Charge and Field | OCR A-Level Physics B (Advancing Physics) (H557)

Charge and Field

123 questions
9 subtopics
The physics content, examined on all three papers
Component 01, Component 02 and Component 03

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.

Sample questions from Charge and Field

Answer each one closed book first, then open the answer.

The uniform electric field

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

Show the answer
F = qE = 1.60 × 10⁻¹⁹ × 5.0 × 10⁴ = 8.0 × 10⁻¹⁵ N.
Radial fields and the inverse square law

Calculate the electric field strength 0.050 m from a point charge of 2.0 × 10⁻⁸ C.

Show the answer
E = kQ/r² = 8.98 × 10⁹ × 2.0 × 10⁻⁸ ÷ (0.050)² = 7.2 × 10⁴ V m⁻¹.
Electric potential and potential energy

Calculate the electrical potential energy of a charge of 3.0 × 10⁻⁹ C placed 0.050 m from a charge of 2.0 × 10⁻⁸ C.

Show the answer
The potential energy is kQq/r = 8.98 × 10⁹ × 2.0 × 10⁻⁸ × 3.0 × 10⁻⁹ ÷ 0.050 = 1.1 × 10⁻⁵ J.
Field and potential graphs

How is the force on a charge found from a graph of its electrical potential energy against distance?

Show the answer
The force is the negative of the gradient of the tangent to the potential energy curve at that distance.
Field lines and equipotential surfaces

What does closely spaced equipotential surfaces tell you about the field there?

Show the answer
Close spacing means the potential changes rapidly with distance, so the field strength is large there.
The electronvolt and accelerating charges

Why is the electronvolt a convenient unit in atomic and particle physics?

Show the answer
Energies of individual particles are tiny fractions of a joule, so using the electronvolt gives numbers of a manageable size and matches the accelerating voltages actually used.
Millikan's experiment and the discreteness of charge

A drop is found to carry a charge of 4.8 × 10⁻¹⁹ C. How many elementary charges does it hold?

Show the answer
Dividing by 1.60 × 10⁻¹⁹ C gives 3, so the drop holds three elementary charges.
Moving charges in magnetic fields

Calculate the force on an electron moving at 2.0 × 10⁶ m s⁻¹ at right angles to a field of flux density 0.30 T.

Show the answer
F = qvB = 1.60 × 10⁻¹⁹ × 2.0 × 10⁶ × 0.30 = 9.6 × 10⁻¹⁴ N.

The 9 subtopics

One subtopic is one session. Work down the list.

Subtopic	What it covers	Questions
The uniform electric field	Recall 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.	14
Radial fields and the inverse square law	Recall 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.	15
Electric potential and potential energy	Recall 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.	15
Field and potential graphs	Recall 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.	13
Field lines and equipotential surfaces	Recall 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.	12
The electronvolt and accelerating charges	Recall 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.	13
Millikan's experiment and the discreteness of charge	Recall 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.	14
Moving charges in magnetic fields	Recall 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.	14
Comparing electric and gravitational fields	Recall 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.	13

Charge and Field is 123 of the 2,455 questions in the guide.Get the guide, £8

How the guide is worked

Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.

Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.

Read the full method, the return schedule and the research behind it.