A field is a way of explaining action at a distance. Massive particles
attract each other. How do they do this, if they are not in contact with
each other? We say that massive particles produce gravitational fields.
A field is a condition in space that can be probed. Massive
particle are also the probes that detect a gravitational field. A
gravitational field exerts a force on massive particles. The magnitude of the
**gravitational field** produced by a massive object at a point
P is the gravitational force per unit mass it exerts on another massive
object located at that point. The direction of the gravitational field is
the direction of that force

The magnitude of the gravitational force near the surface of Earth is F = mg,
the gravitational field has magnitude F/m = g. Its direction is downward.

Charged particles attract or repel each other, even when not in contact with
each other. We say that charged particles produce electric fields.
Charged particles are also the probes that detect an electric field. An
electric field exerts a force on charged particles. The magnitude of the
**electric field ** **E** produced by a
charged particle at a point P is the electric **force per unit positive charge
**it
exerts on another charged particle located at that point. The direction of
the electric field is the direction of that force on a positive charge.
The actual force on a particle with charge q is given by **F** = q**E**.
It points in the opposite direction of the electric field **E** for a
negative charge.

In the presence of many other charges, a charge q is acted on by a net force
**F**,
which is the vector sum of the forces due to all the other charges.
The electric field due to all the other charges at the position of the charge q
is **E **= **F**/q, i.e. it is the vector sum of the electric fields
produce by all the other charges. To measure the electric field
**E** at a point P due
to a collection of charges, we can bring a small positive charge q to the point
P and measure the force on this test charge. The test charge must be
small, because it interacts with the other charges, and we want this interaction
to be small. We divide the force on the test charge by the magnitude of
the test charge to obtain the field.

Consider a
**point charge**** Q** located at the origin.

The force on a test charge q at position **r** is
**F** = (k_{e}Qq/r^{2})
(**r**/r).

The electric field produced by Q is
**E **= **F**/q
= (k_{e}Q/r^{2})
(**r**/r).

If Q is
positive, then the electric field points radially away from the charge.

The electric field decreases with distance as 1/(distance)^{2}.

If
Q is negative, then the electric field points radially towards the
charge.

The electric field decreases with distance as 1/(distance)^{2}.

We obtain the electric field due to a collection of charges using the
principle of superposition. **
E **=

Field lines were introduced by Michael Faraday to help visualize the
direction and magnitude of he electric field. The direction of the field at any
point is given by the direction of a line tangent to the field line, while the magnitude of the
field is given qualitatively by the density of field lines. The field lines
converge at the position of a point charge. Near a point charge their density
becomes very large. The magnitude of the field and the density of the field
lines scale as the inverse of the distance squared. ** **

Field lines start on positive charges and end on negative charges.

Rules for drawing field lines:

- Electric field lines begin on positive charges and end on negative charges, or at infinity.
- Lines are drawn symmetrically leaving or entering a charge.
- The number of lines entering or leaving a charge is proportional to the magnitude of the charge.
- The density of lines at any point (the number of lines per unit length perpendicular to the lines themselves) is proportional to the field magnitude at that point.
- At large distances from a system of charges, the field lines are equally spaced and radial as if they came from a single point charge equal in magnitude to the net charge on the system (presuming there is a net charge).
- No two field lines can cross, since the field magnitude and direction must be unique.

The field lines of an electric **dipole**,
i.e. a positive and a negative charge of equal magnitude, separated by a
distance d.

The electric field decreases
with distance as
1/(distance)^{3}, much faster than the field of a point
charge.

The field lines of two positive charges of equal magnitude separated by a
distance d.

The electric field decreases with distance as 1/(distance)^{2}.

Polar molecules do not have a net charge, but the centers of the positive and negative charge do not coincide. Such molecules produce a dipole field and interact via the electrostatic force with their neighbors.

Every water molecule (H_{2}O) consists of one oxygen atom and
two hydrogen atoms. A water molecule has no net charge because the
number of positively charged protons equals the number of negatively
charged electrons in each atom. In the water molecule, each hydrogen
atom is bound to the oxygen atom by a **covalent
bond**. The hydrogen atom and the oxygen atom share an
electron, which has a slightly higher probability to be closer to the
oxygen atom than to the hydrogen atom. This results in a **polar molecule**, in which the oxygen
"side" of the molecule has a slight negative charge, while the hydrogen
"sides" have a slight positive charge. Because water is a polar
molecule it dissolves many inorganic and organic materials. Organisms
can build macromolecules to attract or repel water as needed simply by
varying the charge on side chains.

The figure on the right shows the electric field lines for a system of two point
charges.

(a) What are the relative magnitudes of the charges?

(b) What are the signs of the charges?

Solution:

(a) There are 32 lines coming from the charge on the left, while there
are 8 converging on that on the right. Thus, the magnitude of the
charge on the left is 4 times
larger than the magnitude of the charge on the right.

(b) The charge on the left is positive; the charge on the right is negative.

Even though charge is quantized, we often can treat it as being continuously
distributed inside some volume, since one quantum of charge is a tiny amount of
charge. On a macroscopic scale we define the
**volume charge density** ρ = lim_{ΔV-->0}(ΔQ/ΔV) as the charge per
unit volume, the **surface charge density** σ = lim_{ΔA-->0}(ΔQ/ΔA) as the charge per
unit area, and the **line charge density** λ = lim_{ΔL-->0}(ΔQ/ΔL) as the charge per
unit length.

We then have for the electric field of a distribution of charges in a volume V

**E**(**r**) = [1/(4πε_{0})]∑_{i}q_{i}(**r**
- **r**_{i})/|**r** - **r**_{i}|^{3}] --> [1/(4πε_{0})][∫_{v'}
dV' ρ(**r'**)(**r** - **r**')/|**r**
- **r**'|^{3}.

Here
(**r**
- **r**_{i})/|**r** - **r**_{i}| is the unit vector pointing from **r**_{i}
to **r**, and
(**r** - **r**')/|**r**
- **r**'| is a unit vector pointing from
the volume element dV' at **r**' to **r**.

If the charge is distributed over a surface, then ρdV --> σdA, where σ is the surface charge
density and dA is an element of surface area. For a line charge distribution we have
ρdV --> λd*l*, where λ
is the line charge density and d*l* is an element length.

Consider a line charge with line charge density λ = Q/2a that extends along the x-axis from x = -a to x = +a. Find the electric field on the y-axis.

Solution:

We are asked to find the electric field due to a line charge distribution.

The field on the y-axis due to an infinitesimal element of charge λdx is given
by

dE_{x} = (k_{e}λdx/(x^{2} + y^{2})) cosθ, dE_{y}
= (k_{e}λdx/(x^{2} + y^{2})) sinθ,

where cosθ = x/(x^{2} + y^{2})^{½}, sinθ = y/(x^{2}
+ y^{2})^{½}.

On the y-axis the field due to the line charge therefore is given by

E_{x} = k_{e}λ∫_{-a}^{a}xdx/(x^{2} + y^{2})^{3/2}
= 0 from symmetry, and

E_{y} = k_{e}λy∫_{-a}^{a}dx/(x^{2} + y^{2})^{3/2}.

Using ∫_{-a}^{a}dx/(x^{2} + y^{2})^{3/2}
= 2a/(y^{2}(a^{2} + y^{2})^{½})

we have E_{y} = k_{e}λ 2a/(y(a^{2} + y^{2})^{½})
= k_{e}Q/(y(a^{2} + y^{2})^{½})

The electric field on the y-axis points in the positive y-direction for y > 0
and in the negative y-direction for y < 0.

As y becomes very large, we can neglect a^{2 }compared to y^{2}
under the square root and then E_{y }= k_{e}Q/y^{2 }∝
1/y^{2}. From very far away, the line charge looks like a point charge.

If, on the other hand, the line is very long, and we y << a, then we can neglect
y^{2 }compared to a^{2} under the square root and then E_{y
}= k_{e}Q/(ay) ∝ 1/y. Near a very long line charge the electric
field falls off as 1/distance, not as 1/distance^{2}.

Simulation: Electric Field Hockey (A homework problem refers to this simulation.)

- This simulation looks down on an air hockey table. Instead of hitting the puck, you move it using charges.
- Use the Practice mode for testing your ideas.
- Clear zeros everything and Reset brings the puck back to the starting point with the same charges.
- You can change the sign of the charge on the puck by checking or un-checking the Puck is Positive option.
- You may want investigate how to use multiple charges to make a goal.

Link to other web material: The Physics Classroom: Static Electricity