This is purely a matter of convention. According to the usual convention, the potential energy of an electric dipole is −p⋅E. Historically, whoever first defined the dipole moment could have defined it with the opposite sign. Then the energy would have been +p⋅E. The sign would also have been reversed in every other equation, e.g., τ=p×E would have become τ=E×p..
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This is purely a matter of convention. According to the usual convention, the potential energy of an electric dipole is −p⋅E. Historically, whoever first defined the dipole moment could have defined it with the opposite sign. Then the energy would have been +p⋅E. The sign would also have been reversed in every other equation, e.g., τ=p×E would have become τ=E×p..
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