Electric Dipole[class-12]

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Electric Dipole

An electric dipole is a pair of equal and opposite point charges q and –q, separated by a distance of 2a.

• Direction from –q to q is the direction of the dipole.
• The mid-point of locations of –q and q is called the center of the dipole.
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• Total charge of an electric dipole is zero but since the charges are separated by some distance the electric field do not cancel out.
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• Dipole moment is the mathematical product of the separation of the ends of a dipole and the magnitude of the charges (2a x q).
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• Some molecules like H2O, have permanent dipole moment as their charges do not coincide. These molecules are called polar molecules.
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• Permanent dipoles have a dipole moment irrespective of any external Electric field.

Electric field of an electric dipole

Electric field of an electric dipole at a point in space depends upon the position of the point.

• The dipole field at a point is inversely proportional to the cube of distance from the center to the point.
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• For a very small dipole, the 2a approaches zero. This is called point dipole.
• Example:
  To calculate electric field at mid point of line joining two charges 3 & -3 μC, which are 20 cm apart, we can use the formla for electric field of a dipole on an axis.
  
• Dipole in a uniform electric field
  When two charges in a dipole are separated by some distance, the forces acting at different points result in torque on the dipole.•    
  • Magnitude of torque = qE x 2a sinθ = 2qaE sinθ = pEsinθ.
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  • Direction of the torque= normal to the plane coming outwards.
  
  
  
  
  
  
  
• The torque tries to align the dipole with electric field. Once aligned, the torque becomes 0.
• Continuous Charge Distribution
  
  

  Type of charge distribution	Denoted by	Value	Unit
  Line Charge	λ (Line charge density)	ΔQ/Δl, Δl is small line element of wire that contains microscopic charged constituents and ΔQ is charge contained in the line element.	C/m
  Surface Charge	σ (surface charge density)	ΔQ/ΔS, ΔSis an area element on the surface of a conductor and ΔQ is charge on that element.	C/m2
  Volume Charge	ρ (volume charge density)	ΔQ/ΔV, ΔV is a volume element which includes a large number of microscopically charged constituents and ΔQ is charge on that element.	C/m3