1. Chapter 17 Table of Contents Section 1 Electric Potential
Electrical Energy and Current
Table of Contents
Section 1 Electric Potential
Section 2 Capacitance
Section 3 Current and Resistance
Section 4 Electric Power

2. Objectives: Section 1 Electric Potential
Chapter 17
Objectives: Section 1 Electric Potential
Distinguish between electrical potential energy, electric potential, and potential difference.
Solve problems involving electrical energy and potential difference.
Describe the energy conversions that occur in a battery.

3. Chapter 17 Electrical Potential Section 1 Electric Potential
Work is done to separate charges, just as work is done to lift an object.
Separated charges possess potential energy, as does an object elevated above a reference point.
Bodies with different concentrations of electrons have a difference in potential between them and are a source of potential energy.
The difference in potential depends on the concentration of electrons and is measured in volts.
Flow of electrons
+ +
- -
Flow of electrons -
- -
Flow of electrons
+ + +

4. Electrical Potential Energy
Section 1 Electric Potential
Chapter 17
Electrical Potential Energy
Electrical potential energy is potential energy associated with a charge due to its position in an electric field.
Electrical potential energy is a component of mechanical energy.
ME = KE + PEgrav + PEelastic + PEelectric
PEelectric = –qEd
electrical potential energy = – (charge)  (electric field strength)  (displacement from the reference point in the direction of the field)
(-) sign indicates that the PE increases for a negative charge.

5. Electrical Potential Energy
Section 1 Electric Potential
Chapter 17
Electrical Potential Energy

6. Chapter 17 Electric Potential Section 1 Electric Potential
Electric Potential equals the work that must be performed against electric forces to move a charge from a reference point to the point in question, divided by the charge.
The electric potential associated with a charge is the electric potential energy divided by the charge:

7. Chapter 17 Electrical Potential Section 1 Electric Potential
PEelectric = –qEd

8. Electric Potential Difference
Section 1 Electric Potential
Chapter 17
Electric Potential Difference
Potential Difference equals the work that must be performed against electric forces to move a charge between the two points in question, divided by the charge.
Potential difference is a change in electric potential.
J/C or volts (V)

9. Electric Potential Difference
Section 1 Electric Potential
Chapter 17
Electric Potential Difference
Units for potential difference are J/C or volts (V).
A 1.5 V battery transfers 1.5 J energy/C of charge.
Every coulomb of charge that moves will do 1.5 J of work.
The D cell battery has more coulombs of charge with which to do work so it will last longer.
In a 12 V car battery every coulomb will do 12 J of work.
In a 120 V electrical outlet every coulomb will do 120 J of work.

10. Section 1 Electric Potential
Chapter 17
Potential Difference

11. Potential Difference, continued
Section 1 Electric Potential
Chapter 17
Potential Difference, continued
The potential difference in a uniform field varies with the displacement from a reference point. Displacement is parallel to the field.
Potential Difference in a Uniform Electric Field
∆V = –Ed
potential difference = –(magnitude of the electric field  displacement)

12. Chapter 17 Sample Problem Potential Energy and Potential Difference
Section 1 Electric Potential
Chapter 17
Sample Problem
Potential Energy and Potential Difference
A charge moves a distance of 2.0 cm in the direction of a uniform electric field whose magnitude is 215 N/C.As the charge moves, its electrical potential energy decreases by 6.9  J. Find the charge on the moving particle. What is the potential difference between the two locations?

13. Sample Problem, continued
Section 1 Electric Potential
Chapter 17
Sample Problem, continued
Potential Energy and Potential Difference
Given:
∆PEelectric = –6.9  10–19 J
d = m
E = 215 N/C
Unknown:
q = ?
∆V = ?
Know:
PEelectric = –qEd
∆V = –Ed

14. Sample Problem, continued
Section 1 Electric Potential
Chapter 17
Sample Problem, continued
Potential Energy and Potential Difference
Use the equation for the change in electrical potential energy.
PEelectric = –qEd
Rearrange to solve for q, and insert values.

15. Sample Problem, continued
Section 1 Electric Potential
Chapter 17
Sample Problem, continued
Potential Energy and Potential Difference
The potential difference is the magnitude of E times the displacement.

16. Chapter 17 Batteries Section 1 Electric Potential
Batteries do work to move charges.
In a voltaic pile, electrons move from one metal to the other through an electrolyte (ions in solution). The ions react with the metals, causing an electrochemical reaction, that transfers electrons.
The two types of metals in a voltaic pile are called electrodes. One metal reacts more strongly than the other, which leaves an electrical potential difference (voltage) between the two types of metals. One metal becomes positively charged (the positive electrode) and the other becomes negatively charged (the negative electrode). This causes electrons to move.

17. Chapter 17 Batteries Section 1 Electric Potential
Zinc-carbon battery - Also known as a standard carbon battery, zinc-carbon chemistry is used in all inexpensive AA, C and D dry-cell batteries. The electrodes are zinc and carbon, with an acidic paste between them that serves as the electrolyte.
Alkaline battery - Alkaline chemistry is used in common Duracell and Energizer batteries, the electrodes are zinc and manganese-oxide, with an alkaline electrolyte.
Lithium-iodide battery - Lithium-iodide chemistry is used in pacemakers and hearing aides because of their long life.
Lead-acid battery - Lead-acid chemistry is used in automobiles, the electrodes are made of lead and lead-oxide with a strong acidic electrolyte (rechargeable).
Nickel-cadmium battery - The electrodes are nickel-hydroxide and cadmium, with potassium-hydroxide as the electrolyte (rechargeable).
Nickel-metal hydride battery - This battery is rapidly replacing nickel-cadmium because it does not suffer from the memory effect that nickel-cadmiums do (rechargeable).
Lithium-ion battery - With a very good power-to-weight ratio, this is often found in high-end laptop computers and cell phones (rechargeable).
Zinc-air battery - This battery is lightweight and rechargeable.
Zinc-mercury oxide battery - This is often used in hearing-aids.
Silver-zinc battery - This is used in aeronautical applications because the power-to-weight ratio is good.

18. Objectives: Section 2 Capacitance
Chapter 17
Objectives: Section 2 Capacitance
Relate capacitance to the storage of electrical potential energy in the form of separated charges.
Calculate the capacitance of various devices.
Calculate the energy stored in a capacitor.

19. Capacitors and Charge Storage
Section 2 Capacitance
Chapter 17
Capacitors and Charge Storage
A capacitor is a device that is used to store electrical potential energy.
Capacitance is the ability of a conductor to store energy in the form of electrically separated charges.
The SI units for capacitance is the farad, F, which equals a coulomb per volt (C/V)
Typical capacitances: 1mF to 1pF

20. Capacitors and Charge Storage
Section 2 Capacitance
Chapter 17
Capacitors and Charge Storage
Capacitance is the ratio of charge to potential difference.
Capacitance depends on the size and shape of a capacitor.
Capacitance for a Parallel-Plate Capacitor in a Vacuum

21. Section 2 Capacitance
Chapter 17
Capacitance

22. Capacitors and Charge Storage
Section 2 Capacitance
Chapter 17
Capacitors and Charge Storage
The material between a capacitor’s plates can change its capacitance.
The effect of a dielectric is to reduce the strength of the electric field in a capacitor. A dielectric is an insulating material.
Earth has an extremely large capacitance.

23. Capacitors in Keyboards
Section 2 Capacitance
Chapter 17
Capacitors in Keyboards

24. Parallel-Plate Capacitor
Section 2 Capacitance
Chapter 17
Parallel-Plate Capacitor

25. Chapter 17 Energy and Capacitors
Section 2 Capacitance
Chapter 17
Energy and Capacitors
The potential energy stored in a charged capacitor depends on the charge and the potential difference between the capacitor’s two plates.

26. Chapter 17 Sample Problem Capacitance
Section 2 Capacitance
Chapter 17
Sample Problem
Capacitance
A capacitor, connected to a 12 V battery, holds 36 µC of charge on each plate. What is the capacitance of the capacitor? How much electrical potential energy is stored in the capacitor?
Given:
Q = 36 µC = 3.6  10–5 C
∆V = 12 V
Unknown:
C = ? PEelectric = ?

27. Sample Problem, continued
Section 2 Capacitance
Chapter 17
Sample Problem, continued
Capacitance
To determine the capacitance, use the definition of capacitance.

28. Sample Problem, continued
Section 2 Capacitance
Chapter 17
Sample Problem, continued
Capacitance
To determine the potential energy, use the alternative form of the equation for the potential energy of a charged capacitor:

29. Objectives Section 3 Current and Resistance
Chapter 17
Objectives Section 3 Current and Resistance
Describe the basic properties of electric current, and solve problems relating current, charge, and time.
Calculate resistance, current, and potential difference by using the definition of resistance.
Distinguish between ohmic and non-ohmic materials, and learn what factors affect resistance.

30. Current and Charge Movement
Section 3 Current and Resistance
Chapter 17
Current and Charge Movement
Electric current is the rate at which electric charges pass through a given area.
Current is the movement of electric charges.
The large copper wire easily allows electrons to move and transfer charge so the leaves quickly collapse.
The thin nichrome wire has fewer free electrons to easily transfer charge so the leaves gradually collapse.

31. Current and Charge Movement
Section 3 Current and Resistance
Chapter 17
Current and Charge Movement
Electric current is the rate at which electric charges pass through a given area.
Current is the movement of electric charges.
Unit: Ampere (A); 1 A = 1 C/s
By convention, the direction of current is opposite the movement of (-) charges.

32. Current and Charge Movement
Section 3 Current and Resistance
Chapter 17
Current and Charge Movement
Charge carriers can be (+), (-), or both.
Most common charge carriers are electrons in metals (copper wire) or ions in solutions (electrolytes).
Current associated with an electron’s change in direction or change in electric field is propagated almost immediately so that when you flip a switch the device responds immediately. Electrons do not move from one part of the conductor (wire) to the device that quickly. It is the change in the electric field that moves quickly.

33. Section 3 Current and Resistance
Chapter 17
Conventional Current

34. Chapter 17 Drift Velocity
Section 3 Current and Resistance
Chapter 17
Drift Velocity
Drift velocity is the the net velocity of a charge carrier moving in an electric field.
Drift speeds are relatively small because of the many collisions that occur when an electron moves through a conductor.

35. Section 3 Current and Resistance
Chapter 17
Drift Velocity

36. Ohm’s Law: DV = IR Chapter 17 Resistance to Current
Section 3 Current and Resistance
Chapter 17
Resistance to Current
Resistance is the opposition to electric current by a material or device.
Resistance is the opposition to the motion of electrons.
The SI units for resistance is the ohm (Ω) and is equal to one volt per ampere.
Resistance
Ohm’s Law: DV = IR

37. Chapter 17 Resistance to Current Section 3 Current and Resistance
For many materials resistance is constant over a range of potential differences. These materials obey Ohm’s Law and are called ohmic materials.
Ohm’s low does not hold for all materials. Such materials are called non-ohmic. Diodes are non-ohmic semiconducting devices used in circuits to control the direction of current by providing low resistance to flow in one direction and high resistance to flow in the other direction.
Resistance depends on length, cross-sectional area, temperature, and material.

38. Factors that Affect Resistance
Section 3 Current and Resistance
Chapter 17
Factors that Affect Resistance

39. Chapter 17 Resistance to Current Section 3 Current and Resistance
Resistors are used to control the amount of current in a conductor.
Human Body:
Salt water and perspiration lower the body's resistance.
Rdry ~ W; R salt H2O ~ 100 W
0.01 A current is imperceptible to causing a slight tingle.
0.15 A current can be fatal.
GVR (galvanic skin response) sensors are used in lie detectors.
Potentiometers have variable resistance.

40. Objectives Section 4 Electric Power
Chapter 17
Objectives Section 4 Electric Power
Differentiate between direct current and alternating current.
Relate electric power to the rate at which electrical energy is converted to other forms of energy.
Calculate electric power and the cost of running electrical appliances.

41. Sources and Types of Current
Section 4 Electric Power
Chapter 17
Sources and Types of Current
Batteries and generators supply energy to charge carriers.
Batteries – chemical
Generators – mechanical (A wire loop is rotated in a magnetic field.)
Wisconsin Valley Improvement Company - How Generators Work
Current can be direct or alternating.
In direct current, charges move in a single direction.
In alternating current, the direction of charge movement continually alternates.
60 Hz supplied to homes

42. Chapter 17 Energy Transfer Section 4 Electric Power BatteriesD
Batteries
A to B no change in potential energy
B to C the potential energy drops to zero across as it is converted to light and heat
C to D no change in potential energy
D to A the battery does work and increases the PEelectrical by QDV where DV is the potential difference across the two terminals. B C A D B C
Energy

43. Electric power = current  potential difference
Section 4 Electric Power
Chapter 17
Energy Transfer
Electric power:
rate of conversion of electrical energy (J/s) to other forms of energy
rate at which charge carriers do work
rate at which PEelectric is converted to other forms of energy
Electric power P = I∆V (C/s)(J/C)
Electric power = current  potential difference
1W = 1J/s (One joule of electrical energy is converted to another form of energy in one second.)
P = W/t and W=DPE
DV = DPE/q so DPE = qDV
P = qDV/t and q/t = I
P = IDV

44. P = IDV Chapter 17 Energy Transfer Section 4 Electric Power
P = W/t and W=DPE
DV = DPE/q so DPE = qDV
P = qDV/t and q/t = I
P = IDV
P = IDV

45. Section 4 Electric Power
Chapter 17
Energy Transfer

46. Relating Kilowatt-Hours to Joules
Section 4 Electric Power
Chapter 17
Relating Kilowatt-Hours to Joules

47. Chapter 17 Energy Transfer Section 4 Electric Power
Power dissipated by a resistor (V= IR)
I2R is called joule heating or I2R loss.
Eloss= Pt = I2Rt (Loss due to resistance in the long wires.)
Electrical energy is transferred at high potential differences to minimize energy loss. P = IDV so for same power use high DV and small I.
Transformers: V stepped down to 4000 V stepped down to 120 V

48. Chapter 17 Energy Consumption Section 4 Electric Power
Electric companies measure energy consumption in kilowatt-hours.
1kW-h = 1000W-h = 1000J/s x h
= 1000J/s x h x 3600s/h = J
How much does it cost to watch an entire World Series (21 h) on a 90.0 W black and white television set? My electric bill charged me $ for 2201 kW-h last month. (Use that information.)

49. Chapter 17 Energy Consumption Section 4 Electric Power
How much does it cost to watch an entire World Series (21 h) on a 90.0 W black and white television set? My electric bill charged me $ for 2201 kW-h last month. (Use that information.)
$215.5/2201 kW–h = $0.10/kW-h
21 h x 3600 s/h x 90 J/s = J
1 kW-h/ J x J = 1.89 kW-h
1.89 kW-h x 0.10/ kW–h = $ 0.19

50. Energy Transfer, continued
Section 4 Electric Power
Chapter 17
Energy Transfer, continued
Electric companies measure energy consumed in kilowatt-hours. 1kW-h = J

51. Chapter 17 Multiple Choice Standardized Test Prep
1. What changes would take place if the electron moved from point A to point B in the uniform electric field?
A. The electron’s electrical potential energy would increase; its electric potential would increase.
B. The electron’s electrical potential energy would increase; its electric potential would decrease.
C. The electron’s electrical potential energy would decrease; its electric potential would decrease.
D. Neither the electron’s electrical potential energy nor its electric potential would change.

52. Section 2 Capacitance
Chapter 17
Charging a Capacitor

53. A Capacitor With a Dielectric
Section 2 Capacitance
Chapter 17
A Capacitor With a Dielectric

54. Factors That Affect Resistance
Section 2 Capacitance
Chapter 17
Factors That Affect Resistance