Skip to main content
Next exam on November 14 – 15, 2026:
All concepts

Electric Field and Field Strength

电场与电场强度

Coulomb's law and charge conservation, the defining equation and superposition of electric field strength, the fields of a point charge and a uniform field, and what field-line density means.

16 real-exam questions · 4 sittings22 in mock papers40 practice

Formulas

  • Coulomb's law

    Law
    F=kq1q2r2F=k\frac{q_1q_2}{r^2}

    Applies when：Point charges in vacuum; k is the electrostatic constant

  • Definition of electric field strength

    Definition
    E=FqE=\frac{F}{q}

    Applies when：A ratio definition: E is set by the field itself, not by the test charge q

  • Field of a point charge

    Formula
    E=kQr2E=k\frac{Q}{r^2}

    Applies when：A point charge in vacuum. Outside a uniformly charged shell or solid sphere the field is the same as if all the charge sat at the centre; inside they differ — the field is zero inside a shell (and inside a conductor in electrostatic equilibrium), but not inside a uniformly charged solid sphere

  • Field strength in a uniform field

    Formula
    E=UdE=\frac{U}{d}

    Applies when：Uniform fields only; d is measured along the field direction

How the real exam tests it

Questions from real CSCA papers that test this concept. Work each one out before you open the answer.

  1. CSCA Physics December 2025 Past Paper
    The electric field lines of an electric field are distributed as shown in the figure. If point charges aa and bb of equal charge are placed at points a and b in the field, the relationship between the electric forces exerted on the two point charges is ( ), Question 25 diagram
    • A．Fa≥FbF _ { a } \geq F _ { b }
    • B．Fa=FbF _ { a } = F _ { b }
    • C．Fa<FbF _ { a } < F _ { b }
    • D．Fa>FbF _ { a } > F _ { b }
    Show answer & solution

    Answer: C

    1 Electric force on a test charge: F=qEF = q E. 2 Field strength is larger where field lines are denser; point bb is in the denser region than point a (from the diagram). 3 So FbF_b > FaF_a, i.e., FaF_a<FbF_b.
    See it in the full CSCA Physics December 2025 Past Paper
  2. CSCA Physics March 2026 Past Paper
    Charges A,B,C\mathrm { A } , \mathrm { B } , \mathrm { C } at vertices of an equilateral triangle. A and B are positive. The resultant force on A from B and C is F_A\mathrm { F } \_ \mathrm { A }, with angle to BA extension <60∘< 60 ^ { \circ }. Which is correct about C ? Question 29 diagram
    • A．Positive, charge < B
    • B．Negative, charge >B> B
    • C．Positive, charge > B
    • D．Negative, charge < B
    Show answer & solution

    Answer: D

    BB repels AA along BAB A extension. Resultant angle <60∘< 60 ^ { \circ } means slight deflection toward CAC A. If CC were positive, its repulsion would push the angle beyond 60∘60 ^ { \circ } - contradiction. So C is negative (attractive), deflecting toward CA．Angle <60∘< 60 ^ { \circ } means C 's attraction <B< \mathrm { B } 's repulsion, so ∣C∣<∣B∣| \mathrm { C } | < | \mathrm { B } |. Answer: D．
    Take the full CSCA Physics March 2026 Past Paper — timed & graded
  3. CSCA Physics April 2026 Past Paper
    A point charge QQ produces an electric field of magnitude EE at point AA. If the distance from the charge to AA is doubled, the new electric field magnitude is
    • A．E2\frac { E } { 2 }
    • B．2E2 E
    • C．E4\frac { E } { 4 }
    • D．4E4 E
    Show answer & solution

    Answer: C

    Since E=kQr2,EE = \frac { k Q } { r ^ { 2 } } , E is inversely proportional to r2r ^ { 2 }. Doubling rr reduces EE to 14\frac { 1 } { 4 } of its original value.
    Take the full CSCA Physics April 2026 Past Paper — timed & graded

Real-exam evidence

Recorded past-paper appearances are listed below; online verification is labelled separately.

16 questions across 4 sittings1 of 4 sittings available to verify free online

22 questions across 6 mock papers — our own papers, so a coverage signal rather than real-exam evidence

40 related practice questions availablePractise free: Electrostatics

Sitting evidence comes from the CSCA past papers on record; questions whose original paper is missing are excluded, and sprint papers are deliberately not counted.

Related concepts

Electric Potential and Potential DifferenceSteady Current and DC Circuits