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Quiz on the math needed today
What is the result of this integral: B
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x
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Chapter 25
Electric Potential
A review of gravitational potential
B
When object of mass m is on ground level B, we define that it has zero gravitational potential energy. When we let go of this object, if will stay in place.
When this object is moved to elevation A, we say that it has gravitational potential energy mgh. h is the distance from B to A. When we let go of this object, if will fall back to level B.
A
When this object is at elevation A, it has gravitational potential energy UA-UB= mgh. UA is the potential energy at point A with reference to point B. When the object falls from level A to level B, the potential energy change: ΔU =UB-UA
The gravitational force does work and causes the potential energy change: W = mgh = UA-UB= -ΔU
h
mg
We also know that the gravitational force is conservative: the work it does to the object only depends on the two levels A and B, not the path the object moves.
Introduction of the electric potential, a special case: the electric field is a constant.
EF
0q
When a charge q0 is placed inside an electric field, it experiences a force from the field:
When the charge is released, the field moves it from A to B, doing work:
Edqq 00W dEdF
If we define the electric potential energy of the charge at point A UA and B UB, then:
UUUEdq BA 0WIf we define UB=0, then UA= q0Ed is the electric potential energy the charge has at point A. We can also say that the electric field has an electric potential at point A. When a charge is placed there, the charge acquires an electric potential energy that is the charge times this potential.
Electric Potential Energy, the general caseWhen a charge is moved from point A to point B in an electric field, the charge’s electric potential energy inside this field is changed from UA to UB: AB UUU
EF
0qThe force on the charge is:
B
A
BA dqWUUU sE
0 force) field the(of
So we have this final formula for electric potential energy and the work the field force does to the charge:
When the motion is caused by the electric field force on the charge, the work this force does to the charge cause the change of its electric potential energy, so: UW
Electric Potential Energy, final discussion
Electric force is conservative. The line integral does not depend on the path from A to B; it only depends on the locations of A and B.
B
A
BA dqUUU sE
0
A BLine integral paths
The electric potential energy of a charge q0 in the field of a charge Q?
Q
q0
R
Reference point:We usually define the electric potential of a point charge to be zero (reference) at a point that is infinitely far away from the point charge.
B
A
BA dqUUU sE
0Applying this formula:
Where point A is where the charge q0 is, point B is infinitely far away.
R
Qkdr
r
Qk
drr
Qkdd
ddˆr
Qk
e
R
e
e
e
2
2
2
and
so
,
rEsE
rsrE
And the result is a scalar!
So the final answer is
R
QqkRU e
0 )(
Electric Potential, the definition The potential energy per unit charge, U/qo, is the
electric potential The potential is a characteristic of the field only
The potential energy is a characteristic of the charge-field system
The potential is independent of the value of qo The potential has a value at every point in an electric field
The electric potential is
As in the potential energy case, electric potential also needs a reference. So it is the potential difference ΔV that matters, not the potential itself, unless a reference is specified (then it is again ΔV).
o
UV
q
Electric Potential, the formula The potential is a scalar quantity
Since energy is a scalar As a charged particle moves in an electric
field, it will experience a change in potential
B
A
BA dVVV sE
reference) (often the
Potential Difference in a Uniform Field
dEsEsE
B
A
B
A
BA ddVVV
The equations for electric potential can be simplified if the electric field is uniform:
BABA VV,VVV
or 0
direction, same the and i.e.,
dE 0, dE
When:
This is to say that electric field lines always point in the direction of decreasing electric potential
Electric Potential, final discussion
The difference in potential is the meaningful quantity
We often take the value of the potential to be zero at some convenient point in the field
Electric potential is a scalar characteristic of an electric field, independent of any charges that may be placed in the field
Electric Potential, electric potential energy and Work
When there is electric field, there is electric potential V.
When a charge q0 is in an electric field, this charge has an electric potential energy U in this electric field: U = q0 V.
When this charge q0 is move by the electric field force, the work this field force does to this charge equals the electric potential energy change -ΔU: W = -ΔU = -q0 ΔV.
Units The unit for electric potential energy is the unit for energy joule
(J). The unit for electric potential is volt (V):
1 V = 1 J/C This unit comes from U = q0 V (here U is electric potential
energy, V is electric potential, not the unit volt) It takes one joule of work to move a 1-coulomb charge through a
potential difference of 1 volt
But from
We also have the unit for electric potential as 1 V = 1 (N/C)mSo we have that 1 N/C (the unit of ) = 1 V/m This indicates that we can interpret the electric field as a
measure of the rate of change with position of the electric potential
B
A
dV sE
E
Electron-Volts, another unit often used in nuclear and particle physics
Another unit of energy that is commonly used in atomic and nuclear physics is the electron-volt
One electron-volt is defined as the energy a charge-field system gains or loses when a charge of magnitude e (an electron or a proton) is moved through a potential difference of 1 volt 1 eV = 1.60 x 10-19 J
Direction of Electric Field, energy conservation
As pointed out before, electric field lines always point in the direction of decreasing electric potential
So when the electric field is directed downward, point B is at a lower potential than point A
When a positive test charge moves from A to B, the charge-field system loses potential energy through doing work to this charge
Where does this energy go?
PLAYACTIVE FIGURE
2502
It turns into the kinetic energy of the object (with a mass) that carries the charge q0.
Equipotentials = equal potentials Points B and C are at a lower
potential than point A Points B and C are at the
same potential All points in a plane
perpendicular to a uniform electric field are at the same electric potential
The name equipotential surface is given to any surface consisting of a continuous distribution of points having the same electric potential
Charged Particle in a Uniform Field, ExampleQuestion: a positive charge (mass m) is released from rest and moves in the direction of the electric field. Find its speed at point B.
Solution: The system loses potential energy: -ΔU=UA-UB=qEd
The force and acceleration are in the direction of the field
Use energy conservation to find its speed:
m
qEdv
qEdmv
2
2
1 2
Potential and Point Charges
A positive point charge produces a field directed radially outward
The potential difference between points A and B will be
1 1B A e
B A
V V k qr r
Potential and Point Charges, cont.
The electric potential is independent of the path between points A and B
It is customary to choose a reference potential of V = 0 at rA = ∞
Then the potential at some point r is
e
qV k
r
Electric Potential of a Point Charge
The electric potential in the plane around a single point charge is shown
The red line shows the 1/r nature of the potential
Electric Potential with Multiple Charges
The electric potential due to several point charges is the sum of the potentials due to each individual charge This is another example of the superposition
principle The sum is the algebraic sum
V = 0 at r = ∞
ie
i i
qV k
r
Immediate application: Electric Potential of a Dipole
The graph shows the potential (y-axis) of an electric dipole
The steep slope between the charges represents the strong electric field in this region
)11
(
r-rr-rqkVVV e
Work on the board to prove it.
Potential Energy of Multiple Charges
Consider two charged particles
The potential energy of the system is
Use the active figure to move the charge and see the effect on the potential energy of the system
1 2
12e
q qU k
r
PLAYACTIVE FIGURE
2509
More About U of Multiple Charges
If the two charges are the same sign, U is positive and external work (not the one from the field force) must be done to bring the charges together
If the two charges have opposite signs, U is negative and external work is done to keep the charges apart
U with Multiple Charges, take 3 as an example
If there are more than two charges, then find U for each pair of charges and add them
For three charges:
The result is independent of the order of the charges
1 3 2 31 2
12 13 23e
q q q qq qU k
r r r
Finding E From V
This is straight forward
sEdVFrom We have V
xxxV )( kjiE
If E is one dimensional (say along the x-axis)dx
dVEx
If E is only a function of r (the point charge case):
r)rE
)(( and ,)( rEdr
dVrE
Find V for an Infinite Sheet of Charge
We know the a constant From
We have The equipotential lines are the
dashed blue lines The electric field lines are the
brown lines The equipotential lines are
everywhere perpendicular to the field lines
02
E
sEdVEdV d
E and V for a Point Charge
The equipotential lines are the dashed blue lines
The electric field lines are the brown lines
The equipotential lines are everywhere perpendicular to the field lines
E and V for a Dipole
The equipotential lines are the dashed blue lines
The electric field lines are the brown lines
The equipotential lines are everywhere perpendicular to the field lines
When you use a computer (program) to calculate electric Potential for a Continuous Charge Distribution:
Consider a small charge element dq Treat it as a point charge
The potential at some point due to this charge element is
e
dqdV k
r
V for a Continuous Charge Distribution, cont.
To find the total potential, you need to integrate to include the contributions from all the elements
This value for V uses the reference of V = 0 when P is infinitely far away from the charge distributions
e
dqV k
r
V for a Uniformly Charged Ring
P is located on the perpendicular central axis of the uniformly charged ring The ring has a radius a
and a total charge Q
2 2
ee
k QdqV k
r a x
V for a Uniformly Charged Disk
The ring has a radius R and surface charge density of σ
P is along the perpendicular central axis of the disk
1
2 2 22 eV πk σ R x x
V for a Finite Line of Charge
A rod of line ℓ has a total charge of Q and a linear charge density of λ
2 2
lnek Q aV
a
Prove that V is everywhere the same on a charged conductor in equilibrium
Inside the conductor, because is 0, , so ΔV=0
On the surface, consider two points on the surface of the charged conductor as shown
is always perpendicular to the displacement
Therefore, Therefore, the potential
difference between A and B is also zero
E
ds
0d E s
E
0d E s
Summarize on potential V of a charged conductor in equilibrium
V is constant everywhere on the surface of a charged conductor in equilibrium ΔV = 0 between any two points on the surface
The surface of any charged conductor in electrostatic equilibrium is an equipotential surface
Because the electric field is zero inside the conductor, we conclude that the electric potential is constant everywhere inside the conductor and equal to the value at the surface
E Compared to V
The electric potential is a function of r
The electric field is a function of r2
The effect of a charge on the space surrounding it: The charge sets up a
vector electric field which is related to the force
The charge sets up a scalar potential which is related to the energy
Cavity in a Conductor
Assume an irregularly shaped cavity is inside a conductor
No charges are inside the cavity
The electric field inside the conductor must be zero (can you prove that?)
Cavity in a Conductor, cont
The electric field inside does not depend on the charge distribution on the outside surface of the conductor
For all paths between A and B,
A cavity surrounded by conducting walls is a field-free region as long as no charges are inside the cavity