IB DP Physics: SL复习笔记5.1.4 Electric Fields

Electric Fields

  • An electric field is a region of space in which an electric charge “feels” a force
    • It is a vector field
    • The direction of the electric field is the same as the direction of the electric force

Electric Field Strength

  • The electric field strength is a measure of the strength of the electric field

The electric field strength is defined as the magnitude of the electric force per unit charge experienced by a small positive test charge placed at that point


The strength of the electric field generated by charge Q is measured by placing a test charge q in the field

  • The equation to calculate the electric field strength at a given point in space is:


  • Where:
    • E = electric field strength in newtons per coulomb N C–1
    • F = electric force in newtons (N)
    • q = electric charge in coulombs (C)

Representing Electric Fields

  • Electric field lines are used to visualise electric fields
  • Arrows along the lines indicate the direction of the field
    • The arrows always point away from the positive charge and towards negative the charge
  • Around a point charge or a charged sphere, the electric field lines are directly radially inwards or outwards:
    • If the charge is positive (+), the field lines are radially outwards
    • If the charge is negative (–), the field lines are radially inwards
  • This shares many similarities to radial gravitational field lines around a point mass
    • The difference being that the gravitational force is always attractive, whilst the electric force can be either attractive or repulsive


Electric field lines around isolated positive and negative point charges

  • The strength of the electric field is proportional to the number of lines per unit cross-sectional area
    • A stronger electric field is represented by arrows that are closer together
  • The field lines between opposite charges are connected, showing attraction
  • The field lines between like repelling charges never connect


Electric field lines showing attraction between a positive and a negative charge, and repulsion between two positive charges

Uniform and Non-Uniform Electric Fields

  • A uniform electric field is a field of force in which the strength of the electric force is the same throughout
    • It is represented by parallel and equally spaced field lines
  • Whenever the spacing between the field lines changes, the electric field is non-uniform


Uniform and non-uniform electric fields between two parallel plates

Worked Example

A charged particle is in an electric field with electric field strength 3.5 × 104 N C-1, where it experiences a force of 0.3 N.Calculate the charge of the particle.

Step 1: Write down the known quantities

    • E = 3.5 × 104 N C-1
    • F = 0.3 N

Step 2: Write down the equation for the electric field strength


Step 3: Rearrange the above equation to calculate the charge q


Step 4: Substitute the numbers into the above equation


q = 8.6 × 10–6 C

Exam Tip

When drawing field lines, make sure they never cross. The electric field can only have one value at any given point.