Electric fields and field lines

Physics A2 · Electric fields. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

An electric field is a region where a charge experiences a force. It is a field of force, like a gravitational field, but it acts on charge and the force can be attractive or repulsive.

Electric field strength E at a point is the force per unit positive charge acting on a small charge placed there. It is a vector. Its direction is the direction of the force on a positive charge, so a negative charge is pushed the opposite way.

Field lines show the field. They start on positive charge and end on negative charge, never cross, and are closer together where the field is stronger. Parallel, equally spaced lines mean a uniform field.

Rules to remember

Common mistake

Students draw the force on an electron in the direction of the field. The field direction is defined for a positive charge, so the force on an electron or any negative charge is opposite to the field lines.

Worked example

A small droplet with a charge of +4.8 × 10−19 C experiences an upward electric force of 9.6 × 10−15 N. What is the electric field strength there, and what is the force on an electron at the same point?

  1. E = F ÷ q = 9.6 × 10−15 ÷ 4.8 × 10−19 = 2.0 × 104 N C−1.
  2. The droplet is positive, so the field is in the direction of the force: upwards.
  3. Force on an electron: F = qE = 1.60 × 10−19 × 2.0 × 104 = 3.2 × 10−15 N.
  4. The electron is negative, so this force is opposite to the field: downwards.

Answer: E = 2.0 × 104 N C−1 upwards; the force on the electron is 3.2 × 10−15 N downwards.

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