Examstyle Questions: Electric Potential

Fig. 5.1 shows electric equipotential lines between a charged sphere and an earthed plate.
a) On Fig. 5.1, draw an arroe to show the direction of the electric filed at the point labelled X.
(2 Marks)
b) Here is a list of potentials
+50 V
0 V
10 V
50 V
The potential of the sphere is +50 V.
State the best value for the potential of the plate.
(1 Mark)
(Marks available: 3)

This question is about the ground state energy of a hydrogen atom.
A hydrogen atom in its ground state has an energy of 2.2 x 10^{18} J. The most probable seperation between electron and proton is 5.3 x 10^{11} m in this state.
a) The electron has electrical potential energy.
Show that the electrical potential is about +27 V at a distance of 5.3 x 10^{11} m from the proton.
e = 1.6 x 10^{19} C, k = 1/4πε_{0} = 9.0 x 10^{9} N m^{2} C^{2}
(2 Marks)
b) Show that the potential energy of the electron is about 4.3 x 10^{18}J at this distance from the proton.
(2 Marks)
(Marks available: 4)
Answer outline and marking scheme for question:

a) at right angles to euipotential through the point (by eye) pointing away from sphere
ACCEPT curved field lines which have the correct direction at X
(2 Marks)
b) 0 V
(1 Mark)
(Marks available: 3)

a) V = kQ/r
V = 9.0 x 10^{9} x 1.6 x 10^{19}/ 5.3 x 10^{11} = 27.2 V
(2 Marks)
b) Ep = QV
Ep = 1.6 x 10^{19} x 27.2 = 4.35 x 10^{18} J
(2 Marks)
(Marks available: 4)