Electric Fields (DP IB Physics: SL)

• 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

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