Electric Dipole Questions

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Very Short Answers Questions [1 Mark Questions]

Ques. _____will act on an electric dipole if it is placed in a uniform electric field.

1. Force only
2. Both torque and force
3. No force but torque
4. None of the above

Ans. The correct answer is c. No force but torque

Explanation: A dipole, as we know, is a grouping of two opposite and equal charges held at a specific distance apart. If both charges are put in a uniform electric field, they will experience the same but opposing forces, resulting in zero net force on the dipole. However, because equal and opposing forces operate in two distinct places, there will be a net torque operating on the dipole.

Ques. The nature of electric flux from the cube surface, if an electric dipole is placed inside a cube will be?

1. There will be no flux at all
2. The electric flux will be released out of the surface
3. The electric flux will be coming into the surface
4. None of the above

Ans. The correct answer is a. There will be no flux at all

Explanation: Because an electric dipole comprises two-point charges with the same amount of charges but different polarity, the sum of total charges in a dipole is always 0. Furthermore, the total charge inside a surface is proportional to the flux from a closed surface. As the total charge within the cube is zero, there will be no flux entering or releasing towards the surface.

Ques. On which among the following factors does dipole moment depend?

1. Dielectric constant and charge of the medium
2. Length only
3. Length and charge of a dipole
4. Charge only

Ans. The correct answer is c. Length and charge of a dipole

Explanation: A dipole is defined as two equal and opposite charges kept at a small distance apart and possessing a dipole moment. And because the dipole moment is the product of the length of the dipole and the electric charge, it is affected by the length and charge of a dipole.

Ques. Which of the following phenomena is experienced by the dipoles in an electric field?

1. Magnetization
2. Polarisation
3. Interference
4. Electromagnetism

Ans. The correct answer is b. Polarisation

Explanation: Dipoles will experience polarisation in any pure electric field. It is the method of dipole moment alignment in accordance with the applied electric field.

Ques. Which among the following statements best defines a dipole?

1. A combination of two equal and opposite charges separated by a small distance
2. A combination of two unequal and like charges placed at the origin
3. A combination of two unlike and unequal charges parted by a large distance
4. A combination of two like and unequal charges positioned at the origin

Ans. The correct answer is a. A combination of two equal and opposite charges separated by a small distance

Explanation: An electric dipole can be described as a pair of opposite and equal charges separated by a small distance.

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Short Answers Questions [2 Marks Questions]

Ques. What is an electric dipole?

Ans. An electric dipole is a pair of equal and opposing charges +q and -q separated by a relatively small distance. The dipole's total charge is zero, but its electric field is not zero since charges q and -q are separated by some distance and the electric field because of them when added is not zero.

H₂O, HCl, and CH₃COOH are examples of electric dipoles.

Ques. Define electric dipole moment.

Ans. The dipole moment of an electric field is a vector quantity that is given by the product of the magnitude of either charge and the separation between two opposite charges.

The formula for electric dipole moment is given by

p = q x 2l

Where

• p is electric dipole moment
• q is the magnitude of either charge
• 2l is the distance between the two charges (dipole length)

The direction of the dipole moment is from the negative charge to the positive charge along the dipole axis.

Ques. What is the difference between electric dipole and electric dipole moment?

Ans. The separation of two equal and opposing charges separated by a small distance is referred to as an electric dipole, whereas the electric dipole moment is used to calculate the strength of an electric dipole. Because an electric dipole exists, an electric dipole moment exists. It determines the electric polarity of a charge system.

Ques. What is an ideal electric dipole?

Ans. If the charge q increases and the distance 2a decreases, the dipole moment, p = q x 2a, has a finite value, and such a dipole of negligibly small size is termed an ideal or point dipole.

Also Read:

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Long Answers Questions [3 Marks Questions]

Ques. What is the significance of electric dipole and electric dipole moment?

Ans. The physical significance of electric dipole and electric dipole moment is discussed below

• In their ground state, atoms are electrically neutral. Atoms have an equal amount of positive and negative charge. Molecules, like atoms, are neutral, but they also have an equal quantity of positive and negative charges.
• When the algebraic total of all the charges in a system is zero, this does not always imply that the electric field produced by the system is zero everywhere. As a result, studying electric dipoles is vital for understanding electrical phenomena in matter.
• Matter is electrically neutral since it is made up of atoms and molecules. When the centers of mass of positive and negative charges coincide, the molecule functions as a non-polar molecule. If the center of mass of positive charges does not coincide with the center of mass of negative charges, the molecule functions as a polar molecule. These polar compounds have dipole moments that are constant. In the absence of an external electric field, these dipole moments are directed at random. When we expose a material with polar molecules to an external electric field, the molecules align in the direction of the field. This causes the formation of a net dipole moment. This specific substance is believed to be polarised.
• Thus, the study of dipoles and dipole moments provides a measure of a net neutral system's polarisation. The study of dipole moments measures a dipole's tendency to align with an external electric field.

Ques. Two charges each of 2 x 10^-7 C but opposite in sign form a system. These charges are located at points A (0, 0, -10) cm and B (0, 0, 10) cm respectively. What is the total charge and electric dipole moment of the system?

Ans. Since the system contains two equal and opposite charges, the net charge of the system is given by

q_net = (2 x 10^-7) + (- 2 x 10^-7) = 0

Now let the charge qA be located at A (0, 0, -10) cm and the charge qB located at B (0, 0, 10) cm. Therefore

The distance between the two charges, l = 10 + 10 = 20 cm = 20 x 10^-2 m

The dipole moment of an electric dipole is given by

p = magnitude of either charge of the dipole (q) x dipole length (l)

⇒ p = 2 x 10^-7 x 20 x 10^-20 = 4 x 10^-8 C m

Ques. Two charges +0.2 µµC and -0.2 µµC are placed 10^-6 cm apart. Calculate the electric field at an axial point at a distance of 10 cm from their midpoint.

Ans. Given

• The magnitude of the charge of either charge, q = 0.2 µµC = 0.2 x 10^-12 C
• Dipole length, l = 10^-6 cm = 10^-8 m

The dipole moment of an electric dipole is given by

p = magnitude of either charge of the dipole (q) x dipole length (l)

⇒ p = 0.2 x 10^-12 x 10^-8 = 0.2 x 10^-20 C m

The electric field at an axial point at a distance r from the midpoint of the dipole is given by

E = 1/4πϵ₀ (2p/r³)

Given, r = 10 cm = 0.1 m

On substituting the values, we get

E = 9 x 10⁹ (2 x 0.2 x 10^-20 / 0.13) = 3.6 x 10^-8 N/C

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Very Long Answers Questions [5 Marks Questions]

Ques. An electric dipole of dipole moment 4 × 10^-9 C m is placed in a uniform electric field of 5 x 10⁴ NC^-1 making an angle of 30° with the direction of the field. Determine the torque exerted by the electric field on the dipole.

Ans. Given

• Electric dipole moment, p = 4 x 10^-9 C m
• Electric field, E = 5 x 10⁴ NC^-1
• The angle between the electric dipole moment and electric field, θ = 30°

The torque experienced by an electric dipole placed in a uniform magnetic field is given by

τ = pE sinθ

On substituting the values, we get

Torque, τ = 4 x 10^-9 x 5 x 10⁴ x sin30

⇒ τ = 10^-4 N m

Hence the magnitude of the torque exerted by the electric field on the dipole is 10-4 N m.

Ques. If the dipole of moment 2.57 × 10⁻¹⁷ C m is placed into an electric field of magnitude 3.0 × 10⁴ N/C such that the field lines are aligned at 30° with the line joining the dipole, what torque will act on the dipole?

Ans. Given

• Electric dipole moment, p = 2.57 × 10⁻¹⁷ C m
• Electric field, E = 3.0 × 10⁴ N/C
• The angle between the electric dipole moment and electric field, θ = 30°

The torque experienced by an electric dipole placed in a uniform magnetic field is given by

τ = pE sinθ

On substituting the values, we get

Torque, τ = 2.57 × 10⁻¹⁷ x 3.0 × 10⁴ x sin30

⇒ τ = 3.855 x 10^-13 N m

Hence the magnitude of the torque exerted by the electric field on the dipole is 3.855 x 10^-13 N m.

Ques. A dipole consisting of an electron and a proton separated by a distance of 4 × 10^-10 m is situated in an electric field of intensity 3 × 10⁵ NC^-1 at an angle of 30° with the field. Calculate the dipole moment and the torque acting on it. Charge on an electron = 1.602 × 10^-19 C.

Ans. Given

• The magnitude of either charge of the dipole, q = 1.602 × 10^-19 C
• Distance between the charges, l = 4 x 10^-10 m
• Magnitude of electric field intensity, E = 3 x 10⁵ NC^-1
• The angle between the dipole and the electric field, θ = 30°

The dipole moment of an electric dipole is given by

p = magnitude of either charge of the dipole (q) x dipole length (l)

⇒ p = 1.602 × 10^-19 x 4 x 10^-10 = 6.408 x 10^-29 C m

The torque experienced by an electric dipole placed in a uniform magnetic field is given by

τ = pE sinθ

On substituting the values, we get

Torque, τ = 6.408 x 10^-29 x 3 × 10⁵ x sin30

⇒ τ = 9.612 x 10^-24 N m

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