22.2 Coulomb’s Law and Field Strength
1. This question is about electric forces.
A very small negatively-charged conducting sphere is suspended by an insulating thread
from support S. It is placed close to a vertical metal plate carrying a positive charge. The
sphere is attracted towards the plate and hangs with the thread at an angle of 20
degrees to the vertical as shown in fig 1.1.
fig 1.1
a) Draw at least five field lines on fig 1.1 to show the pattern of the field between the
plate and the sphere.
(3)
b) The sphere of weight 1.0 x 10-5N carries a charge of –1.2 x 10-9C.
i) Show that the magnitude of the attractive force between the sphere and the plate is
about 3.6 x 10-6N.
(3)
ii)
Hence show that the value of the electric field strength at the sphere, treated as
a point charge, is 3.0 x 103 in SI units. State the unit.
Unit: ……………… (3)
c) The plate is removed. Fig 1.2 shows an identical sphere carrying a charge of +1.2 x
10-9C, mounted on an insulating stand. It is placed so that the hanging sphere
remains at 20 degrees to the vertical.
fig 1.2
Treating the spheres as point charges, calculate the distance r between their centres.
r = ………………….m (3)
d) On fig 1.2, sketch the electric field pattern between the two charges. By comparing
this sketch to your answer to (a), suggest why the distance between the plate and
the sphere in fig1.1 is half of the distance between the two spheres in fig 1.2.
(2)
2. a) Define electric field strength at a point in space.
(2)
Fig 2.1 shows two point charges of equal magnitude, 1.6 x 10-19C, and opposite sign,
held a distance 8.0 x 10-10m apart at points A and B. The charge A is positive.
b)
8.0 x 10-10m
A
i)
ii)
B
Fig 2.1
On Fig 2.1, draw the electric field lines to represent the field in the region around
the two charges.
(3)
Calculate the magnitude of the electric field strength at the mid point between
the charges. Give a suitable unit for your answer.
Electric field strength = ……………………….. unit ……………….. (5)
c)
Imagine two equal masses, connected by a light rigid link, carrying equal but
opposite charges. This is a system called a dipole. Fig 2.2 and 2.3 show the dipole
placed in different orientations between two uniformly and oppositely charged
plates.
fig 2.2
fig 2.3
Any effects of gravity are negligible.
i)
ii)
Describe the electric forces acting on the charges by drawing suitable arrows on
the diagrams.
(2)
Explain the motion, if any, of the dipole when it is released from rest
in fig 2.2
(1)
in fig 2.3
(2)