1. PHYS219; Fall Semester 2014 Lecture 03: Electric Fields
Dimitrios GIANNIOS
Purdue University

2. Practical issues No Class next Monday (Labor’s Day)
First Homework assignment due next Friday

3. CHiP in general Due on Fridays before Recitation (10:30am)
(50% credit for homework done up to 24 hours later)

4. Electric field E
Both gravity and electrostatic forces are non-contact forces. They act on objects that are not touching each other.
The concept of a force field was developed to better describe this
action-at-a-distance feature of these forces.
Gravitational Field
Electric Field

5. Electric field considerations
Electric field is a property of space around charges: it is there all the time!
CANNOT depend on the charge qtest one uses to measure it
electric force alone FE is NOT a good measure of electric field!
suppose you want to define the electric field at distance r away from charge Q:
2FE FE + r
2qtest
qtest Q
Units E[N/C]

6. Electric field The presence of a charge produces an electric field
Shown by the arrows in the figure
A positive charge produces field lines that radiate outward
For a negative charge the field lines are directed inward, toward the charge
The electric field is a vector and denoted by

7. Electric field and test charge
Consider a point in space where the electric field is
If a charge q is placed at the point, the force is given by
The charge q is called a test charge
By measuring the force on the test charge, the magnitude and direction of the electric field can be inferred

8. Electric field, cont.
The electric force is either parallel or antiparallel to the electric field
Parallel if q is positive and antiparallel if q is negative
SI units of the electric field are N/C
Coulomb’s Law can be used to find the magnitude of the electric field, where Q is the charge producing the field and q is the test charge

9. Why bother introducing the Electric Field?
An electric field is property of space: it is present even when there is no second (or test) charge present to experience the electric force
The electric field helps explain how the Coulomb force can act between two charges that are separated by large distances
The electric field is essential for understanding electromagnetic waves

10. Electric Field Lines
We visualize an electric field is with electric field lines
Field lines are a set of continuous lines that are always parallel to the electric field

11. Drawing Electric Field Lines
Field lines must always begin on positive charges
Field lines must always end on negative charges

12. Drawing Electric Field Lines (more)
The Larger the charge the more lines start/end there
QA < QB < QC

13. Another Example: 3 charges
Field lines must always begin on positive charges
Field lines must always end on negative charges
The density electric field lines shows field strength
low field, weak force --lines further apart
q1 = q3 = q2/2 + +
strong field, force lines close together

14. software to visualize Electric fields

15. Quiz time: where are the charges?
Are there more negative or positive charges?
How many charges are there?
How many positive charges?
How many negative charges?

16. Electric Fields and Multiple Charges
To find the electric field due to multiple charges use the principle of superposition
Find the electric fields due to each charge
Add them as vectors

17. Example: E produced by 3 chargesθ1 θ3 θ2

18. Example: Analyze its charge separately

19. Final Answer

20. The origin of Inverse Square Laws
The force between two point charges falls off as 1/r2
Where r is the separation between the charges
The electric field also falls off as 1/r2
The electric force also obeys an inverse square law
The charged particle “sets up” field lines in its neighborhood
The density of the field lines is proportional to the amount of charge on the particle

21. Inverse Square Laws, cont.
The electric field lines emanate outward from a point charge
They intercept larger and larger surface areas
The surfaces are spherical
Their areas increase as A r2
The number of field lines per unit area falls as 1/r2 r