Tues. Oct. 6, 2009 Physics 208 Lecture 10 1 Last time… Fields, forces, work, and potential Electric forces and work + + Potential energy stored in electric field
Dec 30, 2015
Tues. Oct. 6, 2009 Physics 208 Lecture 10 1
Last time… Fields, forces, work, and potential
Electric forces and work+ +
Potential energy stored in electric field
Tues. Oct. 6, 2009 2
Work, KE, and potential energy
When particle is not isolated,
€
Wexternal = ΔK + ΔUWork done on system
Change in kinetic energy
Change in electric potential energy
Works for constant electric field if
Only electric potential energy difference Sometimes a reference point is chosen
E.g. Then for uniform electric field
€
ΔU = −qr E ⋅Δ
r r
€
Ur r ( ) = 0 at
r r = (0,0,0)
€
Ur r ( ) = −q
r E ⋅
r r
Tues. Oct. 6, 2009 3
Electric potential V
Electric potential difference ΔV is the electric potential energy / unit charge = ΔU/q
For uniform electric field,
€
ΔVr r ( ) =
ΔUr r ( )
q=
−qr E ⋅Δ
r r
q= −
r E ⋅Δ
r r
This is only valid for a uniform electric field
Tues. Oct. 6, 2009 4
Check for uniform E-field
++
Push particle against E-field, or across E-field
Which requires work? Constant electric potential in this direction
Decreasing electric potential in this direction
Increasing electric potential in this direction
Tues. Oct. 6, 2009 5
Quick QuizTwo points in space A and B have electric potential VA=20
volts and VB=100 volts. How much work does it take to move a +100µC charge from A to B?
A. +2 mJ
B. -20 mJ
C. +8 mJ
D. +100 mJ
E. -100 mJ
Tues. Oct. 6, 2009 Physics 208 Lecture 10 6
Potential from electric field
dV largest in direction of E-field.
dV smallest (zero) perpendicular to E-field
€
dV = −r E • d
r l
€
dr l
€
rE
V=Vo
€
V = Vo −r E d
r l
€
V = Vo +r E d
r l
€
dr l
€
dr l
€
V = Vo
Tues. Oct. 6, 2009 7
Electric potential: general
Electric field usually created by some charge distribution. V(r) is electric potential of that charge distribution
V has units of Joules / Coulomb = Volts
€
ΔU =r F Coulomb • d
r s ∫ = q
r E • d
r s ∫ = q
r E • d
r s ∫
Electric potential energy difference ΔU proportional to charge q that work is
done on
€
ΔU /q ≡ ΔV =Electric potential difference
Depends only on charges that create E-fields
€
= r
E • dr s ∫
Tues. Oct. 6, 2009 8
Electric potential of point charge
Electric field from point charge Q is
What is the electric potential difference?
€
rE =
kQ
r2ˆ r
€
ΔV =r E • d
r s
start
end
∫ = kQ
r2dx
rinitial
rfinal
∫
= −kQ
r rinitial
rfinal
= kQ
rinital
− kQ
rfinal
Define
€
V r( ) = kQ
rfor point charge
€
V r = ∞( ) = 0 Then
Tues. Oct. 6, 2009 Physics 208 Lecture 10 9
Equipotential lines
Lines of constant potential In 3D, surfaces of constant potential
Tues. Oct. 6, 2009 Physics 208 Lecture 10 10
Topographic map
Each lines is constant elevation
Same as constant gravitational potentialgh (energy = mgh)
Height interval between lines constant
Tues. Oct. 6, 2009 Physics 208 Lecture 10 11
Electric field from potential
Spell out the vectors:
€
dW =r F ext • d
r s = −
r F Coulomb • d
r s ⇒
dV = −r E • d
r s
€
dV = − Exdx + Eydy + E zdz( )
€
Ex = −dV
dx, Ey = −
dV
dy, E z = −
dV
dz
Usually written
€
r E = −
r ∇V = −
dV
dx,dV
dy,dV
dz
⎛
⎝ ⎜
⎞
⎠ ⎟
€
r E = −
r ∇V = −
dV
dx,dV
dy,dV
dz
⎛
⎝ ⎜
⎞
⎠ ⎟
Said before that
This works for
Tues. Oct. 6, 2009 Physics 208 Lecture 10 12
Quick Quiz
Suppose the electric potential is constant everywhere. What is the electric field?
A) Positive
B) Negative
C) Increasing
D) Decreasing
E) Zero
Tues. Oct. 6, 2009 Physics 208 Lecture 10 13
Electric Potential - Uniform Field
Constant E-field corresponds to linearly decreasing (in direction of E) potential
€
dV = −r E • d
r s
€
=− EdxA
B
∫ = −E dxA
B
∫ = −E xB − xA( )
A
B
x
Here V depends only on x, not on y
€
⇒ VB −VA = −r E
A
B
∫ • dr s = −Eˆ x
A
B
∫ • dr s
Tues. Oct. 6, 2009 Physics 208 Lecture 10 14
Check of basic cases
Previous quick quiz: uniform potential corresponds to zero electric field
Linear potential corresponds to constant electric field
€
E = −∇V = −∇ constant( ) = 0
€
E = −∇V = −∇ −Ex( ) =∂
∂xEx,
∂
∂yEx,
∂
∂zEx
⎛
⎝ ⎜
⎞
⎠ ⎟= Eˆ x
Tues. Oct. 6, 2009 Physics 208 Lecture 10 15
Potential and charge
Have shown that a conductor has an electric potential, and that potential depends on its charge
For a charged conducting sphere:
+ +++
+++
++
++
€
V R( ) −V ∞( ) = kQ
R=
k
RQ
Electric potential proportional to total chargeElectric potential proportional to total charge
Tues. Oct. 6, 2009 Physics 208 Lecture 10 16
Quick Quiz
Consider this conducting object. When it has total charge Qo, its electric potential is Vo. When it has charge 2Qo, its electric potential
A. is Vo
B. is 2Vo
C. is 4Vo
D. depends on shape
Tues. Oct. 6, 2009 Physics 208 Lecture 10 17
Capacitance
Electric potential of any conducting object proportional to its total charge.
€
V =1
CQ
C = capacitance Large capacitance: need lots of charge to change
potential Small capacitance: small charge can change
potential.
Tues. Oct. 6, 2009 Physics 208 Lecture 10 18
Capacitors
Where did the charge come from? Usually transferred from another conducting
object, leaving opposite charge behind
A capacitor consists of two conductors Conductors generically called ‘plates’ Charge transferred between plates
Plates carry equal and opposite charges Potential difference between plates
proportional to charge transferred Q
Tues. Oct. 6, 2009 Physics 208 Lecture 10 19
Definition of Capacitance Same as for single conductor
but ΔV = potential difference between plates Q = charge transferred between plates
SI unit of capacitance is farad (F) = 1 Coulomb / Volt This is a very large unit: typically use F = 10-6 F, nF = 10-9 F, pF = 10-12 F
€
ΔV =1
CQ
Tues. Oct. 6, 2009 Physics 208 Lecture 10 20
How was charge transferred? Battery has fixed electric potential difference
across its terminals Conducting plates connected to battery
terminals by conducting wires.
ΔVplates = ΔVbattery across plates
Electrons move from negative battery terminal to -Q plate from +Q plate to positive battery terminal
This charge motion requires work The battery supplies the work
ΔV
€
Q = CΔV
Tues. Oct. 6, 2009 Physics 208 Lecture 10 21
Requires work to transfer charge dq from one plate:
Work done to charge a capacitor
€
dW = ΔVdq =q
Cdq
€
dW = ΔVdq =q
Cdq
€
W =q
C0
Q
∫ dq =Q2
2C
€
W =q
C0
Q
∫ dq =Q2
2C
Work done stored as potential energy in capacitor
Total work = sum of incremental work
€
U =Q2
2C=
1
2QΔV =
1
2C ΔV( )
2
€
U =Q2
2C=
1
2QΔV =
1
2C ΔV( )
2
Tues. Oct. 6, 2009 Physics 208 Lecture 10 22
Example: Parallel plate capacitor
Charge Q moved from right conductor to left conductor
Charge only on inner surfaces Plate surfaces are charge
sheets, each producing E-field
+Q -Q
d
innerouter
€
E left + E right = η /2εo + η /2εo = η /εo
Uniform field between plates
Tues. Oct. 6, 2009 Physics 208 Lecture 10 23
Quick Quiz
Electric field between plates of infinite parallel-plate capacitor has a constant value /o. What is the field outside of the plates?
A. /o
B. /2o
C. - /2o
Δ. /4o
E. 0
Tues. Oct. 6, 2009 Physics 208 Lecture 10 24
What is potential difference?
-Q+Q
d
+++
+
+
+++
+
++
+++
+
---
-
-
---
-
--
---
-
Potential difference = V+-V-
= - (work to move charge q from + plate to - plate) / q
€
=−−qEd( ) /q
ΔV = Ed = ηd /εo = Qd
εoA
⎛
⎝ ⎜
⎞
⎠ ⎟
€
ΔV = V+ −V− = Qd
εoA
⎛
⎝ ⎜
⎞
⎠ ⎟
€
rE
Tues. Oct. 6, 2009 Physics 208 Lecture 10 25
What is the capacitance?
+Q
-Q
d
€
ΔV = V+ −V− = Qd
εoA
⎛
⎝ ⎜
⎞
⎠ ⎟
€
ΔV = Q /C
€
C =εoA
dThis is a geometrical factor
€
U =1
2C ΔV( )
2=
1
2
εoA
dEd( )
2=
1
2Ad( )εoE 2
Energy stored in parallel-plate capacitor
€
U / Ad( ) =1
2εoE 2
Energy density
Tues. Oct. 6, 2009 Physics 208 Lecture 10 26
Human capacitors Cell membrane:
‘Empty space’ separating charged fluids (conductors)
~ 7 - 8 nm thick In combination w/fluids, acts as
parallel-plate capacitor
Cytoplasm
Extracellular fluid
Plasma membrane
100 µm
Tues. Oct. 6, 2009 Physics 208 Lecture 10 27
Modeling a cell membrane
Charges are +/- ions instead of electrons
Charge motion is through cell membrane (ion channels) rather than through wire
Otherwise, acts as a capacitor ~0.1 V ‘resting’ potential
Cytoplasm
Extracellular fluid
Plasma membrane
Na+ Cl-
K+ A-
- - - - - -
+ + + + + +
7-8 nm
ΔV~0.1 V
Ionic charge at surfaces of conducting fluids
Capacitance:
€
oA
d=
8.85 ×10−12 F /m( )4π 50 ×10−6 m( )2
8 ×10−9 m= 3.5 ×10−11F = 35 pF
100 µm sphere surface area
~ 3x10-4 cm2
~0.1µF/cm2
Tues. Oct. 6, 2009 Physics 208 Lecture 10 28
Cell membrane depolarization
Cell membrane can reverse potential by opening ion channels.
Potential change ~ 0.12 V
Ions flow through ion channels Channel spacing ~ 10xmembrane thickness (~ 100 channels / µm2 )
How many ions flow through each channel?
Cytoplasm
Extracellular fluid
Plasma membrane
Na+ Cl-
K+ A-
- - - - - -+ + + + + +
7-8 nm
ΔV~0.1 V
+ + + + + +- - - - - -
ΔV~-0.02 V
(100 channels/µm2)x4π(50 µm)2=3.14x106 ion channels
Charge xfer required ΔQ=CΔV=(35 pF)(0.12V) =(35x10-12 C/V)(0.12V)
= 4.2x10-12 Coulombs1.6x10-19 C/ion -> 2.6x107 ions flow
Ion flow / channel =(2.6x107 ions) / 3.14x106 channels ~ 7 ions/channel
Tues. Oct. 6, 2009 Physics 208 Lecture 10 29
Cell membrane as dielectric
Membrane is not really empty
It has molecules inside that respond to electric field.
The molecules in the membrane can be polarized
Cytoplasm
Extracellular fluid
Plasma membrane
Na+ Cl-
K+ A-
- - - - - -
+ + + + + +
7-8 nm
Dielectric: insulating materials can respond to an electric field by generating an opposing field.
Tues. Oct. 6, 2009 Physics 208 Lecture 10 30
Effect of E-field on insulators If the molecules of the dielectric are non-polar
molecules, the electric field produces some charge separation
This produces an induced dipole moment
+
-
+
-
E=0E
Tues. Oct. 6, 2009 Physics 208 Lecture 10 31
Dielectrics in a capacitor An external field can polarize
the dielectric
The induced electric field is opposite to the original field
The total field and the potential are lower than w/o dielectric E = E0/ and V = V0/
The capacitance increases C = C0
E0
Eind
Tues. Oct. 6, 2009 Physics 208 Lecture 10 32
Cell membrane as dielectric
Without dielectric, we found 7 ions/channel were needed to depolarize the membrane. Suppose lipid bilayer has dielectric constant of 10. How may ions / channel needed?
Cytoplasm
Extracellular fluid
Plasma membrane
Na+ Cl-
K+ A-
- - - - - -
+ + + + + +
7-8 nm
C increases by factor of 1010 times as much charged needed to reach potential
A. 70
B. 7
C. 0.7