A LevelPhysicsAQARevision Notes7. Fields & Their Consequences7.4 Electric Fields7.4.3 Electric Field Strength

Electric Field Strength (AQA A Level Physics)

Revision Note

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Electric Field Strength

  • An electric field is a region of space in which an electric charge experiences a force
  • The electric field strength at a point is defined as:

The force per unit charge experienced by a small positive test charge placed at that point

  • The electric field strength can be calculated using the equation:

E space equals space F over Q

  • Where:
    • E = electric field strength (N C−1)
    • F = electric force on the charge (N)
    • Q = magnitude of the charge (C)
  • Note that the definition specifies that a positive test charge is used
  • This sets a clear convention for the direction of an electric field, for example, in a field of strength E:
    • A positive charge plus Q experiences a force E Q in the direction of the field
    • A negative charge negative Q experiences a force E Q in the opposite direction
  • Hence, electric field strength is a vector quantity and is always directed:
    • Away from a positive charge
    • Towards a negative charge

Worked example

A charged particle experiences a force of 0.3 N at a point where the magnitude of electric field strength is 3.5 × 104 N C−1.

Calculate the magnitude of the charge on the particle.

Answer:

Step 1: Write down the equation for electric field strength

E space equals space F over Q

Step 2: Rearrange for charge Q

Q space equals space F over E

Step 3: Substitute in the values and calculate:

Q space equals space fraction numerator 0.3 over denominator 3.5 cross times 10 to the power of 4 end fraction space equals space 8.571 cross times 10 to the power of negative 6 end exponent space equals space 8.6 cross times 10 to the power of negative 6 end exponent C (2 s.f.)

  • The particle has a charge of 8.6 × 10−6 C or 8.6 μC

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    Katie M

    Author: Katie M

    Expertise: Physics

    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.