Chap – 2: Static and Current Electricity
Practice Problems
Some important Physical Constants:
𝑒 = 1.6 × 10−19 𝐶
𝜖0 = 8.85 × 10
−12
𝐶2
𝑁 ∙ 𝑚2
1
𝑁 ∙ 𝑚2
9
= 8.99 × 10
4𝜋𝜖0
𝐶2
Charge and Coulomb’s Law
1.
2.
3.
4.
5.
6.
7.
8.
Calculate the number of electrons that contributes one
Coulomb charge?
Calculate the total electric charge of protons of Avogadro
number (6.023 × 1023 ).
Calculate the force between electron moving in first orbit
and proton situated at nucleus. The radius of first orbit is
0.53 𝐴̇. (1 𝐴̇ = 10−10 𝑚)
A point charge (𝑞1 ) has a magnitude of 3 × 10−6 𝐶. A second
charge (𝑞2 ) has a magnitude of −1.5 × 10−6 𝐶 and is located
0.12 𝑚 from the first charge. Determine the electrostatic
force each charge exerts on the other.
What will be the electrostatic force between the two-point
charges of charges +2μC and +4μC when they are placed
10 m away from each other. What will be the type of this
force?
Determine the electrostatic force between the two charges
of magnitude 2 C and -1 C separated by a distance 1 m in
air. Also decide the type of force.
Consider a system of two charges of magnitude 2 × 10−7 𝐶
and 4.5 × 10−7 𝐶 which is acted upon by a force of 0.1 𝑁.
What is the distance between the two charges?
Determine the magnitude of the two identical charges,
when the electrostatic force between these two identical
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
charges is 1000 N and are separated by a distance of 0.1
m.
A specific charge Q is split into two components, 𝑞, and
𝑄 − 𝑞. What is the relation between 𝑄 and 𝑞 if the two
portions are separated by 𝑟 and have the greatest Coulomb
repulsion?
Two like and equal charges are at a distance of 𝑑 = 5 𝑐𝑚 and
exert a force of 𝐹 = 9 × 10−3 𝑁 on each other. Find the
magnitude of each charge?
A point charge of 4 𝜇𝐶 is 3 cm apart from the charge 1 𝜇𝐶.
(a) Find the magnitude of the Coulomb force which one
particle
exerts
on
the
other.
(b) Is the force attractive or repulsive?
What is the magnitude of the force that a 25 𝜇𝐶 charge
exerts on a −10 𝜇𝐶 charge at 8.5 𝑐𝑚 away?
Two point charged particles are 4.41 cm apart. They are
moved and placed in a new position. The force between
them is found to have been tripled. How far apart are they
now?
Two point-charges 𝑞1 = +2 𝜇𝐶 and 𝑞2 = +8 𝜇𝐶 are 30 𝑐𝑚
apart from each other. Another charge 𝑄 is placed so that
the three charges are brought to a balance. What is the
magnitude and sign of the charge 𝑄?
Two point-charges of 𝑞1 = +2 𝜇𝐶 and 𝑞2 = −8 𝜇𝐶 are at a
distance of 10 cm. Where must a third charge 𝑞3 be placed
so that the net Coulomb force acted upon it is zero?
Two balloons are charged with an identical quantity and
type of charge: −0.0025 𝐶. They are held apart at a
separation distance of 8 𝑚. Determine the magnitude of the
electrical force of repulsion between them.
Two charged boxes are 4 meters apart from each other. The
blue box has a charge of +0.000337 𝐶 and is attracting the
red box with a force of 626 Newtons. Determine the charge
of the red box. Remember to indicate if it is positive or
negative.
A piece of Styrofoam has a charge of −0.004 𝐶 and is placed
3.0 𝑚 from a comb with a charge of −0.003 𝐶. How much
electrostatic force is produced? Identify the type of force
produced.
19. Two coins lie 1.5 𝑚 apart on a table. They carry identical
electric charges. Approximately how large is the charge on
each coin if each coin experiences a force of 2.0 𝑁?
20.The total number of electrons in the human body is typically in the order
of 1028 . Calculate the electrostatic force between you and your friend
separated at a distance of 1 m.
21.
CAPACITOR & CAPACITANCE
1. A capacitor consists of two circular metal plates, each with a radius of 5
cm. The plates are parallel to each other and separated by a distance of 1
mm. You connect a 9 volt battery across the plates. Find: the capacitance
of the capacitor, the charge on each plate.
2. Determine the amount of charge stored on either plate of a capacitor of
4 × 10−6 𝐹 when connected across a 12 volt battery.
3. A capacitor of capacitance 5 𝜇𝐹 is connected to a 6 V supply. What
charge is stored in the capacitor?
4. A 400 𝑝𝐹 capacitor carries a charge of 2.5 × 10−8 𝐶. What is the
potential difference across the plates of the capacitor?
5. A parallel plate capacitor is constructed of metal plates, each with an area
of 0.2 𝑚2 . The capacitance is 7.9 𝑛𝐹. Determine the plate separation
distance.
6. If the plate separation for a capacitor is 2.0 × 10−3 𝑚, determine the area
of the plates if the capacitance is exactly 1 F.
7. Calculate the voltage of a battery connected to a parallel plate capacitor
with a plate area of 2.0 𝑐𝑚2 and a plate separation of 2 𝑚𝑚 if the charge
stored on the plates is 4.0 𝑝𝐶
8. A typical capacitor in a memory cell may have a capacitance of 3 ×
10−14 𝐹. If the voltage across the capacitor is 0.5 𝑣𝑜𝑙𝑡, determine the
number of electrons that must move on the the capacitor to charge it.
C
Q
V
9. A 4700 𝜇𝐹 capacitor is connected to
4.5 volt battery. When it is fully
charged: (a) what is the charge on the
positive plate of the capacitor?
(b)what is the potential difference
across the capacitor?
10.
Fill
the
blanks
by
1000 F
12 V
2 mC
32 F
0.64 mC
33 mF
6V
1.25 C
10 mF
attempting
250 V
25 kV
2.5 C
necessary
calculations.