Electric Dipole: Overview, Questions, Preparation

Current Electricity 2024

Salviya Antony

Salviya AntonySenior Executive - Content

Updated on Aug 25, 2023 16:13 IST

An electric dipole is an important concept in the chapter Electric charges and fields. It plays an important role in understanding the behaviour of electric charges and their interactions in various physical systems. This concept is extensively explored in Class 12 physics, providing students with insights into the principles underlying electric fields and their applications.

Electric Dipole: Definition

We can define an electric dipole as a pair of equal and opposite electric charges separated by a certain distance. This charge separation results in the creation of an electric dipole moment, represented by the symbol "p". The dipole moment is a vector quantity pointing from the negative charge to the positive charge and is given by the product of the magnitude of either charge (q) and the separation distance (d) between them.

p = q⋅d

The direction of electric dipoles in space is always from negative charge “-q” to positive charge “q”. The midpoint “q” and “–q” is known as the centre of the dipole. A pair of electric charges of two opposite signs and equal magnitude separated by distance, is an example of electric dipole. 

Characteristics of Electric Dipole are given below.

  • Magnitude of Electric Dipole Moment: The magnitude of the electric dipole moment quantifies the strength of the dipole. Greater charge magnitude or increased separation distance leads to a larger dipole moment.
  • Direction of Electric Dipole Moment: The direction of the dipole moment is from the negative charge to the positive charge, following the path of the electric field lines.
  • Net Charge of an Electric Dipole: An electric dipole as a whole is neutral. i.e it has no net charge. This is because the positive and negative charges cancel out each other.

Behaviour in Electric Fields

When placed in an external electric field, an electric dipole experiences a torque that tends to align the dipole moment with the direction of the electric field. The torque (τ) acting on the dipole is proportional to the product of the dipole moment (p) and the electric field strength (E), and the sine of the angle (θ) between the dipole moment and the electric field direction:

We can write τ = p⋅E⋅sin(θ)

This torque attempts to align the dipole in the direction of the electric field, resulting in two equilibrium positions: one when the dipole moment is aligned with the field (stable equilibrium) and another when it is perpendicular to the field (unstable equilibrium).

Electric Potential due to an Electric Dipole

At different points in space, an electric dipole will create an electric potential which is the sum of potentials due to its positive and negative charges. The electric potential (V) due to an electric dipole at a distance (r) from the centre of the dipole is given by gthe equation V = k⋅p⋅cos(θ)/r2

Here, k denotes the electrostatic constant 

θ denotes the angle between the dipole moment and the line joining the dipole to the point of interest.

When θ = 0, V = kp/r2

When θ = 900, V = 0

Electric Dipole: Applications

Some applications of Electric dipoles are given below.

  • Electromagnets: Electric dipoles play a role in the creation of electromagnets, where the alignment of molecular dipoles contributes to the overall magnetic behavior.
  • Microwave Ovens: The rotation of water molecules in food due to an oscillating electric field, a result of the dipole nature of water, leads to efficient heating in microwave ovens.
  • Dielectric Materials: In capacitors and dielectric materials, the alignment of electric dipoles in response to an external electric field increases the capacitance and reduces the electric field within the material.

The concept of an electric dipole is a foundational building block in understanding the behaviour of charges and electric fields. Its properties, behavior in electric fields, and applications in various real-world scenarios make it an important topic for Class 12 students studying Electric charges and fields.

FAQs on Electric Dipole

 

Q: What is the force between two small charged spheres having charges of 2 ×10^(-7) C and 3 × 10^(-7)C placed 30 cm apart in air?

A: Charge of the first sphere, q1 = 2 × 10-7 C

Charge of the second sphere,q2  = 3 × 10-7 C

Distance between the spheres, r = 30 cm = 0.3 m

Electrostatic force between the spheres is given by the relation,

F = 1/4π∈0 X q1q2 / r2, where ∈0 = Permittivity of free space = 8.854 X 10-12 C2N-1m-2

Hence, F = 1 / 4Xπ x 8.854 X 10-12 X 2 × 10-7 X 3 × 10-7 / (0.3)2 = 5.99 X 10-3 N

Therefore the force between the two small charged spheres is 5.99 X 10-3  N. The charges are of same nature. Hence, force between them will be repulsive.

Q: In a certain region of space, electric field is along the z-direction throughout. The magnitude of electric field is, however, not constant but increases uniformly along the positive z-direction, at the rate of 10^5 NC^(-1) per meter. What are the force and torque experienced by a system having a total dipole moment equal to 10^(-7) Cm in the negative z-direction?

A: Dipole moment of the system, p = q Xdl = 10-7 Cm

Rate of increase of electric field per unit length, dE/dl  = 105 NC

Force experienced by the system is given by the relation, F = qE = q  X dE/dl X dl

= (q X dl) X dE/dl = p X dE/dl = -10-7 X105   = -10-2 N

The force is  -10-2N in the negative z-direction i.e. opposite to the direction of electric field. Hence, the angle between electric field and dipole moment is 180 .

Torque (ζ ) is given by the relation, ζ = pE Sin 180°  = 0

Therefore, the torque experienced by the system is zero.

Q: Suppose that the particle in Exercise in 1.33 is an electron projected with velocity v_x = 2.0 × 10^6 m s^–1. If E between the plates separated by 0.5 cm is 9.1 × 10^2 N/C, where will the electron strike the upper plate? (|e|=1.6 × 10^–19 C, me = 9.1 × 10^–31 kg.)

 

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