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Physics
Electric Fields and Field Lines
Field Lines, Force, and Potential
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An electric field is a region of space where a charged object experiences a force. We visualize fields using field lines: arrows that show the direction a positive test charge would be pushed. Lines emerge from positive charges and terminate on negative charges. Where lines are dense, the field is strong; where they're sparse, it's weak.
Coulomb's law describes the force between two point charges, and from it we derive the field strength . Electric potential (voltage) is related but different: it tells you the potential energy per unit charge at a point in the field. Equipotential lines (surfaces where voltage is constant) are always perpendicular to field lines.
Key Facts
- Coulomb's Law: ()
- Electric field strength: (units: N/C or V/m)
- Field lines run from positive to negative charges.
- Field lines never cross each other.
- Between parallel plates, the field is uniform:
- Electric potential energy: (potential energy = charge voltage)
Vocabulary
- Electric field (E)
- Force per unit positive charge at a point in space; measured in N/C.
- Field line
- A line whose direction shows the direction of the electric force on a positive test charge.
- Coulomb's Law
- The force between two point charges is proportional to their charges and inversely proportional to the square of the distance between them.
- Electric potential (V)
- Electric potential energy per unit charge at a point, measured in volts.
- Equipotential
- A surface or line along which the electric potential is constant; always perpendicular to field lines.
Common Mistakes to Avoid
- Confusing electric field (a vector, force per charge) with electric potential (a scalar, energy per charge). They are related but not the same thing.
- Assuming field lines show the path a charge would travel. A positive charge accelerates along field lines only if released from rest; moving charges curve due to changing field directions.
- Forgetting that field strength falls off as 1/r² for a point charge - doubling distance reduces the field to one-quarter.
- Thinking work done moving along an equipotential surface is nonzero. No work is done along an equipotential because the potential doesn't change.
Practice Questions
- 1 Two charges of +2 μC and -2 μC are 0.1 m apart. Find the force between them.
- 2 What is the electric field strength 0.3 m from a +5 μC point charge?
- 3 Sketch the field line pattern for two equal positive charges placed 4 cm apart.