01 · Which way does force point?
How does charge sign affect field and force direction?
Will a negative test charge accelerate with or against the field?
- Place one positive source and inspect field arrows.
- Reverse its sign and release a test particle.
- Change the test charge and compare acceleration.
Measurement
E_x and E_y in N/C; F=qE in N.
Field direction is defined using a positive test charge. A negative test charge feels the opposite force. The field itself depends on the sources, not the test charge.
Q is source charge, q test charge in C; r in m. 1 nC=10⁻⁹ C.
E = kQ r/|r|³; F = qE; k ≈ 8.988×10⁹ N m²/C²
Worked example 1
For Q=1 nC at r=1 m, E=8.99 N/C outward.
Worked example 2
In that field q=−2 nC gives F=−17.98 nN, toward the source. These unsoftened calculations apply away from the core.
What happens to field strength when distance triples?
Assumptions
Static sources and Coulomb superposition. A softened core regularises singularities. Field lines are visual constructions whose density depends on seeding. This is not a time-dependent Maxwell solver.