General Procedure for Calculating Electric Field of Distributed Charges

1. Cut the charge distribution into pieces for which the field is known

2. Write an expression for the electric field due to one piece(i) Choose origin(ii) Write an expression for E and its components

3. Add up the contributions of all the pieces(i) Try to integrate symbolically(ii) If impossible – integrate numerically

4. Check the results:(i) Direction(ii) Units(iii) Special cases

Distance dependence:

Far from the ring (z>>R):

Close to the ring (z<<R): Ez~z

Ez~1/z2

A Uniformly Charged Thin Ring

A total charge Q is uniformly distributed over a half ring with radius R. The total charge inside a small element dθ is given by:

1. Choice One

2. Choice Two

3. Choice Three

4. Choice Four

5. Choice Five

6. Choice Six

θdθ

Q

R

dRQ

2

A.

dRQ

B.

RQ

C.

dQD.

dQ2

E.

Clicker Question

A total charge Q is uniformly distributed over a half ring with radius R. The y component of electric field at the center created by a short element dθ is given by:

1. Choice One

2. Choice Two

3. Choice Three

4. Choice Four

θdθ

Q

R

sin2R

kQdA.

cos2R

kQdB.

sin3R

kQdC.

cos3R

kQdD.

+yClicker Question

A Uniformly Charged Disk

Close to the disk (0 < z < R)

Along z axis

Approximations:

If z/R is extremely small

Very close to disk (0 < z << R)

Field Far From the Disk

Exact

For z>>R

Point Charge

Uniformly Charged Disk Edge On

Two disks of opposite charges, s<<R: charges distribute uniformly:

+Q-Q

s

A single metal disk cannot be uniformly charged: charges repel and concentrate at the edges

We will calculate E both inside and outside of the disk close to the center

Two uniformly charged metal disks of radius R placed very near each other

Almost all the charge is nearly uniformly distributed on the inner surfaces of the disks; very little charge on the outer surfaces.

Capacitor

+Q-Q

s

We know the field for a single disk There are only 2 “pieces”

E-

E+Enet

Step 1: Cut Charge Distribution into Pieces

Step 2: Contribution of one Piece

Origin: left disk, center

E-

E+Enet

sz

0

Location of disks: z=0, z=sDistance from disk to “2”

z, (s-z)

Left:

Right:

Step 3: Add up Contributions

Location: “2” (inside a capacitor)

Does not depend on z

Step 3: Add up Contributions

E-

E+Enet

sz

0

Location: “3” (fringe field)

For s<<R: E1=E30

Far from the capacitor (z>>R>>s): E1=E3~1/z3 (like dipole)

Fringe field is very small compared to the field inside the capacitor.

E-

E+Enet

sz

0Units:

Inside:

Fringe:

Step 4: check the results:

Electric Field of a Capacitor

Which arrow best represents the field at the “X”?

A)

B)

C) E=0

D)

E)

Clicker Question

Field inside:

Field outside: (like point charge)

Electric Field of a Spherical Shell of Charge

E1+E4

E2E3

E6 E5

Divide into 6 areas:

Direction: radial - due to the symmetry

E of a Sphere Outside

Magnitude: E=0

Note: E is not always 0 inside – other charges in the Universemay make a nonzero electric field inside.

E of a Sphere Inside

E=0: Implications

Fill charged sphere with plastic.Will plastic be polarized? No!

Solid metal sphere: since it is a conductor, any excess charges on the sphere arranges itself uniformly on the outer surface.There will be no field nor excess charges inside!

In general: there is no electric field inside metals

E of a Sphere Inside

Divide shell into rings of charge, each delimited by the angle and the angle +From ring to point: d=(r-Rcos)Surface area of ring:

Integrating Spherical Shell

R

R

Rcos

Rsin

r

d

Q

A mess of math

A solid metal ball bearing a charge –17 nC is located near a solid plastic ball bearing a uniformly distributed charge +7 nC (on surface). Show approximate charge distribution in each ball.

What is electric field field inside the metal ball?

Metal-17 nC

Plastic+7 nC

Exercise

Two uniformly charged thin plastic shells.

Find electric field at 3, 7 and 10 cm from the center

3 cm: E=0

7 cm:

10 cm:

Exercise

What if charges are distributed throughout an object?

Step 1: Cut up the charge into shells

r E

RFor each spherical shell:

outside:

inside: dE = 0

Outside a solid sphere of charge:

for r>R

A Solid Sphere of Charge

Inside a solid sphere of charge:

E

R

r

for r<R

Why is E~r?

On surface:

A Solid Sphere of Charge

What is in the box?

no charges? vertical charged plate?

Patterns of Fields in Space

Box versus open surface

Seem to be able to tellif there are charges inside

…no clue…

Gauss’s law: If we know the field distribution on closed surface we can tell what is inside.

Patterns of Fields in Space