The true face of the Coulomb force

Announcements Posted on
TSR looking different to you this week? Find out why here. 02-12-2016
    • Thread Starter
    Offline

    0
    ReputationRep:

    Hey guys,
    I have troubles with understanding the Couloumb's law.
    Well I get that (example used is the picture above):
    1) The two charges q1 and q2 interact through a single force called the coulomb force.
    2) Even though there are two electrostatic force vectors F12 and F21, there is only "one electrostatic force dragging two charges closer or further away" that is presenting.

    ==> That's my understanding ! Am I correct ? If so, if we use a machine that measuring electrostatic force (imaginary, I don't know if it's real), the value of Newtons we get is |F| and not 2|F|, right ? I just want to know and to be sure that according to the Couloumb's law, there are two vectors of forces but there is only one force that drags the two charges closer or farther away from each other.
    Offline

    2
    ReputationRep:
    I'm not sure that it is helpful to try to think about "the force", or even what that means. It makes more sense to think about the force that is felt by a particular object. So, in the top picture, charge q1 feels an electrostatic force because of charge q2, and the size of that charge is F1. Newton's third law then tells us that charge q2 feels a force because of charge q1 which is of the same magnitude and points in the opposite direction.

    If we think about a similar but easier to understand situation - as I sit here in my chair, the Earth pulls on me, and I feel this force acting on me (my weight). At the same time, I pull on the Earth with the same force, but this time towards me. The Earth doesn't seem to be caring much, but then, it's bigger than I am. What would you mean by "the one force that drags us together"? I don't really see what this would mean.
    Offline

    3
    ReputationRep:
    (Original post by Pangol)
    Newton's third law then tells us that charge q2 feels a force because of charge q1 which is of the same magnitude and points in the opposite direction.
    I don't think this is related to newton's third law.

    Both charges create electric fields that affect the other charge
    • Thread Starter
    Offline

    0
    ReputationRep:
    Oh ok I get your point .= I also grasp the Newton Third's Law because it was interesting when I realized the reason why it hurts when you punch a wall = The newton third law according to my understand: if you exert a force on an object, the object exerts a force on you. Like when you use a knife to stab a metal blade, the point that the knife touch the plate has two force, the force towards the wall and another force of opposite direction of the wall. Another example with a diagram is the picture below, call F12 Force of wall on finger and F21 Force of finger on wall. Then the forces will be exerted on the point where finger and wall meet. If we convert force into energy in this case. Yes we will get the energy consumed from F12 and also F21 (the forces eliminated each other to create balance eventually). This is like when we sit on a chair (out weight make force (consume energy) and the chair exert (consume an amount of energy) another force of opposite way (energy) to maintain its structure.______________________ ________________________________ ______________________________In terms of electric charge=>You guys meant that (for example we have two charges q1 and q2) if we pick q1 then we have the force exerted on q1 due to q2 and the same goes for q2, the force exerted on q2 due to q1.-But this is what I am confused, for example q1 positive, q2 negative. And they attracts each other, and thus vectors F12 and F21 exists. This is my confusion after this quote:" q1 and q2 interacts with each other through A Single Force called the Electrostatic Force"" the electrostatic force between two charges Q and q is repulsive or attractive".--> So what I mean is that if we pick the charge q1 we get F21, and then pick q2, we get F12. And according to everyone's opinion, the magnitude of force we get of the diagarm if measuring is not |F| but 2|F|, am I right !? If so and if we convert the force into energy, then the energy we get is both from F12 and F21 and that make 2|F| = |F12| + |F21|. If that's correct then my opinion below will be wrong.--> My opinion is that in reality there are "two equal but opposite direction" forces apply on "two charges". And I am sorry for not specifize my idea of the machine. I mean (for example if q1 and q2 attract), and that machine can measure the force's magnitude and the energy that the force make. I mean the eventual value we get is not 2|F| but |F|.
    Offline

    2
    ReputationRep:
    (Original post by langlitz)
    I don't think this is related to newton's third law.

    Both charges create electric fields that affect the other charge
    Your last point is true, of course, but I don't see why Newton's third law doesn't apply. I would say that your point is an explanation as to why it does apply!
    • Thread Starter
    Offline

    0
    ReputationRep:
    Oh ! I get the points != I also grasp the Newton Third's Law because it was interesting when I realized the reason why it hurts when you punch a wall = The newton third law according to my understand: if you exert a force on an object, the object exerts a force on you. Like when you use a knife to stab a metal blade, the point that the knife touch the plate has two force, the force towards the wall and another force of opposite direction of the wall. Another example with a diagram is the picture below, call F12 Force of wall on finger and F21 Force of finger on wall. Then the forces will be exerted on the point where finger and wall meet. If we convert force into energy in this case. Yes we will get the energy consumed from F12 and also F21 (the forces eliminated each other to create balance eventually). This is like when we sit on a chair (out weight make force (consume energy) and the chair exert (consume an amount of energy) another force of opposite way (energy) to maintain its structure.______________________ ________________________________ ______________________________In terms of electric charge=>You guys meant that (for example we have two charges q1 and q2) if we pick q1 then we have the force exerted on q1 due to q2 and the same goes for q2, the force exerted on q2 due to q1.-But this is what I am confused, for example q1 positive, q2 negative. And they attracts each other, and thus vectors F12 and F21 exists. This is my confusion after this quote:" q1 and q2 interacts with each other through A Single Force called the Electrostatic Force"" the electrostatic force between two charges Q and q is repulsive or attractive".--> So what I mean is that if we pick the charge q1 we get F21, and then pick q2, we get F12. And according to everyone's opinion, the magnitude of force we get of the diagarm if measuring is not |F| but 2|F|, am I right !? If so and if we convert the force into energy, then the energy we get is both from F12 and F21 and that make 2|F| = |F12| + |F21|. If that's correct then my opinion below will be wrong.--> My opinion is that in reality there are "two equal but opposite direction" forces apply on "two charges". And I am sorry for not specifize my idea of the machine. I mean (for example if q1 and q2 attract), and that machine can measure the force's magnitude and the energy that the force make. I mean the eventual value we get is not 2|F| but |F|.
    Offline

    3
    ReputationRep:
    (Original post by Pangol)
    Your last point is true, of course, but I don't see why Newton's third law doesn't apply. I would say that your point is an explanation as to why it does apply!
    Ok yes I guess it still is technically an example of newtons 3rd law. But consider the Lorentz force, where's the opposing force there?
    Offline

    2
    ReputationRep:
    (Original post by langlitz)
    Ok yes I guess it still is technically an example of newtons 3rd law. But consider the Lorentz force, where's the opposing force there?
    I'm not a sufficiently advanced physicist to know how to answer this confidently. But if you mean the combined electric and magnetic force on a point charge in an electromagnetic field, surely it would be the force exerted by the point charge on whatever causes the field in the first place.
 
 
 
Write a reply… Reply
Submit reply

Register

Thanks for posting! You just need to create an account in order to submit the post
  1. this can't be left blank
    that username has been taken, please choose another Forgotten your password?
  2. this can't be left blank
    this email is already registered. Forgotten your password?
  3. this can't be left blank

    6 characters or longer with both numbers and letters is safer

  4. this can't be left empty
    your full birthday is required
  1. Oops, you need to agree to our Ts&Cs to register
  2. Slide to join now Processing…

Updated: October 19, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

Today on TSR
Poll
How are you feeling about doing A-levels?
Study resources

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

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