Test: Electric Flux & Electric Dipole (NCERT)
10 Questions MCQ Test Physics Class 12 | Test: Electric Flux & Electric Dipole (NCERT)
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As electric flux, φ = E.Δs
∴ unit of φ is N C-1 m2
The dimensional formula of electric flux is
Electric flux, φ= 
The dimension of φ = Dimension of E x dimension of s
[φ] = [M1L1T-2][AT]-1[L2] = [M1L3T-3A-1]
A circular plane sheet of radius 10 cm is placed in a uniform electric field of 5 x 105N C-1, making an angle of 600 with the field. The electric flux through the sheet is
Here, r = 10 cm 0.1 m, E = 5 x 105 N C-1
As the angle between the plane sheet and the electric field is 600, angle made by the normal to the plane sheet and the electric field is θ = 900 - 600 = 300
φE = ES cos θ = E x πr2 cos θ
= 5 x 105 x 3.14 x (0.1)2 cos 300 = 1.36 x 104 N m2C-1
A uniform electric field E = 2 x 103 N C-1 is acting along the positive x-axis. The flux of this field through a square of 10cm on a side whose plane is paralled to the yz plane is
Here, E = 2 x 103 N C-1 is along + x-axis
Surface area, s = (10 cm)2 = 102 x 10-4 m2 = 10-2 m2
When plane is parallel to yz plane, θ = 0°
So φ = E s cosθ = 2 x 103 x 10-2 cos 0° = 20N C-1 m2
In the question number 66, the flux through the same square if the line normal to its plane makes a 60° angle with the x-axis is
When normal to the plane makes an angle of 60° with x-axis, then θ = 60°
φ = Es cosθ = 2 x 103 x 10-2 cos 60° = 10 N C-1N C-1m2
Which of the following statements about dipole moment is not true?
Dipole moment is a vector quantity and has magnitude of 2qa and it is in the direction of the dipole axis from -q to q.
The unit of electric dipole moment is
The unit of electric dipole moment is: 2ql = C − m = Debye
Two charges + 20 μC and -20 μC are placed 10 mm apart. The electric field at point P, on the axis of the dipole 10 cm away from its centre O on the side of the positive charge is
Here, q = ±20μC = ±20 x 10-6C, 2a = 10 mm = 10 x 10-3m
r = OP = 10 cm = 10 x 10-2 m
= q x 2a = 20 x 10-6 x 10 x 10-3 m = 2 x 10-7 m
The electric field along BP, 
As a < < r,
Consider a region inside which, there are various types of charges but the total charge is zero. At points outside the region
When there are various types of charges in a region, but the total charge is zero, the region can be supposed to contain a number of electric dipoles. Therefore, at points outside the region, the dominant electric field ∝ (1/r3) for large r.
Two point charges of 1μC and -1μC are separated by a distance of 100 Å. A point P is at a distance of 10 cm from the midpoint and on the perpendicular bisector of the line joining the two charges. The electric field at P will be
The Point lies on equatorial line of a short dipole,
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