Electric potential and field patternsEdexcel A-Level Physics: Subtopic test

1. 1
   Two large parallel metal plates are 0.040 m apart in a vacuum. A potential difference of 600 V is maintained across them. Away from the edges of the plates the field between them can be treated as uniform.

   (a)

   Which statement describes the electric field lines in the central region between the plates?

   [1 mark]

   • AParallel to each other, equally spaced and at right angles to the plates
   • BCurved, spreading out from the positive plate
   • CParallel to the plates and equally spaced
   • DRadial, closer together near the positive plate

   (b)

   Which statement describes the equipotentials in the central region between the plates?

   [1 mark]

   • APlanes at right angles to the plates
   • BPlanes parallel to the field lines
   • CConcentric spheres centred on the positive plate
   • DEqually spaced planes parallel to the plates, at right angles to the field lines

   (c)

   Calculate the electric field strength between the plates and the magnitude of the force on an electron in the field.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A small sphere carries a charge of +8.0 nC. It may be treated as a point charge in a vacuum. Point P is 0.30 m from the centre of the sphere.

   (a)

   Which statement about the electric potential at P is correct?

   [1 mark]

   • AIt is negative, because the field points away from the sphere
   • BIt is zero, because the field is radial
   • CIt is positive, and it would decrease if P were moved further from the sphere
   • DIt is positive, and it would increase if P were moved further from the sphere

   (b)

   P is replaced by a point 0.60 m from the sphere. By what factor does the potential change compared with its value at 0.30 m?

   [1 mark]

   • AIt is one quarter
   • BIt is one half
   • CIt is doubled
   • DIt is unchanged

   (c)

   Calculate the electric potential at P.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A tiny oil droplet of mass 2.6 × 10⁻¹⁵ kg carries a negative charge. It is held at rest between two horizontal parallel plates, 15 mm apart, by a potential difference of 480 V across the plates.

   (a)

   Calculate the electric field strength between the plates and state which plate is at the higher potential.

   [3 marks]

   (b)

   Calculate the charge on the droplet, and show that it is about five times the elementary charge.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Two small spheres carry charges of +6.0 nC and −6.0 nC. They are fixed in a vacuum with their centres 0.40 m apart. Point M is at the midpoint of the line joining the centres. The spheres may be treated as point charges.

   (a)

   Calculate the electric potential and the electric field strength at M, stating the direction of the field.

   [6 marks]

   (b)

   A student says that the field strength at M must be zero because the potential at M is zero. Evaluate this statement.

   [6 marks]

   Total for question 4: 12 marks

End of questions