Two point charges 5nC and -5nC, seperated by a distance d= 2.0 mm constitute an

Question

Two point charges 5nC and -5nC, seperated by a distance d= 2.0 mm constitute an electric dipole, The dipole is placed in an external electric field E= 3.5x105 N/C I as shown below.

a) is the system at equilibrium? Give reasons for your answer

b) What is the electric dipole moment (note that it is a vector)

c) Why does the electric field rotate the dipole? Find the magnitude of the torque that the electric field exerts on the dipole at the orientation.

d) If the dipole is in the same direction at the field, what is the potential energy?

e) What is the potential energy of the dipole at this orientation

E= 3.5x106 N/C i

Explanation / Answer

a) since this dipole will experiences the torque then

the system is not in equilibrium

b) electric dipole moment is p = q*d = 5*10^-9*2*10^-3=10*10^-12 C-m

c) since two equal force try to rorate the two charges ,these non collineasr forces constitute a couple which causes rotation of the dipole

Torque T = p*E*sin(theta) = 10*10^-12*3.5*10^5*sin(20) = 1.2*10^-6 N-m

d) potential energy U = p*E= 10*10^-12*3.5*10^5 = 3.5*10^-6 J

e) U = p*E*cos(20) = 10*10^-12*3.5*10^5*cos(20) = 3.3*10^-6 J

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