(Forces on a dipole): Can one exert an electrostatic force on an electric dipole
ID: 2271230 • Letter: #
Question
(Forces on a dipole):
Can one exert an electrostatic force on an electric dipole, which is
overall neutral? To investigate this question, suppose you have a NaCl molecule (electric
dipole) and a proton (p) with charge +q = e = 1.6x10-19 C lined up the way it is shown
below, so the Cl- ion is a distance R from the proton and the Na+
ion is a distance R+d from the proton.
proton (+)---------------R----------------Cl (-)------d------Na (+)
1) Using a free body diagram show that the net force on the dipole is due to the proton. Remember that the charge on a proton is +e. Is this net force attractive or repulsive.
2) Now assume R = 1 mm and (d=10^-11 m) and calculate the magnitude of the net electrostatic force
on the dipole by the proton.
3) Next, calculate the magnitude of the electrostatic force on the proton by the
dipole. Is the magnitude of this force larger than, smaller than or equal to the net
force you found in part 2)? Which physical principle justifies your answer?
4) Suppose in our arrangement we rotate the NaCl molecule by 180 degrees. Thus,
now the proton is a distance R from the Na+ ion and a distance R+d from the Cl ion. Would the magnitudes and directions of the electrostatic forces change,
which you calculated in parts 2) and 3)? Why or why not?
Explanation / Answer
1)the net force on the NACL molecule will be attractive as there are more electrons in CL- ion than protons and the attractive force is more in compared to that of the force between Na+
3) According to Newtons third law every action as an equal and and opposite reaction so the force exerted by proton on dipole will be same as foece exerted by dipole on proton
4) Yes they will change as the distance between proton and respective ion changes and the net force by proton on the dipole changes entirely
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