may present some problems.

28. Now what do you think?
If a light bulb replaced the two metal plates and the battery was connected, electrons would flow out of the negative and into the positive terminal. Will this also occur with the two metal plates?
If not, why not?
If so, is the flow similar or different from that with the light bulb? Explain.
Electrons will flow due to PE differences between the battery and the metal plates. The flow will begin normally but, as charge builds on the plates, the flow will gradually slow down and eventually stop. As electrons build on the left plate, they repel the incoming electrons until the force is great enough to stop the flow. As electrons leave the right plate, the remaining positive charge attracts the electrons as they try to leave the plate. Eventually, the force is great enough to stop the outward flow of electrons.
With a light bulb, the flow would be constant until the chemicals in the battery are depleted.

29. Today’s special HW check; Q & A Saturday is SAT practice test day!
Engineering for oreo’s
Lab: Ohm’s Law V = I*R
Lab due next time!

30. Today’s special Saturday is SAT practice test day!
Engineering for oreo’s
Lab: Ohm’s Law V = I*R
Continue lab with partners
Lab due next time!

31. Today’s special Turn in labs now! Physics of the long jump Notes II
HW II due next time
SAT question of the day

32. What do you think?
The term resistance is often used when describing components of electric circuits.
What behavior of the components does this term describe?
Do conductors have resistance?
If so, are all conductors the same? Explain.
What effect would increasing or decreasing the resistance in a circuit have on the circuit?
When asking students to express their ideas, you might try one of the following methods.
(1) You could ask them to write their answers in their notebook and then discuss them.
(2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups.
The most important aspect of eliciting students’ ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally.
You may wish to build a simple circuit with a battery or two, a bulb, and a switch with connecting wires. You could use this circuit as a focal point for the discussion of resistance. Ask students to think about what resistance might mean and how it would apply to the different components they see (bulb, wire, insulator on the wire, switch, and so on).

34. Electric Current
Electric current (I) is rate at which charges flow through an area.
SI unit: coulombs/second (C/s) or amperes (A)
1 A = 6.25  1018 electrons/second

35. Conventional Current
Conventional current (I) is defined as the flow of positive charge.
The flow of negative charge as shown would be equivalent to an equal amount of positive charge in the opposite direction.
In conducting wires, I is opposite the direction of electron flow.
Current flow in wires is opposite the direction of electron flow. Problem solving is the same with either convention. Stress this fact to students and teach them to label current as the flow of positive charge even though it is really electrons that are flowing though the wires.

37. Velocity of Electrons Through Wires
When you turn on a wall switch for a light, electrons flow through the bulb. Which speed below do you believe most closely approximates that of the electrons?
The speed of light ( m/s)
1 000 m/s
10 m/s
m/s
Why do you think so?
Students may believe that the electrons must travel at high speeds to get from the switch to the bulb. You might ask them if the wire is already “filled” with electrons or if the electrons lighting the bulb are those that are “waiting” at the switch for it to close.

38. Drift Velocity Electrons undergo collisions with atoms in the metal.
They “drift” through the wire.
Drift velocity for a copper wire with a current of 10 A is m/s.
The E field moves through the wire near the speed of light, causing all electrons in the wire to move nearly instantly.
The electrons in a light bulb begin moving as soon as they experience the force of the electric field.

39. Resistance to Current Resistance is opposition to the flow of charge.
SI unit: volts/ampere (V/A) or ohms ()
Ohm’s Law : V = IR
Valid only for certain materials whose resistance is constant over a wide range of potential differences
Ohm’s law does not apply to all materials. For example, when using a tungsten filament flashlight bulb, the current does not double when the potential difference doubles. The resistance of the filament changes when more voltage is applied and more current flows though it.

40. Classroom Practice Problems
A typical 100 W light bulb has a current of A. How much charge flows through the bulb filament in 1.0 h? How many electrons would flow through in the same time period?
Answers: 3.0  103 C, 1.9  1022 electrons
This same 100 watt bulb is connected across a 120 V potential difference. Find the resistance of the bulb.
Answer: 1.4  102 
For problems, it is a good idea to go through the steps on the overhead projector or board so students can see the process instead of just seeing the solution. Allow students some time to work on problems and then show them the proper solutions. Do not rush through the solutions. Discuss the importance of units at every step. Problem solving is a developed skill and good examples are very helpful.
Q = Current x time = 0.83 C/s x (3600 s) = 2988 C = 3.0 x 103 C
(3.0 x 103 C) x (6.25 x 1018 electrons/C) = 1.9 x 1022 electrons
R = V/I = 120 V / 0.83 A =  = 1.4 x 102 

41. Resistance of a Wire
On the next slide, predict the change necessary to increase the resistance of a piece of wire with respect to:
Length of wire
Cross sectional area or thickness of the wire
Type of wire
Temperature of the wire
When showing the next slide, the answers will be revealed one at a time with each mouse click.

42. Ask students which types of wire has the least resistance.
Silver has the least resistance. Copper, gold and aluminum also have low values for resistivity.

43. Applications Resistors in a circuit can change the current.
Variable resistors (potentiometers) are used in dimmer switches and volume controls.
Resistors on circuit boards control the current to components.
The human body’s resistance ranges from  (dry) to 100  (soaked with salt water).
Currents under 0.01 A cause tingling.
Currents greater than 0.15 A disrupt the heart’s electrical activity.

44. Now what do you think?
The term resistance is often used when describing components of electric circuits.
What behavior of the components does this term describe?
Do conductors have resistance?
If so, are all conductors the same? Explain.
What effect would increasing or decreasing the resistance in a circuit have on the circuit?
Resistance is opposition to the flow of charge.
Conducting wires do have a very small resistance. This would be a good time to review how the resistance of wires depends on length, thickness, temperature, and type of wire.
Non-conductors have much greater resistance to the flow of electrons.
Adding resistance to a circuit will decrease the flow of current. Resistance is used to control the flow of current.

45. What do you think?
Hair dryers, microwaves, stereos, and other appliances use electric power when plugged into your outlets.
What is electric power?
Is electric power the same as the power discussed in the chapter “Work and Energy?”
Do the utility companies bill your household for power, current, potential difference, energy, or something else?
What do you think is meant by the terms alternating current (AC) and direct current (DC)?
Which do you have in your home?
When asking students to express their ideas, you might try one of the following methods.
(1) You could ask them to write their answers in their notebook and then discuss them.
(2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups.
The most important aspect of eliciting students’ ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally.
There are several different questions posed on this slide. Ask students to explain their answers as well as they can based on experiences or prior knowledge about electricity.

46. Types of Current - Direct
Batteries use chemical energy to give electrons potential energy.
Chemical energy is eventually depleted.
Electrons always flow in one direction.
Called direct current (DC)

47. Types of Current - Alternating
Generators change mechanical energy into electrical energy.
Falling water or moving steam
Electrons vibrate back and forth.
Terminals switch signs 60 times per second (60 Hz).
Called alternating current (AC)
AC is better for transferring electrical energy to your home.
Electron flow switches so rapidly that we do not see light bulbs flicker as the current passes though zero twice each cycle.

48. Energy Transfer
Is the electrical potential energy gained, lost, or unchanged as the electrons flow through the following portions of the circuit shown:
A to B
B to C
C to D
D to A
Explain your answers.
After students consider the different segments of the circuit, show the graph of PE on the next slide, followed by the Visual Concept on slide 6 to reinforce the concept.
Make sure they explain why they believe energy is gained or lost in the wires, bulb and battery.

49. Energy Transfer A to B (unchanged) B to C (lost in bulb)
C to D (unchanged)
D to A (gained in battery)
See video on next slide.

50. Electric Power
Power is the rate of energy consumption (PE/t ). For electric power, this is equivalent to the equation shown below.
SI unit: joules/second (J/S) or watts (W)
Current (I) is measured in amperes (C/s).
Potential difference (V) is measured in volts (J/C).
Substitute using Ohm’s law (V = IR) to write two other equations for electric power.
The Student Edition shows how the equation given here is derived from the definition of power as W/t or PE/t. You may wish to challenge students to work from the definition to derive the equation shown on the slide. This will reinforce the concept that we are using the same definition of power as that used in the chapter “Work and Energy.”
For the last bullet point, students should come up with P = I2R and P = V2/R with simple substitutions.

51. Classroom Practice Problems
A toaster is connected across a 120 V kitchen outlet. The power rating of the toaster is 925 W.
What current flows through the toaster?
What is the resistance of the toaster?
How much energy is consumed in 75.0 s?
Answers: 7.7 A, 16 , 6.94  104 J
For problems, it is a good idea to go through the steps on the overhead projector or board so students can see the process instead of just seeing the solution. Allow students some time to work on problems and then show them the proper solutions. Do not rush through the solutions. Discuss the importance of units at every step. Problem solving is a developed skill and good examples are very helpful.
I = P/V = 925 W / 120 V = 7.7 A
R = V/I = 120 V/ 7.7 A = 16 
Energy = Pt = (925 W)(75.0 s) = 6.94  104 J

52. Household Energy Consumption
Power companies charge for energy, not power.
Energy consumption is measured in kilowatt•hours ( kw•h).
The joule is too small.
A kw•h is one kilowatt of power for one hour.
Examples of 1 kw•h:
10 light bulbs of 100 W each on for 1 h
1 light bulb of 100 W on for 10 h
1 kw•hr = J or 3.6 x 106 J
Find the cost of 1 kw•h from your power company. It is likely to be a around $0.10. This provides an opportunity to discuss the cost of leaving appliances turned on for extended periods of time.

53. Electrical Energy Transfer
Transfer of energy from power plants to your neighborhood must be done at high voltage and low current.
Power lost in electrical lines is significant.
P = I2R
Power lines are good conductors but they are very long.
Since power companies can’t control the resistance (R), they control the current (I) by transferring at high voltage.
Alternating current can be changed from low voltage/high current and back using transformers. This allows transmission over large distances at high voltage/low current to reduce power loss. Close to its destination, the energy is converted back to low voltage/high current.

54. Now what do you think?
Hair dryers, microwaves, stereos, and other appliances use electric power when plugged into your outlets.
What is electric power?
Is electric power the same as the power discussed in the chapter “Work and Energy?”
Do the utility companies bill your household for power, current, potential difference, energy, or something else?
What do you think is meant by the terms alternating current (AC) and direct current (DC)?
Which do you have in your home?
Power is the rate of energy consumption, just as we defined it in the chapter “Work and Energy” and with the same units (J/s or W).
Utilities do not bill homes for power but instead bill for the total energy used each month. Power companies use units of kw•h (1 kw•h = 3,600,000 J).
With DC, electrons always flow in one direction. With AC, electrons vibrate back and forth. AC is used in homes, and DC is used in batteries.

55. Today’s Special HW Check; Q & A SAT QOTD 5 fun physics phenomena
Concept map
Buddy review
5 fun physics phenomena explained
Test next time!!!

56. Today’s special Test 17 Enter MC answers in Smart response
Get Vocab sheet & Lab when finished.