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Electric fields Cambridge International AS & A Level Physics revision

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In plain words

Rub a balloon on your jumper and it will pull your hair towards it without touching. The balloon is surrounded by an electric field: a region where any charge feels a force. The field strength says how many newtons each coulomb would feel.

4 things to know

  1. An electric field is a region in which a charge experiences a force. Electric field strength E = F ÷ Q: the force per unit positive charge, in N/C. So the force on a charge is F = QE.
  2. Field lines show the direction of the force on a positive charge: away from positive charge and towards negative charge. Lines closer together mean a stronger field.
  3. Round a point charge or a charged sphere the field is radial. Between two parallel charged plates it is uniform: the lines are parallel and evenly spaced.
  4. A negative charge feels a force in the opposite direction to the field.

Worked example

An electron is in an electric field of strength 2.0 × 10⁴ N/C. Find the force on it and its acceleration. (Mass of an electron = 9.11 × 10⁻³¹ kg.)

  1. F = QE = 1.60 × 10⁻¹⁹ × 2.0 × 10⁴ = 3.2 × 10⁻¹⁵ N.
  2. a = F ÷ m = 3.2 × 10⁻¹⁵ ÷ 9.11 × 10⁻³¹.
  3. = 3.5 × 10¹⁵ m/s², in the opposite direction to the field.

Tips and tricks

  • The direction of a field is the direction of the force on a positive charge.
  • Field lines never cross. They start on positive charge and end on negative charge.
5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Electric fields: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

  1. What is electric field strength?
    • force per unit positive charge (the answer)
    • energy per unit charge
    • charge per unit area
    • force per unit mass

    E = F ÷ Q.

  2. Which way do electric field lines point?
    • from positive charge towards negative charge (the answer)
    • from negative charge towards positive charge
    • in circles round the charges
    • always upwards

    They show the force on a positive charge.

  3. How does the force on an electron compare with the direction of the field it is in?
    • It is in the opposite direction. (the answer)
    • It is in the same direction.
    • It is at right angles.
    • There is no force.

    An electron has a negative charge.

  4. A charge of 2.0 µC is in a field of 3000 N/C. What is the force on it?
    • 1.5 × 10⁻³ N
    • 6.0 × 10⁻³ N (the answer)
    • 6.0 × 10³ N
    • 1.5 × 10⁹ N

    F = QE.

  5. What do parallel, evenly spaced field lines show?
    • a uniform field (the answer)
    • a radial field
    • no field
    • a field that is getting weaker

    The field has the same strength and direction everywhere.

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