Static electricity & electric fields

Learning goals

What students should be able to explain, calculate and investigate after completing Static Electricity, Coulomb's Law and Electric Fields.

• Explain how objects become charged by friction and by induction.
• State the law of conservation of charge.
• State Coulomb's law and use it to calculate the force between two point charges.
• Define electric field strength and describe electric fields using field lines.
• Define potential difference (voltage) in terms of work done per unit charge.
• Describe experiments demonstrating static electricity and charge distribution.

Key definitions

Precise terms, symbols and SI units used in Static Electricity, Coulomb's Law and Electric Fields.

Charging Objects

Ordinary matter is electrically neutral because it contains equal numbers of positive (protons, bound in the nucleus) and negative (electrons) charges.

In conductors, some electrons are free to move; in insulators, all electrons stay bound to their atoms. Charging by friction transfers electrons from one insulating material to another when they are rubbed together, leaving one object with a surplus of electrons (negative) and the other with a deficit (positive). Charging by induction (single sphere): a charged rod is brought near (but not touching) an isolated metal sphere; free electrons in the sphere are attracted or repelled, redistributing the charge on its surface without any charge transferring to or from the rod. The sphere is then connected to earth (grounded) while the rod is still nearby: electrons flow to or from the earth as needed to neutralise the near side. The earth connection is removed first, then the rod is taken away — leaving the sphere with a net charge opposite in sign to the rod, spread over its surface.

• A lightning conductor protects a structure mainly by providing a preferred interception point and a low-resistance path that carries lightning current safely to Earth. The sharp tip also concentrates the electric field locally, but it should not be described as simply “attracting lightning”.

Conservation of Charge

Whenever objects are charged (by friction or induction), charge is not created — it is simply transferred from one object to another (or to/from the earth).

An object becomes negatively charged when it gains electrons and positively charged when it loses electrons. The algebraic total charge of the complete isolated system remains constant.

• Electric charge can move between objects but is not created or destroyed in an isolated system.
• Charging by friction transfers electrons; it does not transfer protons between nuclei.
• The two objects gain equal and opposite changes in charge.

Coulomb's Law

Coulomb’s law gives the magnitude of the inverse-square electrostatic force between two point charges; the charge signs determine whether the force is attractive or repulsive.

Coulomb’s law states that the magnitude of the electrostatic force between two point charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them. The force acts along the line joining the charges. Like charges repel and unlike charges attract.

• For two point charges, F ∝ |q₁q₂| and F ∝ 1/r².
• Like charges repel and unlike charges attract.
• Doubling the separation reduces the force to one quarter.

Electric Fields and Field Strength

An electric field describes the force that a positive test charge would experience at each point around charged objects.

Electric field strength, E = F/Q, is the force experienced per unit positive charge at a point; it is a vector, pointing in the direction a small positive charge would be pushed. Field lines point away from positive charges and towards negative charges; closer field lines mean a stronger field. Between two oppositely charged parallel plates, the field is uniform (equally spaced, parallel field lines) except near the edges.

Potential Difference

Potential difference (voltage) between two points is the work done per unit charge moving between them: V = W/Q.

This is the same relationship whether applied to an isolated field (electric potential) or to a circuit (potential difference across a component) — in both cases, it measures energy transferred per unit charge.

Diagrams and formulae

Key diagrams and equations for reviewing Static Electricity, Coulomb's Law and Electric Fields.

• Charging a single sphere by induction: the earth connection is removed first (while the rod is still nearby), and only then is the rod taken away, leaving the sphere charged opposite to the rod.
• Electric field lines point away from positive charges and towards negative charges. Between oppositely charged parallel plates they are evenly spaced and parallel in the central region; they curve near the edges because of fringing.

Charging a single sphere by induction: the earth connection is removed first (while the rod is still nearby), and only then is the rod taken away, leaving the sphere charged opposite to the rod.

Electric field lines point away from positive charges and towards negative charges. Between oppositely charged parallel plates they are evenly spaced and parallel in the central region; they curve near the edges because of fringing.

Worked Example

A step-by-step calculation applying the central relationships in Static Electricity, Coulomb's Law and Electric Fields.

Two point charges, +2 × 10⁻⁶ C and −3 × 10⁻⁶ C, are 0.5 m apart in air. Calculate the force between them. (ε₀ = 8.85 × 10⁻¹² F m⁻¹.)

• F = Q₁Q₂ ÷ (4πε₀d²)
• F = (2 × 10⁻⁶ × 3 × 10⁻⁶) ÷ (4π × 8.85 × 10⁻¹² × 0.5²)
• F = (6 × 10⁻¹²) ÷ (2.78 × 10⁻¹¹) ≈ 0.216 N
• Since the charges have opposite signs, the force is attractive.

Practical / Investigation

Experimental planning, measurement and evaluation for Static Electricity, Coulomb's Law and Electric Fields.

Experiment: Demonstrating charging by friction and induction

• Sources of error: humidity in the air allows charge to leak away over time, reducing the effect; nearby charged objects can interfere with the demonstration.
• Improvement: carry out the demonstration in dry conditions; keep other charged or earthed objects away from the apparatus during the demonstration.

• Observation: the inside test gives little or no deflection, while the outside test gives a clear deflection, supporting the conclusion that excess charge lies on the outer surface.

• For charging by induction, keep the charged rod near the conductor while earthing it; remove the earth connection before removing the rod.
• Use a charged electroscope or known test charge to distinguish attraction caused by polarisation from evidence of charge sign.
• Work in dry conditions, discharge apparatus between trials and keep high-voltage supplies away from this demonstration.

Quick-Check Questions

Short retrieval questions covering the essential ideas in Static Electricity, Coulomb's Law and Electric Fields.

1. State the law of conservation of charge.

2. Why must the earth connection be removed before the charging rod in the induction method?

3. State Coulomb's law in words.

4. Define electric field strength.

5. What is the direction of the electric field around a negative point charge?

Common mistakes and exam tips

Common misconceptions and exam-technique checks for Static Electricity, Coulomb's Law and Electric Fields.

• Avoid: Removing the charged rod before disconnecting the earth during induction. Correct idea: Disconnect the earth while the rod is still nearby, then remove the rod.
• Avoid: Touching the conductor with the charging rod during induction. Correct idea: The rod remains separated; charge flows between the conductor and Earth, not from the rod.
• Avoid: Drawing electric field lines that cross. Correct idea: Field direction is unique at each point, so field lines cannot cross.

• Give induction steps in the correct order.
• After a Coulomb-law calculation, state whether the force is attractive or repulsive.
• Draw field arrows away from positive charge and towards negative charge.

Exam-style questions

Five exam-style questions with worked answers on Static Electricity, Coulomb's Law and Electric Fields.

Further required content

Additional required content for all students. This is not a separate Higher Level requirement.

• Use the separation between the charges for r.
• Like charges repel and unlike charges attract.
• For more than one force, determine each direction before combining the forces.

End-of-unit revision

Formulae, definitions, experiments and mixed revision questions for Static Electricity, Coulomb's Law and Electric Fields.

• F = Q₁Q₂ / (4πε₀d²) (Coulomb's Law)
• E = F/Q
• F = QE
• V = W/Q

• Conservation of charge
• Coulomb's Law
• Electric field strength
• Potential difference

• Charging a sphere by induction, in the correct step order
• Electric field lines around point charges and between parallel plates

• Demonstrating charging by friction and by induction, and testing the resulting charge

• Removing the rod before the earth connection in induction
• Thinking the rod transfers charge directly to the sphere
• Forgetting Coulomb's law forces can attract or repel
• Confusing E (field strength) with V (potential difference)

1. Two charges of +5 × 10⁻⁶ C are placed 0.2 m apart. Calculate the force between them, and state whether it is attractive or repulsive.

2. A charge of 3 × 10⁻⁶ C has 6 × 10⁻⁴ J of work done on it moving between two points. Calculate the potential difference between the points.

3. Explain, using the idea of free electrons, why metals can be charged by induction but plastics cannot.

4. State what happens to the force between two point charges if the distance between them is halved.

1. F = Q₁Q₂/(4πε₀d²) = (5×10⁻⁶)² ÷ (4π × 8.85×10⁻¹² × 0.2²) ≈ 5.62 N; repulsive, since both charges are positive (like charges).

2. V = W/Q = (6×10⁻⁴) ÷ (3×10⁻⁶) = 200 V.

3. Metals contain free (delocalised) electrons that can move through the material in response to a nearby charge, allowing induction to redistribute charge; plastics (insulators) have no free electrons, so charge cannot redistribute in the same way.

4. Since F ∝ 1/d², halving the distance increases the force by a factor of 2² = 4.

The table below shows which equations from this unit appear in the official State Examinations Commission Formulae and Tables book, and where.

The current specification includes Coulomb's law, electric fields and potential difference for all students.