Translate between field, force and field-line diagrams, including conductor shielding.
Method and worked examples
Electric field is force per unit positive test charge: E=qF. Its direction is the force direction on a positive test charge; a negative charge feels the opposite force. An isolated point charge produces magnitude E=r2k∣Q∣. Field arrows point outward from positive charge and inward toward negative charge. Closer field-line spacing represents stronger field.
Fields add as vectors. For equal like charges, the fields cancel at the midpoint; for equal unlike charges, they reinforce there. In electrostatic equilibrium, the field is zero within conducting material and an empty conducting cavity. External fields meet a conductor’s surface normally. Strong fields near sharp points can ionise air, explaining why conductor shape affects corona discharge.
Example 1
A+3.00μC charge is in a 400NC−1 field east. Find force.
F=qE=3.00×10−6×400=1.20×10−3N east. A negative charge of equal magnitude would feel the opposite force.
Example 2
Two perpendicular field components are 600 and 800NC−1. Find magnitude.
E=√6002+8002=1000NC−1.
Watch out:Its direction is defined for positive charge; a negative charge’s electric force is opposite the field.
2. Electric fields, superposition and conductors | Year 12 Physics | Ascent Arcade