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LU 1A: ELECTROSTATIC
(Subtopic 1.1 & 1.2)
- State or define the following terms:
a) Coulomb’s Law
b) Electric Field
c) Electric Field Strength
- Figure 1 shows four point charges 𝑄 1
2
3
and 𝑄
4
with 𝑞 = + 1 nC are located on the
corners of a rectangle. The length and width of the rectangle are 60 cm and 15 cm
respectively. Calculate the magnitude and direction of the resultant electric force
exerted on 𝑄
4
[Given Coulomb’s constant, 𝑘 = 9. 0 × 10
9
N m
2
C
―
]
Figure 1
Answer: 4. 83 × 10
− 6
- An electron in a uniform electric field is initially at rest and accelerates to the
north at a rate of 120 m/s
2
. Determine the magnitude and direction of the electric
field?
Answer: 6. 82 × 10
− 10
60 cm
15 cm
Q
4
= + 4 q
Q 3 = + 3 q
Q 2 = + 2 q
Q 1 = + 1
- Figure 2 shows 3 g identical metal balls q 1
and q 2
with unknown positive charge, q
hanging in equilibrium by two identical ropes. The distance between the two metal
balls when they are in equilibrium is 14 cm.
Determine the values of unknown charge, q.
[Given Coulomb’s constant, k = 9.0 x 10
9
N m
2
C
]
Figure 2
Answer: 1. 77 × 10
− 7
5. Q
1
= +35.0 nC and Q 2
= +22.0 nC are placed 4.0 cm apart as shown in Figure 3.
Calculate the resultant electric field strength at their midpoint, point P.
[Given Coulomb’s constant, k = 9.0 x 10
9
N m
2
C
]
Figure 3
Answer: 0. 2925 × 10
6
- Two equal charges q 1
and q 2
of opposite signs are separated by a vertical distance
of 26 cm. The net electric field at the midpoint, point P of the line joining both charges
is. Calculate the magnitude of each charge.
[Given Coulomb’s constant, k = 9.0 x 10
9
N m
2
C
]
Answer: 6. 10 × 10
6
6 1
- 5 10 NC
−
P
Q
1
=+ 35.0 nC Q 2
= + 22.0 nC
4.0 cm
14 cm
16 cm
q 2
q 1
LU 1B: ELECTROSTATIC
(Subtopic 1.3, 1.4 & 1.5)
- Define the following terms:
(a) Electric Potential
(b) Potential Difference between two points
(c) Electric Potential Energy of the system
(d) Equipotential Surface
(a)
Electric potential is defined as the work done in bringing positive test charge from
infinity to that point in the electric field.
(b)
Potential difference between two points in an electric field is defined as the work done
in bringing a positive test charge from a point to another point in the electric field.
(c)
Electric potential energy of the system of point charges is the work done to bring all the
charges from infinity to the points where the charges are placed.
(d)
An equipotential surface is defined as a surface where all points on the surface that
have the same electric potential
- Two charged parallel plates are separated by a distance of 5 cm. The potential difference
between the plates is 2.0 kV. Determine the electric field strength between the plates.
Explain whether the force on an electron in the electric field is affected by distance of
the electron from the negative plate.
Answer: 4 × 10
4
- An electron travelling at speed of 1.6 x 10
7
m s
enters the space between two parallel
plates 70 mm long as shown in Figure 1. Given the electric field between the plates is
4.0 x 10
3
V m
Figure 1
(a) Sketch the path of the electron after emerging from the space between the
plates.
(b) Calculate the magnitude of acceleration of the electron between the plates.
Answer:
14
03 10
Two point charges, Q 1
= +40 μC and Q 2
= - 60 μC respectively, are placed 50 cm apart.
Point P is situated at a point along the line joining the two point charges. Determine
the distance from Q 1
to point P if the resultant potential at point P is zero?
Answer: 0. 2
- Three charged are placed along a straight line as shown in Figure 2. The charges are
q 1
= - 7.5 μC, q 2
= - 2.5 μC and q 3
= - 4.0 μC.
Figure 2
Compute
(a) the electric potential at point A
(b) the electric potential at point B
(c) the work done to move a charge q = 3.2 μC;
i. from point A to point B
ii. from point B to infinity
Answer: −1.94 × 10
6
, −2.69 × 10
6
70 mm
20 mm
4 cm
2 cm
7 cm
3 cm
q
q
q
A
B
- Figure 5 shows two point charges 𝑞 1
= + 3. 4 nC and 𝑞
2
= − 5. 6 nC are 0.10 m apart.
Given point A is midway between them and point B is 0.08 m from 𝑞
1
0.06 m from 𝑞
2
[Given Coulomb constant, 𝑘 = 9 × 10
9
N m
2
C
− 2
]
Figure 5
Calculate
(a) electric potential at point A.
(b) electric potential at point B.
(c)
work done by the electric field on a charge of 2.5 nC that travels from point A to point B.
Answer: − 3. 96 , − 457. 5 , − 1. 54 × 10
− 7
Figure 5
Figure 5 above shows two point chargers, 𝑄
1
= + 2 μC and 𝑄
2
= + 6 μC are separated
by a distance of 15 cm.
(a) Draw the direction of electric field strength at point A due to 𝑄
1
and due to 𝑄
2
(b) Calculate the distance from point charge 𝑄
2
to point A where the resultant
electric field at point A is zero.
Answer: 9. 5
B
A
2
1
A
2
1
