+

+ + +

+ 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.

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

What if we replace the proton with a neutron?

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

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

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.

Electric field of one point charge q Units of electric field: Newtons per Coulomb (N/C)

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

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

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

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, -8.64 x 103, 0> N/C. The only charged particle in the surroundings has charge -3 nC. What is its location? Observation location Source location

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

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

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

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

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.