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Electric Field and Electric Field Lines

Electric field at a point is the electrostatic force experienced per unit positive test charge placed at that point: E = F/q0. It is a vector quantity directed along the force on a positive test charge.

In one line: Field direction is the direction a tiny positive charge would move initially due to electric force.

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Understand it

  • An electric charge creates a field in the space around it.
  • A small positive test charge is used conceptually to detect the field without significantly disturbing the source charge.
  • The SI unit of electric field is N C−1, also written as V m−1 in later electrostatics.
  • Field lines are visual tools: they start from positive charges and end on negative charges, their tangent gives field direction, and closer lines mean stronger field.
  • Field lines never intersect because electric field at a point has only one direction.

Example

The electric field due to a +Q point charge is radially outward. The electric field due to a -Q point charge is radially inward.

Formula and rule

E = F/q0, where E is electric field in N C−1, F is force in newton on a positive test charge, and q0 is test charge in coulomb.

For a point charge, E = (1/4πε0)(Q/r2), directed away from positive Q and toward negative Q.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

A test charge of +2 × 10−6 C experiences a force of 0.04 N at a point.

  1. 1Electric field E = F/q0 = 0.04/(2 × 10−6) = 2.0 × 104 N C−1.
  2. 2Direction is the same as the force on the positive test charge.

Method you can reuse

  1. 1For field magnitude, identify F and q0 or source charge Q and distance r.
  2. 2Use the correct formula.
  3. 3Attach unit N C−1.
  4. 4For direction, imagine a positive test charge and state the direction of force on it.

Visual tip

Required diagrams: radial field lines around a positive charge, radial field lines around a negative charge, and field lines between unlike charges. Labels should include source charge, arrow direction, denser region, and weaker field region.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why can two electric field lines never cross each other?

Think first, then show answer
Two field lines cannot cross because the tangent to a field line gives the direction of electric field. If two lines crossed, one point would have two field directions, which is impossible for a unique electric field.

Common wrong answer

Saying field direction near a positive charge is inward is wrong; a positive test charge would be repelled outward.

Don't mix this up

Students sometimes say field lines are actual physical wires or paths followed by charges. They are only a representation of field direction and relative strength.

Exam tip

In diagrams, arrows on field lines matter. For a positive source charge arrows go outward; for a negative source charge arrows go inward.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Often asked through field-line property questions, diagram-based reasoning, and short numericals using E = F/q0 or E due to a point charge.
  1. 1

    Enough for 1 mark

    Electric field at a point is the electrostatic force experienced per unit positive test charge placed at that point: E = F/q0. It is a vector quantity directed along the force on a positive test charge.

  2. 2

    Add for 2 marks

    Example: The electric field due to a +Q point charge is radially outward. The electric field due to a -Q point charge is radially inward.

  3. 3

    Add for 3 marks

    An electric charge creates a field in the space around it.

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