A-Level Physics: Field and Potential Graphs

Gravitational and electric fields are usually taught one after the other, but the most demanding A level questions treat them together. They ask what a potential–distance graph tells you, what the area under a field-strength graph means, how far apart equipotentials are, and how the two kinds of field are alike and where they differ. This year-13 material is built around those links; the separate gravitational fields and electric fields materials in this category cover the basic formulae for each field on its own.

The quiz (12 questions) begins with equipotentials: their spacing between charged parallel plates, why no work is done along them, why field lines cross them at right angles, and why they spread out around a point mass. You then use the potential gradient in both directions. You find a gravitational field strength from a change in potential over a height, find a uniform electric field strength and its direction from two equipotentials, and read the gradient of a V–r graph as the magnitude of g. The area under a g–r graph gives the work needed to move a satellite between orbits. Further calculations use the 1/r dependence of potential at two Earth radii, the work done moving a charge between two potentials near a point charge, the neutral point between the Earth and the Moon, the ratio of electric to gravitational force between two protons, and the speed an electron gains when accelerated through a potential difference. A multiple-answer question sets out the similarities and differences between the two kinds of field.

The flashcards (12 cards) give the potential formulae for point masses and point charges, field strength from potential, work done in a field, equipotential surfaces, E = V/d, the area under a field–distance graph, the nature of each field, the neutral point condition and the inverse-square law.

The written work (8 questions) asks you to describe and explain equipotential patterns, interpret a V–r graph for the Earth and calculate the energy to raise a satellite, compare gravitational and electric fields point by point, explain why gravity controls astronomical motion despite being so weak, locate the Earth–Moon neutral point and discuss the potential there, work through a parallel-plate problem with an electron, estimate a potential change from a g–r graph with the trapezium rule and compare it with the exact value, and analyse the field of a point charge from one potential reading.

The content is based on the fields sections common to A level Physics specifications in England (for example AQA 3.7.2 gravitational fields and 3.7.3 electric fields, including the comparison between them). Constants are given in each question.

• Describe equipotentials in uniform and radial fields and explain why no work is done along them
• Use potential gradient to find gravitational and electric field strengths and their directions
• Interpret the gradient of a V–r graph and the area under a g–r or E–r graph
• Calculate work done moving masses and charges between points of known potential
• Compare gravitational and electric fields, including relative strength and neutral points

Practice material written by Zestly, based on the DfE GCE AS and A level subject content for physics (DFE-00356-2014): gravitational fields, electric fields and their comparison.