1. The Electric Field
Figure Force exerted by charge Q on a small test charge, q, placed at points A, B, and C.

2. The Electric Field: Introduction
The Electric Force is
a Field Force.
Field forces can act through all space.
Their effects are produced even with
no physical contact between objects.
Faraday developed the concept of a
force in terms electric fields.
Figure Force exerted by charge Q on a small test charge, q, placed at points A, B, and C.

3. The Electric Field  The direction of the Electric Field due
The Electric Field is defined as the force
on a small positive charge q, (a “Test Charge”)
divided by the magnitude of the charge: 
Figure Force exerted by charge Q on a small test charge, q, placed at points A, B, and C.
The direction of the Electric Field due
to a positive charge +Q is radially
outward from the charge.

4.  The direction of the Electric Field due to a positive charge +Q is
radially outward from the charge.
Figure Force exerted by charge Q on a small test charge, q, placed at points A, B, and C.

5. An Electric Field surrounds every charge.
An Electric Field exists
in the region of space
around a charged object.
This charged object is the
Source Charge,
When another charged object, the
Test Charge,
enters this electric field, an electric
force acts on it.
Figure An electric field surrounds every charge. P is an arbitrary point.

6. In terms of ε0,the permittivity of free space:
The Electric Field due to a single point
charge Q is:
In terms of ε0,the permittivity of free space:

7. The direction of E is that of the force on a positive test charge.
We can also say that an electric field
exists at a point if a test charge at that
point experiences an electric force.
The SI units of E
are N/C (or V/m).

8. This is valid for a point charge only. If q is negative,
The Force on a positive point
charge q in an electric field E:
Electric
Field E
Force F on a positive point charge q.
This is valid for a point
charge only. If q is negative,
the Force F & the Electric
Field E point in the opposite
direction than when q is
positive.
Figure (a) Electric field at a given point in space. (b) Force on a positive charge at that point. (c) Force on a negative charge at that point.
Force F on a negative point charge -q.

9. Electric Field, Vector Form
Remember that Coulomb’s Law,
between the source & test charges is
Then, the electric field will be
Figure (a) Electric field at a given point in space. (b) Force on a positive charge at that point. (c) Force on a negative charge at that point.

10. More On Electric Field Direction
If a Charge q is Positive, the Force is
Directed Away from q.
(Fig. a)
The Electric Field Direction is
Also Away from a Positive Source Charge.
(Fig. b)
Figure (a) Electric field at a given point in space. (b) Force on a positive charge at that point. (c) Force on a negative charge at that point.

11. Also Towards a Negative Source Charge.
If a Charge q is Negative, the Force is
Directed Towards q.
(Fig. c)
The Electric Field Direction is
Also Towards a Negative Source Charge.
(Fig. b)
Figure (a) Electric field at a given point in space. (b) Force on a positive charge at that point. (c) Force on a negative charge at that point.

12. Electric Fields from Multiple Charges
At any point P, the total electric
field E due to a group of source
charges qi equals
The Vector Sum of the electric fields of all of the charges:
Figure (a) Electric field at a given point in space. (b) Force on a positive charge at that point. (c) Force on a negative charge at that point.

13. Electric Forces & Electric Fields
Problem Solving in Electrostatics:
Electric Forces & Electric Fields
Sketch a Diagram showing all charges, with signs, & electric fields & forces with directions.
Calculate Forces using Coulomb’s Law.
Add Forces Vectorially to get the result.
Check Your Answer!

14. Field Lines The electric field is stronger where the field lines are
The electric field can be represented by field
lines. These lines start on a positive charge & end
on a negative charge. The number of field lines
Starting (ending) on a positive (negative) charge is
proportional to the magnitude of the charge.
The electric field
is stronger where
the field lines are
closer together.

15. 2 equal charges, opposite in sign:
Electric Dipole:
2 equal charges, opposite in sign:

16. The electric field between two closely spaced, oppositely charged parallel plates is constant.

17. Summary of Field Lines:
Field lines indicate the direction of the
field; the field is tangent to the line.
The magnitude of the field is
proportional to the density of the lines.
Field lines start on positive charges &
end on negative charges; the number is
proportional to the magnitude of the
charge.

18. Electric Fields and Conductors (Electrostatics)
The static electric field inside a
conductor is zero – if it were not, the
charges would move.
So, the net charge on
a conductor is on
its surface (under
static conditions)

19. Under static conditions, the electric field is
perpendicular to the surface of a conductor.
Again, if it were not, the charges would move.