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B What is the *truth* of the electrostatic force

  1. Oct 17, 2016 #1
    Hey guys, I want to ask a question about the Coulomb force or the electrostatic force.

    As you can see, there are these statement,
    1) The electric force is a "vector quantity"
    2) The electric force between two stationary charged objects is called the Electrostatic force.
    3) The electrostatic force between two charges can be repulsive or attractive.

    And my problems are for each question:
    1) If an electric force is a vector quantity. Then why do we call the force between two charges "force" because it should be "forces" as there are two vectors in the case of "Coulomb's law"

    2)+3) I only know what is called "the magnitude of the electrostatic force is F=k q1 q2/ r^2. But what is the electrostatic force.

    Does the term "electrostatic force means (for example in the picture)" ?
    1. The vector FQq and the vector FqQ combined.
    2. Either the vector FQq or the vector FqQ.
  2. jcsd
  3. Oct 17, 2016 #2

    ##1)## Consider either one of the charged particles. How many forces act on it?
    ##2)## What is the definition of a vector?
  4. Oct 17, 2016 #3
    1) Each charge has one forced applied on (talking about electric force).
    2) A vector quantity is a quantity with direction and magnitude.

    I don't think you get me right. I am discussing the quantities in this situation.
    "The electrostatic force between two charges is repulsive or attractive depend on their positive and negative charge", that's the statement in the Coulomb Law. My question why do we not say "electrostatic forces" since there are two vectors or two eletric forces appearing in this particular case !?

  5. Oct 17, 2016 #4
    Coulomb's law expains the interaction between two charged particles; however, further, it gives an expression for the force that acts on either one of the particles due to this interaction.

    So you see, Coulomb's law gives the force that acts on one of the particles, due to their mutual attraction or repulsion.
  6. Oct 17, 2016 #5


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    The singular refers to the general phenomenon of electrostatic repulsion/attraction.
  7. Oct 17, 2016 #6


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    This seems to be a question of the utmost triviality. I cannot think of any way in which the answer to this question possibly matters.

    This is purely a matter of writing style and word preference of the author and does not change any of the physics. If you prefer one word choice over another then learn physics well and write your own textbook and use your preferred wording. It makes no difference.

    I can guarantee that nobody will choose one textbook over another because one says "force" and the other says "forces".
  8. Oct 17, 2016 #7


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    I think your concerns are valid.
    In my experience, when I have asked students for the "force between two charges", I sometimes see a single vector drawn near the midpoint for two point charges. So, I have discouraged the use of "between" in this context, and prefer to specify---depending on what I want---"force on charge1 due to charge2", "force on charge2 due to charge1", or both forces. ("Interaction between" is okay... but should not translate to "force between" or "forces between".)

    The magnitude of the electrostatic force [on charge1 or on charge2] is F=k |q1| |q2|/ r^2 (note the absolute value signs).
    The direction of the "electrostatic force on charge1" [with tail at charge1]
    away from charge2 when q1*q2>0 (i.e. like-signed charges repel), and
    towards charge2 when q1*q2<0 (i.e. unlike-signed charges attract).
  9. Oct 18, 2016 #8

    Thanks a lot Robphy. Your explanation is very helpful :)
  10. Oct 18, 2016 #9
    Ms/Mr Robphy. Could I ask one more question ?. If we had a machine ( I don't know if there is one or not) measuring the whole magnitude of force from the interaction between two charges, we can only get the value of |F| right. Despite there are two vectors but we won't get the value of 2|F| but the |F|. Am I right !?
  11. Oct 18, 2016 #10


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    You will have to describe this machine in more detail.

    If, for instance, you design a machine that determines the mass and acceleration of one charge and multiplies to get |F|, determines the mass and acceleration of the other charge and multiples to get |F|, adds the two products and reports the sum, the reported output will naturally be 2|F|.

    Newton's third law has something to say about this.
  12. Oct 18, 2016 #11


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    Whatever "the whole magnitude of force from the interaction between two charges" is, if the machine is calibrated to measure it, then it will measure it.
  13. Oct 18, 2016 #12


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    This elaborates on the replies to your recent question.

    Maybe it's best to first recognize that, in preparation for using Newton's Laws,
    that a force is applied to an object of interest (call it the "target") by another object (call it the "source").

    Let's first agree on what the target-object of interest is.
    The target-object is feeling the push or pull by the source-object, which is doing the pushing or pulling on the target-object.
    This is a more complete and more useful description that just saying "force" or even "force from the interaction".

    Now devise a way to measure this force on the target-object by the source-object.
    For example, maybe we apply another force on the target-object to balance the other forces on it (e.g. a spring scale).

    So, if you apply this idea to charge1 as the target, then you get "the electric force on charge1 due to charge 2".
    This has a clear physical interpretation.

    Now, if [for some reason] you just care about the size of that electric force,
    you could neglect the direction of that force (e.g., focus on the spring-scale reading, ignoring the orientation of the spring-scale).

    Next, you could use this idea in a more complicated way to, say, also measure the magnitude of "the electric force on charge2 due to charge1".
    [Note how we are describing this fancier device in detail.]
    So, then you would have two magnitudes.... but then what would you do with these two numbers?
    You could compare them and find they are equal (because of the symmetry of Coulomb's law). Ok fine... a symmetry-checking machine.

    Since they are numbers, one can add them together (or perform some other mathematical operation with these numbers)...
    but that quantity likely has no interesting physical interpretation [especially in a more generally setting].
    So, maybe this fancier device isn't so useful.
  14. Oct 18, 2016 #13
    Oh ok I get all your 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|.

  15. Oct 19, 2016 #14
    truth of electrostatic force is :-
    fundamentally electrostatics is a study of body which on rubbing accquire attractive property ,they are said to be charged.
    they get charge by friction.
    when these charges flow it is said to be current electricity.
    force by which body on rubbing gets charged when they attract or repel is called eletrostatic force
  16. Oct 19, 2016 #15
    I 've read it once I again I finally understand my misunderstanding :). Sorry everyone and thank you very much
  17. Oct 19, 2016 #16


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    You are confusing force and energy here. They are not interchangeable. The chair exerts force to hold your butt up. It does not consume energy to do so, because it doesn't move. Energy is force times distance.
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