Charge and Field

What Charge and Field covers

Charge and Field is examined in all three written papers — the specification states that Components 01, 02 and 03 each assess content from across all the teaching modules, so nothing is confined to one paper. It covers the uniform electric field, radial fields and the inverse square law, electric potential and potential energy, field and potential graphs, field lines and equipotential surfaces, the electronvolt and accelerating charges, Millikan's experiment and the discreteness of charge, moving charges in magnetic fields and comparing electric and gravitational fields.

The 9 subtopics

Each one has its own page.

• The uniform electric fieldRecall questions on electric field strength E = F/q and E = V/d between parallel plates, V m⁻¹ and N C⁻¹, field, force and acceleration calculations for an electron, the field's direction and pattern, changing the plate separation, work done W = qV and kinetic energy gained, parabolic beam paths, and deflection with doubled voltage.Open →
• Radial fields and the inverse square lawRecall questions on F = kqQ/r² and E = kQ/r², the electric force constant and the permittivity of free space ε₀, force and field calculations, attraction and repulsion, tripling the separation, field patterns around a negative charge and a pair of opposite charges, charged spheres as point charges, zero field inside a hollow conductor, surface charge, and sharp points.Open →
• Electric potential and potential energyRecall questions on defining potential and the energy of a charge pair, why zero sits at infinity, signs around a negative charge, work done moving a charge, and removing one to infinity.Open →
• Field and potential graphsRecall questions on the areas under field– and force–distance graphs, field or force from a gradient, shapes for a point charge and a charged sphere, and uniform fields between plates.Open →
• Field lines and equipotential surfacesRecall questions on equipotential surfaces and why field lines cross them at right angles, equipotentials around a point charge and between plates, what close spacing shows, where field lines begin and end and why they never cross, conductor surfaces as equipotentials, the field of two equal positive charges, the neutral point for unequal charges, and the contour-line analogy.Open →
• The electronvolt and accelerating chargesRecall questions on what the unit means, its size in joules, converting energies, speeds gained by electrons and alpha particles, why plate separation does not matter, and MeV and GeV.Open →
• Millikan's experiment and the discreteness of chargeRecall questions on what the oil drop experiment showed, its apparatus, q = mgd/V for a stationary drop and a worked calculation, counting elementary charges, finding a drop's mass from terminal velocity, why oil is used, how drops are charged, the evidence for quantised charge, the elementary charge, measuring many drops, balancing a drop, and a doubtful result.Open →
• Moving charges in magnetic fieldsRecall questions on the force equation and its condition, its direction, why the path curves and no work is done, radius and period calculations, crossed fields, and particles spiralling towards the poles.Open →
• Comparing electric and gravitational fieldsRecall questions on what the two inverse-square fields share, their units, the force ratio between two protons, why only one can be shielded, and why gravity still governs planets.Open →

What you need to know

Answer each one before you reveal it.

Take Charge and Field further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

• Electric field strength E vs Electric potential V

  Electric field strength E: the force per unit charge at a point, a vector quantity measured in N C⁻¹ that falls away as 1/r² from a point charge. Electric potential V: the work done per unit charge in bringing a charge to that point from infinity, a scalar quantity measured in volts that falls away as 1/r.

• Electrical potential energy vs Electric potential

  Electrical potential energy: what a pair of charges jointly hold, equal to kQq/r, measured in joules and taken as zero at infinite separation. Electric potential: what a single point in space is at, equal to kQ/r, measured in volts and equal to one joule per coulomb.

• Force on a charge in an electric field vs Force on a charge in a magnetic field

  Force on a charge in an electric field: F = qE, acting along the line of the field whether the particle happens to be moving or not. Force on a charge in a magnetic field: F = qvB, acting only while the particle moves across the field, and at right angles to both the velocity and the field.

Questions from this topic

Each one with the model answer underneath.

1. 1

   Define electric field strength.

   Model answer

   Electric field strength is the force per unit positive charge acting at a point, E = F/q, and it is a vector in the direction of the force on a positive charge.

2. 2

   Write the equation for the field strength between two parallel plates.

   Model answer

   For a uniform field between parallel plates E = V/d, where V is the potential difference and d the separation.

3. 3

   Show that the volt per metre is equivalent to the newton per coulomb.

   Model answer

   One volt is one joule per coulomb, so V m⁻¹ is J C⁻¹ m⁻¹, and since a joule is a newton metre this becomes N C⁻¹.

4. 4

   Two parallel plates 5.0 mm apart have a potential difference of 250 V across them. Calculate the field strength between them.

   Model answer

   E = V/d = 250 ÷ 5.0 × 10⁻³ = 5.0 × 10⁴ V m⁻¹.

5. 5

   Calculate the force on an electron in a uniform field of strength 5.0 × 10⁴ V m⁻¹.

   Model answer

   F = qE = 1.60 × 10⁻¹⁹ × 5.0 × 10⁴ = 8.0 × 10⁻¹⁵ N.

6. 6

   In which direction does the electric field point between two charged parallel plates?

   Model answer

   The field points from the positive plate towards the negative plate, which is the direction of the force on a positive charge.

What is inside Levo

Six ways to work the same material.

Charge and Field revision questions

What does Charge and Field cover in OCR A-Level Physics B?

9 subtopics: the uniform electric field, radial fields and the inverse square law, electric potential and potential energy, field and potential graphs, field lines and equipotential surfaces, the electronvolt and accelerating charges, millikan's experiment and the discreteness of charge, moving charges in magnetic fields and comparing electric and gravitational fields. Each one has its own page with questions and model answers.

How is Charge and Field organised?

By the OCR H557 specification, subtopic by subtopic, so it matches the order you are taught it in.