1. +

2. + + +

3. + We can divide the world into two parts:
The source charges, which are causing the force
The test charge, which feels the force
This is a bit like the system/surroundings idea in mechanics.

4. What if we replace the proton with an electron?- - -

5. What if we replace the proton with a neutron?

6. What if we remove the test charge?+ + +
The “influence” of the source charges is still there… we call it the electric field.

7. Electric field
is a vector field. It has a value and a direction at every position in space and every moment in time:

8. Electric field Two steps when thinking about electric forces:
Calculate the field caused by the source charges.
Calculate the effect the field has on other charges.

9. Electric field of one point chargeq
Units of electric field: Newtons per Coulomb (N/C)

10. +
q is positive:
the direction of the field is away from the charge.

11. -
q is negative:
the direction of the field is towards the charge.

13. There is an electric field at point A, in the direction shown by the arrow.
An electron is placed at point A. In which direction is the force on the electron? A

14. The electric field at location <-0. 13, 0
The electric field at location <-0.13, 0.14, 0> m is found to be <6.48 x 103, x 103, 0> N/C. The only charged particle in the surroundings has charge -3 nC. What is its location?
Observation location
Source location

15. Superposition Principle 电场强度叠加原理+ q2 - + q3 q1
What is the force on q3?

16. Superposition Principle 电场强度叠加原理+ q2 -
Location of q3 q1
Step 1: Find the total field at the location of q3 due to the other charges

17. Superposition Principle 电场强度叠加原理+ q2 - + q3 q1
Step 2: Find the force on q3 due to the total field

18. Superposition Principle 电场强度叠加原理+ q2 - + q3 q1
Location of q4

19. Superposition Principle 电场强度叠加原理
In general:
or:
The net field at a location in space is the vector sum of the fields contributed by all charged particles at other locations.