Electric field strength (A-level only) — AQA A-Level Physics

Electric field strength is a fundamental concept in A-level Physics that quantifies the force experienced by a unit positive charge in an electric field. It builds on earlier work on electric fields and Coulomb's law, extending to both uniform fields between charged parallel plates and radial fields around point charges. Understanding E allows you to analyse motion of charged particles, capacitors, and the behaviour of fields in various contexts.

This topic is crucial for later studies of electric potential, capacitance, and the motion of charged particles in fields. It also underpins many practical applications, from particle accelerators to inkjet printers. Mastery of electric field strength is essential for tackling synoptic questions that combine electricity with mechanics and energy.

• →Electric field strength E is defined as the force per unit positive charge: E = F/Q, measured in N C^-1. It is a vector quantity with direction given by the force on a positive charge.
• →For a uniform field between parallel plates, E = V/d, where V is the potential difference and d is the perpendicular separation. The field is constant in magnitude and direction.
• →For a radial field around a point charge Q, E = kQ/r^2, where k = 1/(4πε0) ≈ 8.99 × 10^9 N m^2 C^-2. The field follows an inverse-square law.
• →Electric field lines are used to represent fields: they point in the direction of the force on a positive charge, and their density indicates field strength.
• →The force on a charge q in an electric field is F = qE. For a negative charge, the force is opposite to the field direction.

• Draw field lines with arrows showing the direction of force on a positive test charge.
• Show radial field lines around an isolated point charge, with spacing increasing with distance to indicate decreasing field strength.
• Draw straight, parallel, equally spaced lines between oppositely charged parallel plates to represent a uniform field.
• State that field lines never cross because the electric field has a unique direction at any point.
• Use line spacing or density to compare field strength in different regions of a diagram.
• Defines electric field strength as force per unit positive charge: E = F/Q, with E in N C⁻¹ or V m⁻¹.
• States that electric field strength is a vector quantity and gives its direction as the direction of the force on a positive test charge.
• Applies E = kQ/r² for the field of a point or spherical charge, using k = 1/(4πε₀) and r as distance from the centre.
• Uses E = V/d for a uniform field between parallel plates, recognising that V is the potential difference across the plates and d is their separation.
• Combines electric fields from two or more charges by vector addition, including cases where fields are equal and opposite and cancel to zero.
• Uses correct units and prefixes, converting distances to metres and charges to coulombs before substitution.

• 💡Always add arrowheads to field lines, as direction is part of the representation.
• 💡Use a ruler for parallel-plate fields so lines are straight and evenly spaced.
• 💡When comparing field strengths, refer explicitly to the spacing or density of the lines you have drawn.
• 💡Write the defining equation E = F/Q before substituting values, and include the direction of E in your final answer where the question asks for it.
• 💡For point charges, check whether the field is due to one charge or several; if several, draw arrows to show each field direction before adding them.
• 💡Keep units consistent: convert cm to m, μC to C, and nC to C before calculating, and give the unit N C⁻¹ or V m⁻¹.
• 💡Always write the definition of electric field strength precisely: 'force per unit positive charge'. Missing 'positive' or 'per unit' loses marks.
• 💡When calculating E using E = V/d, ensure d is in metres and is the perpendicular distance between the plates. Double-check unit conversions.
• 💡For radial fields, remember that E follows an inverse-square law. If distance doubles, E quarters. Use this to check your calculations.

• Drawing arrows pointing towards a positive charge; correct by directing arrows away from positive and towards negative charges.
• Showing field lines crossing near a charge; correct by ensuring each point has only one field direction.
• Drawing curved lines between parallel plates; correct by using straight, parallel, equally spaced lines for a uniform field.
• Treating electric field strength as a scalar and adding magnitudes without direction; correction: resolve fields as vectors and state the resultant direction.
• Using the distance between two charges instead of the distance from the charge to the point when applying E = kQ/r²; correction: identify r as the separation between the source charge and the point where E is required.
• Confusing electric field strength with electric potential or force; correction: remember E is force per unit positive charge, while potential is energy per unit charge, and force is QE.
• Students often think electric field strength and electric potential are the same thing. In fact, E is force per unit charge (a vector), while potential is energy per unit charge (a scalar).
• Many believe that in a uniform field, E varies with distance from the plates. Actually, E is constant between the plates (ignoring edge effects).
• Some students forget that E is a vector and only give magnitude when asked to state the field. Always include direction, e.g., 'towards the negative plate'.

1. 1Day 1-2: Revise the definition of electric field strength and the formulas E = F/Q, E = V/d, and E = kQ/r^2. Learn the units and vector nature.
2. 2Day 3-4: Practice calculations for uniform and radial fields, including unit conversions and direction. Use past paper questions.
3. 3Day 5-6: Study electric field lines and how they represent field strength and direction. Draw field patterns for point charges and parallel plates.
4. 4Day 7-8: Work through exam-style questions, focusing on definitions, calculations, and explanations. Review mark schemes to understand expectations.
5. 5Day 9-10: Consolidate with mixed problems and synoptic questions. Create flashcards for key formulas and definitions.

• 📋Definition and explanation questions: e.g., 'Define electric field strength and state its unit.' Advice: Use precise wording and mention vector nature.
• 📋Calculation questions: e.g., 'Calculate the electric field strength at a point due to a point charge.' Advice: Show all steps, convert units, and give direction if asked.
• 📋Uniform field problems: e.g., 'Calculate the force on a charged particle between parallel plates.' Advice: Use E = V/d and F = qE, and consider the sign of the charge for direction.
• 📋Graph and field line interpretation: e.g., 'Sketch the electric field lines between two point charges.' Advice: Ensure lines are directed correctly and spacing indicates relative strength.