phy outline ch17
TRANSCRIPT
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Preview
• Objectives
• Electrical Potential Energy • Potential Difference
• Sample Problem
Chapter 17 Section 1 Electric Potential
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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.
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Section 1 Electric Potential Chapter 17
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
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Section 1 Electric Potential Chapter 17
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)
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Visual Concept
Chapter 17 Section 1 Electric Potential
Electrical Potential Energy
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Section 1 Electric Potential Chapter 17
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
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Section 1 Electric Potential Chapter 17
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
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Visual Concept
Chapter 17 Section 1 Electric Potential
Potential Difference
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Section 1 Electric Potential Chapter 17
Potential Difference, continued
• 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)
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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 10 -19 J. Find the charge on the moving particle. What is the potential difference between the two locations?
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Section 1 Electric Potential Chapter 17
Sample Problem, continued
Potential Energy and Potential Difference
Given:
∆PE electric = –6.9 10 –19 J
d = 0.020 mE = 215 N/C
Unknown:
q = ?
∆V = ?
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Section 1 Electric Potential Chapter 17
Sample Problem, continued
Potential Energy and Potential Difference
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
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Section 1 Electric Potential Chapter 17
Sample Problem, continued
Potential Energy and Potential Difference
The potential difference is the magnitude of E timesthe displacement.
– –(215 N/C)(0.020 m)
–4.3 V
V Ed
V
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Section 1 Electric Potential Chapter 17
Potential Difference, continued
• 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.
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Section 1 Electric Potential Chapter 17
Potential Difference, continued
• 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
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Visual Concept
Chapter 17 Section 1 Electric Potential
Superposition Principle and Electric Potential
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Preview
• Objectives
• Capacitors and Charge Storage • Energy and Capacitors
• Sample Problem
Chapter 17 Section 2 Capacitance
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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.
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Section 2 Capacitance Chapter 17
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)
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Section 2 Capacitance Chapter 17
Capacitors and Charge Storage, continued
• Capacitance is the ratio of charge to potentialdifference.
magnitude of charge on each platecapacitance =
potential difference
Q C V
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Visual Concept
Chapter 17 Section 2 Capacitance
Capacitance
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Section 2 Capacitance Chapter 17
Capacitors and Charge Storage, continued
• 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
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Section 2 Capacitance Chapter 17
Capacitors and Charge Storage, continued
• The material between acapacitor’s plates canchange its capacitance.
• The effect of a dielectricis to reduce the strengthof the electric field in a
capacitor.
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Chapter 17
Capacitors in Keyboards
Section 2 Capacitance
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Visual Concept
Chapter 17 Section 2 Capacitance
Parallel-Plate Capacitor
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Section 2 Capacitance Chapter 17
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
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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 = ? PE electric = ?
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Chapter 17
Sample Problem, continued
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
Section 2 Capacitance
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Chapter 17
Sample Problem, continued
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 2 Capacitance
Section 3 Current and
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Preview
• Objectives
• Current and Charge Movement
• Drift Velocity
• Resistance to Current
Chapter 17Section 3 Current andResistance
Section 3 Current and
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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.
Section 3 Current and
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Section 3 Current andResistance Chapter 17
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
Section 3 Current and
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Visual Concept
Chapter 17Section 3 Current andResistance
Conventional Current
Section 3 Current and
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Section 3 Current andResistance Chapter 17
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.
Section 3 Current and
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Visual Concept
Chapter 17Section 3 Current andResistance
Drift Velocity
Section 3 Current and
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Section 3 Current andResistance Chapter 17
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
Section 3 Current and
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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.
Section 3 Current and
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Visual Concept
Chapter 17 Resistance
Factors that Affect Resistance
Section 3 Current and17
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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
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Preview
• Objectives
• Sources and Types of Current
• Energy Transfer
Chapter 17 Section 4 Electric Power
Section 4 Electric PowerC 17
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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
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Section 4 Electric Power Chapter 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
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Section 4 Electric Power Chapter 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
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Visual Concept
Chapter 17 Section 4 Electric Power
Energy Transfer
Section 4 Electric PowerCh t 17
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Section 4 Electric Power Chapter 17
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
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Visual Concept
Chapter 17 Section 4 Electric Power
Relating Kilowatt-Hours to Joules