Practise › Questions › Coulomb's law and electric field strength
Coulomb's law and electric field strength questions
Charge gets its own inverse-square law and its own field strength. It also brings a trick gravity never showed you, an almost perfectly uniform field between two plates, where a moving charge replays projectile motion with the field cast as gravity.
19 original questions · 52 marks · the coulomb's law and electric field strength notes · Electric fields
Every question here is written for this library rather than taken from a past paper. Write your answer out before opening a mark scheme: the schemes award marks point by point, and the marks are easier to see when you have something of your own to compare against.
State Coulomb's law for the force between two point charges.
Mark scheme
The force is proportional to the product of the charges (1); and inversely proportional to the square of their separation, F = kQ1Q2/r2 where k = 1/4πε0 (1).Two point charges, each of +2.0 μC, are 0.10 m apart. Calculate the force between them.
k = 8.99 × 109 N m2 C−2Mark scheme
F = kQ1Q2/r2 = (8.99 × 109 × (2.0 × 10−6)2)/0.102 (1)
F = 3.60 N (repulsive) (1)A charge of 2.0 × 10−6 C experiences a force of 6.0 × 10−3 N at a point in an electric field. Calculate the electric field strength at that point.
Mark scheme
E = F/Q = (6.0 × 10−3)/(2.0 × 10−6) (1)
E = 3000 N C−1 (1)The electrostatic force between two point charges is F. The separation of the charges is doubled. State the new force in terms of F.
Mark scheme
F/4, because the force follows an inverse-square law (1).Electric field strength can be given in N C−1 or in V m−1. Show that these two units are equivalent.
Mark scheme
1 V = 1 J C−1, so 1 V m−1 = 1 J C−1 m−1 (1). 1 J = 1 N m, so 1 J C−1 m−1 = 1 N m C−1 m−1 = 1 N C−1 (1).Two charged conducting spheres, each of radius 2.0 cm, are placed with their surfaces 4.0 cm apart. State the separation r that should be used in Coulomb's law for the force between them, and the property of charged spheres that justifies it.
Mark scheme
r = 8.0 cm (0.080 m), the distance between the centres (1). A charged sphere behaves as if all its charge were concentrated at its centre (1).In a hydrogen atom the electron and the proton are separated by 5.3 × 10−11 m. Calculate the electrostatic force between them.
e = 1.60 × 10−19 CMark scheme
F = ke2/r2 = (8.99 × 109 × (1.60 × 10−19)2)/(5.3 × 10−11)2 (1)
F = 8.19 × 10−8 N (1)Two parallel plates separated by 0.050 m have a potential difference of 2000 V across them. Calculate the electric field strength between the plates and the force on a charge of 3.0 × 10−6 C placed between them.
Mark scheme
E = V/d = 2000/0.050 = 40000 V m−1 (2)
F = EQ = 40000 × 3.0 × 10−6 = 0.12 N (1)Calculate the electric field strength at a distance of 0.20 m from a point charge of 5.0 × 10−9 C.
A +3.0 μC charge and a −2.0 μC charge are 0.15 m apart. Calculate the magnitude of the force between them and state whether it is attractive or repulsive.
Determine the separation at which the electrostatic repulsion between two protons is 2.3 × 10−4 N.
k = 8.99 × 109 N m2 C−2, e = 1.60 × 10−19 CAt a distance of 0.15 m from the centre of a small charged sphere, the electric field strength is 8.0 × 103 N C−1. Determine the charge on the sphere (k = 8.99 × 109 N m2 C−2).
The charge on each of two point charges is doubled and their separation is halved. Determine the factor by which the electrostatic force between them changes.
An electron enters a uniform electric field of strength 1.0 × 104 N C−1. Calculate the acceleration of the electron.
e = 1.60 × 10−19 C, me = 9.11 × 10−31 kgA point charge of 8.0 × 10−9 C is fixed in position. Calculate the electric field strength at a point 0.30 m from the charge, and hence the force on a +2.0 × 10−9 C charge placed at that point.
A charged particle enters a uniform electric field at right angles to the field lines. Describe the path of the particle in the field and explain why it has this shape.
An electron enters the uniform field between two horizontal parallel plates, midway between them and moving parallel to them at 2.0 × 107 m s−1. The plates are 40 mm long and 20 mm apart, with a pd of 80 V across them. Assume that gravitational forces are negligible. Determine the deflection of the electron, perpendicular to the plates, by the time it leaves the field.
e = 1.60 × 10−19 C, me = 9.11 × 10−31 kgAir conducts, producing a spark, wherever the electric field strength exceeds 3.0 × 106 V m−1. The dome of a school Van de Graaff generator is a sphere of radius 15 cm carrying a charge of 8.0 μC. Deduce whether the air at the surface of the dome will break down.
k = 8.99 × 109 N m2 C−2Describe the electric field (a) between two oppositely charged parallel plates and (b) around an isolated positive point charge. In each case state how the field strength varies from place to place.
The same practice on paper: the printable workbook for this topic, questions and a worked answer book.
Practise coulomb's law and electric field strength one question at a time
The player marks nothing for you. It shows one question, waits, then shows the scheme so you can mark yourself, and brings a question back sooner when it went badly.