Question 11
Two charged particles of charges 4 nC and
– 4 nC are kept 0.2 m cm apart. What is the
magnitude and nature of the electrostatic force? (Permittivity of free space = 8.85 x 10
-12
F/m)
A. -3.6 x 10
-6
N and attractive
B. -2.7 x 10
-6
N and attractive
C. -6.7 x 10
-6
N and attractive
D. -4.5 x 10
-7
N and repulsive
E. 4.5 x 10
-7
N and attractive
Correct Answer: A
Explanation:
Electrostatic force in a region is given as
F = [(1 / 4πε
o
) (q
1
q
2
/ R)] -------------- (1)
Standard value of 1 / 4πε
o
is 9 x 10
9
Nm
2
C
-2
We are given values of charges in nC so we need to convert them into C.
1 nC = 10
-9
C or -4 nC = - 4x10
-9
C
Putting the values in equation 1 we get
F = 9 x 10
9
x [(4 x 10
-9
* 4 x 10
-9
) / 0.2
2
]
Or F = -3.6 x 10
-6
N
As force is negative, that suggests nature of force is attractive.
Hence, A is the correct answer option.
Question 12
What is the current density of electrons flowing through a steel block of length 1 m, if potential
difference applied across the conductor is 200 V? (Conductivity of steel = 1.45 x 10
6
Ω
-1
m
-1
)
A. 2.9 x 10
8
A/m
2
B. 1.4 x 10
8
A/m
2
C. 2.2 x 10
8
A/m
2
D. 3.4 x 10
9
A/m
2
E. 1.45 x 10
4
A/m
2
Correct Answer: A
Explanation:
Current density is given as
J = σE -------------- (1)
Electric field is given as
E = V / l --------------- (2)
Putting the values in equation 2 we get
E = 200/1 Or E = 200 v/m
Putting the values in equation 1 we get
