An electric field is invisible, but field lines make its shape and strength visible. The single rule, force per unit positive charge, generates everything else.
An electric field is a region where a charge feels a force. The field strength is the force per unit positive charge, E = F / q, so the force on a charge is F = qE. Field lines run from positive to negative charges.
Switch between a single charge, a dipole, like charges and parallel plates. The gold test charge always feels a force F = qE directed along the local field line, away from positive and toward negative.
The field strength E = F/q is defined using a positive test charge, so the force on a positive charge is along the field and on a negative charge is opposite to it. Field lines never cross, and where they crowd together the field is stronger.
Four quick checks on the definition of field strength and how field lines behave. Each correct answer earns XP and lights this skill on your star map.
The electric field strength at a point is defined as:
The force on a charge q placed in a field of strength E is:
Electric field lines:
Where field lines are drawn closer together, the field is:
Field strength is defined with a positive test charge, so a negative charge feels a force opposite to the field. Field lines never cross (the field has one direction at each point), and their spacing, not their length, shows the field strength.
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