Equipotential surfaces and field lines
Equipotential surfaces
Equipotential surfaces are mathematically speaking hypersurfaces
The potential in Cartesian coordinates is a function V=V(x,y,z)
With V(x,y,z)=const we define a 2D surface z=z(x,y) in 3D space
We can better picture the situation in 2D
Example: contour lines in toporaphic maps
z
z( x, y )  x  y
2
2
2D surface
in 3D space
z( x, y )  x  y  2
2
2
Contour line
1D equi-value “surface”
y
x
2
2
Note that one could consider z ( x , y )  x 2  y 2 as equi-value surface of f ( x , y , z )  x  y  z  0
Some properties of equipotential surfaces
-in general an equipotential surface is a hypersurface defined by V(x,y,z)=const
-per definition V is the same everywhere on the surface
If you move a test charge q0 on this surface the potential energy U= q0V
remains constant
no work done
if E-field does no work along path of test charge on surface
E-field normal surface E d r  0

It is a general property of the gradient of a function


that  f ( x , y , z )    f ,  f ,  f   f ( x , y , z )  const
b
E
dr
a
2.0
 x y z 
1.5
 f  2 x, 2 y 
1.0
0.5
0.0
f ( x, y )  x  y  1
Y
2
Simple 2D example for this general property
f ( x, y )  x  y
2
 f  2 x, 2 y 
2
-0.5
-1.0
-1.5
-2.0
-2.0
-1.5
-1.0
-0.5
0.0
X
0.5
1.0
1.5
2.0
2
-no point can be at different potentials
equipotential surfaces never touch
or intersect
-field lines and equipotential surfaces are always perpendicular
Examples from our textbook Young and Freedman University Physics page 799
point charge
E-field lines
Cross sections of equipotential surfaces
How do we get the circle solution ?
Remember V ( r )  Q
V ( x , y , z  0) 
4  0 r
Q
4  0
x  y
2
 const
2


Q
x  y 

 4  0 const 
2
2
Similar to higher field line density indicating stronger E-field
Higher density of equipotential contour lines indicates a given
change in the potential takes place with less distance
visualization of stronger E-field because
E   V
2
Electric dipole
Note, the electric field is in general not the
same for points on an equipotential surface
2 equal positive charges
Equipotentials and Conductors
When all charges are at rest, the surface of a conductor is always
an equipotential surface.
Proof:
We use the facts that
i) the E-field is always perpendicular to an equipotential surface
ii) E=0 inside a conductor
We use ii) to show that (when all charges at rest) the E-field outside a
conductor must be perpendicular to the surface at ever point
With i) that implies that surface of a conductor is equipotential surface
E=0 inside a conductor because otherwise charges would move
E tangent to surface inside conductor zero
E tangent to surface (E) outside conductor zero
E=0 just outside the
conductor surface to ensure
Since E=0 inside conductor
Remember E is conservative and work along closed path must be zero
 Ed r  0
in the absence of any tangential component, E,
E can only be perpendicular to the conducting surface
Conductor
Surface of a
conductor