Question 1

What is Coulomb's law for two point charges?

Accepted Answer

For point charges in vacuum, the magnitude of the electrostatic force is F = k|q1 q2|/r^2, where r is their separation and k = 1/(4 pi epsilon0), approximately 9 x 10^9 N m^2/C^2. Equal signs repel, opposite signs attract, and the force acts along the line joining the charges.

Question 2

How is electric field defined?

Accepted Answer

Electric field at a point is the electrostatic force on a small positive test charge q0 placed at that point, divided by q0: E = F/q0. For a point charge Q, its magnitude is k|Q|/r^2, where r is the distance from Q.

Question 3

Why does a negative test charge experience force opposite to the electric field?

Accepted Answer

Any test charge in an electric field experiences F = qE. The electric-field arrow is defined using a positive test charge, so a positive q feels force parallel to E. A negative q has the opposite sign and therefore experiences force opposite to E.

Question 4

What happens when the distance between the charges is doubled?

Accepted Answer

With both charges unchanged, doubling the separation makes r^2 four times larger. The force on the test charge and the electric field at that location both decrease to exactly one quarter of their original magnitudes, while their directions remain the same.

Question 5

What does the field pattern of an isolated point charge look like?

Accepted Answer

The pattern is radially symmetric in three dimensions. For a positive source charge, electric field points outward and lines begin at the charge; for a negative source charge, the field points inward and lines terminate at the charge. Field lines do not intersect.

Coulomb's Law and Electric Field due to a Point Charge — 3D Force Map

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