Phy Outline Ch17

8/3/2019 Phy Outline Ch17

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Preview

• Objectives

• Electrical Potential Energy • Potential Difference

• Sample Problem

Chapter 17 Section 1 Electric Potential



Section 1 Electric Potential Chapter 17

Objectives

• Distinguish between electrical potential energy,electric potential, and potential difference.

• Solve problems involving electrical energy andpotential difference.

• Describe the energy conversions that occur in a

battery.




Electrical Potential Energy

• Electrical potential energy is potential energyassociated with a charge due to its position in anelectric field.

• Electrical potential energy is a component ofmechanical energy.

ME = KE + PE grav + PE elastic + PE electric




Electrical Potential Energy, continued

• Electrical potential energy can be associated with acharge in a uniform field.

• Electrical Potential Energy in a Uniform Electric Field

PE electric = –qEd electrical potential energy = –(charge) (electric field strength) (displacement from the reference point in the direction of the field)



Click below to watch the Visual Concept.

Visual Concept


Electrical Potential Energy

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http://70675.html/




Potential Difference

• Electric Potential equals the work that must beperformed against electric forces to move a chargefrom a reference point to the point in question,divided by the charge.

• The electric potential associated with a charge is theelectric energy divided by the charge:

V PE electric

q




Potential Difference, continued

• Potential Difference equals the work that must beperformed against electric forces to move a chargebetween the two points in question, divided by thecharge.

• Potential difference is a change in electric potential.

change in electric potential energy

potential differenceelectric charge

electric PE V

q




Visual Concept


Potential Difference

http://70676.html/

http://70676.html/





• The potential difference in a uniform field varies withthe displacement from a reference point.

• Potential Difference in a Uniform Electric Field∆V = –Ed

potential difference = –(magnitude of the electricfield displacement)




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 10 -19 J. Find the charge on the moving particle. What is the potential difference between the two locations?




Sample Problem, continued


Given:

∆PE electric = –6.9 10 –19 J

d = 0.020 mE = 215 N/C

Unknown:

q = ?

∆V = ?






Use the equation for the change in electrical potentialenergy.

PE electric = –qEd

Rearrange to solve for q, and insert values.

–19

–19

(–6.9 10 J) – –

(215 N/C)(0.020 m)

1.6 10 C

electric PE

q Ed

q






The potential difference is the magnitude of E timesthe displacement.

– –(215 N/C)(0.020 m)

–4.3 V

V Ed

V





• At right, the electric poten-tial at point A depends onthe charge at point B and

the distance r.

• An electric potential existsat some point in an electric

field regardless of whetherthere is a charge at thatpoint.





• The reference point for potential difference near apoint charge is often at infinity.

• Potential Difference Between a Point at Infinity and a

Point Near a Point Charge

• The superposition principle can be used to calculatethe electric potential for a group of charges.

value of the point chargepotential difference = Coulomb constant

distance to the point charge

C

q V k

r




Visual Concept


Superposition Principle and Electric Potential

http://70543.html/

http://70543.html/



Preview

• Objectives

• Capacitors and Charge Storage • Energy and Capacitors

• Sample Problem

Chapter 17 Section 2 Capacitance



Section 2 Capacitance Chapter 17

Objectives

• Relate capacitance to the storage of electricalpotential energy in the form of separated charges.

• Calculate the capacitance of various devices.

• Calculate the energy stored in a capacitor.




Capacitors and Charge Storage

• A capacitor is a device that is used to store electricalpotential energy.

• Capacitance is the ability of a conductor to storeenergy in the form of electrically separated charges.

• The SI units for capacitance is the farad, F, which

equals a coulomb per volt (C/V)




Capacitors and Charge Storage, continued

• Capacitance is the ratio of charge to potentialdifference.

magnitude of charge on each platecapacitance =

potential difference

Q C V




Visual Concept


Capacitance

http://70677.html/

http://70677.html/





• Capacitance depends on the size and shape of acapacitor.

• Capacitance for a Parallel-Plate Capacitor in aVacuum

–12 2

0

0

area of one of the plates

capacitance = permittivity of a vacuumdistance between the plates

of the medium 8.85 10 C /N mpermittivity

AC

d





• The material between acapacitor’s plates canchange its capacitance.

• The effect of a dielectricis to reduce the strengthof the electric field in a

capacitor.



Chapter 17

Capacitors in Keyboards

Section 2 Capacitance




Visual Concept


Parallel-Plate Capacitor

http://70544.html/

http://70544.html/




Energy and Capacitors

• The potential energy stored in a charged capacitor depends on the charge and the potential differencebetween the capacitor’s two plates.

1electrical potential energy = (charge on one plate)(final potential difference)

2

12

electric PE Q V




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 = ? PE electric = ?



Chapter 17


Capacitance

To determine the capacitance, use the definition ofcapacitance.

–5

–6

3.6 10 C12 V

3.0 10 F 3.0 µF

Q C V

C




Chapter 17


Capacitance

To determine the potential energy, use thealternative form of the equation for the potentialenergy of a charged capacitor:

2

–6 2

–4

1( )

2

1(3.0 10 F)(12 V)

22.2 10 J

electric

electric

electric

PE C V

PE

PE


Section 3 Current and



Preview

• Objectives

• Current and Charge Movement

• Drift Velocity

• Resistance to Current

Chapter 17Section 3 Current andResistance




Section 3 Current andResistance Chapter 17

Objectives

• Describe the basic properties of electric current, andsolve problems relating current, charge, and time.

• Distinguish between the drift speed of a charge

carrier and the average speed of the charge carrierbetween collisions.

• Calculate resistance, current, and potentialdifference by using the definition of resistance.

• Distinguish between ohmic and non-ohmicmaterials, and learn what factors affect resistance.





Current and Charge Movement

• Electric current is the rate at which electric chargespass through a given area.

charge passing through a given areaelectric current =

time interval

Q I t





Visual Concept


Conventional Current


http://70678.html/

http://70678.html/




Drift Velocity

• Drift velocity is the thenet velocity of a chargecarrier moving in anelectric field.

• Drift speeds arerelatively small becauseof the many collisions

that occur when anelectron moves througha conductor.





Visual Concept


Drift Velocity


http://70679.html/

http://70679.html/




Resistance to Current

• Resistance is the opposition presented to electriccurrent by a material or device.

• The SI units for resistance is the ohm (Ω) and isequal to one volt per ampere.

• Resistance

potential difference

resistancecurrent

V R I




Sect o 3 Cu e t a dResistance Chapter 17

Resistance to Current, continued

• For many materials resistance is constant over arange of potential differences. These materials obeyOhm’s Law and are called ohmic materials.

• Ohm’s low does not hold for all materials. Suchmaterials are called non-ohmic .

• Resistance depends on length, cross-sectional area,temperature, and material.





Visual Concept

Chapter 17 Resistance

Factors that Affect Resistance

Section 3 Current and17

http://70429.html/

http://70429.html/



Resistance Chapter 17

Resistance to Current, continued

• Resistors can be used to control the amount ofcurrent in a conductor.

• Salt water and perspiration lower the body'sresistance.

• Potentiometers have variable resistance.

17 Section 4 Electric Power



Preview

• Objectives

• Sources and Types of Current

• Energy Transfer

Chapter 17 Section 4 Electric Power

Section 4 Electric PowerC 17



Section 4 Electric Power Chapter 17

Objectives

• Differentiate between direct current and alternatingcurrent.

• Relate electric power to the rate at which electricalenergy is converted to other forms of energy.

• Calculate electric power and the cost of running

electrical appliances.

Section 4 Electric PowerCh 17




Sources and Types of Current

• Batteries and generators supply energy to chargecarriers.

• 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.

Section 4 Electric PowerCh 17




Energy Transfer

• Electric power is the rate of conversion of electricalenergy.

• Electric powerP = I ∆V

Electric power = current potential difference

Ch t 17 Section 4 Electric Power




Visual Concept


Energy Transfer

Section 4 Electric PowerCh t 17

http://70545.html/

http://70545.html/




Energy Transfer, continued

• Power dissipated by a resistor

• Electric companies measure energy consumed in kilowatt-hours.

• Electrical energy is transferred at high potentialdifferences to minimize energy loss.

22 ( )V

P I V I R R

Ch t 17 Section 4 Electric Power




Visual Concept


Relating Kilowatt-Hours to Joules

http://70121.html/

http://70121.html/

Phy Outline Ch17

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