Turn on thread page Beta
    • Thread Starter
    Offline

    5
    ReputationRep:
    Hi can anyone explain in the simplest terms why for the lennard jones potential (or just potential energy in general??), a positive force implies repulsion between particles and vice versa? where Force= -del(Potential)
    Offline

    10
    (Original post by darren86)
    Hi can anyone explain in the simplest terms why for the lennard jones potential (or just potential energy in general??), a positive force implies repulsion between particles and vice versa? where Force= -del(Potential)
    It's purely convention.
    Once you define potential energy as zero at infinity, then if the force is repulsive you have to do work on the object to move it towards the other particle. (Against the repulsion) If you do work on something, by convention, you give it energy. In this case potential energy. It's the same reasoning behind the idea of potential energy being stored in a spring when you compress it. You have to do work to do this. The work done on the spring is positive and the energy transferred to the spring is positive.
    The reason Force is negative potential gradient is that the potential energy is a maximum nearest the particle and goes to zero at infinity. So the potential gets less as distance from the particle gets greater. This is a negative gradient. The force is repulsive so acts in the positive direction, away from the particle. So the force is positive and the gradient is negative for a repulsive force.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: October 14, 2013

University open days

  • University of East Anglia
    UEA Mini Open Day Undergraduate
    Fri, 23 Nov '18
  • Norwich University of the Arts
    Undergraduate Open Days Undergraduate
    Fri, 23 Nov '18
  • Edge Hill University
    All Faculties Undergraduate
    Sat, 24 Nov '18
Poll
Black Friday: Yay or Nay?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.