# B What do you mean by force "between" two charges?

1. May 7, 2017

### Young wolf

I can understand force being exerted by an object on another object but I can't understand what all this force between two objects is. Can anyone give the complex and simple explanation, please? I am completely freaking out not being able to understand. Analogies and visualizations are a great help too.

2. May 7, 2017

### Staff: Mentor

Forces are always reciprocal - Newton's third law. If A exerts a force on B, then B also exerts a force on A. Sometimes you can ignore one direction (no matter how much you push a wall, you might move, but the wall won't move in any relevant way).

3. May 7, 2017

### Sturk200

Start from the basics. Things exist. Now, of the things that exist, some of them interact with one another. Of the possible interactions that things have with one another, some involve accelerations of the participating things. Such interactions constitute a force by definition of force as that which causes acceleration. Hence we have an interaction between two or more things that constitutes a force, or a force between the things.

4. May 8, 2017

### bahamagreen

Does Newton's 3rd apply between a field and an object (is the force reciprocal for the field? Does the field move in the opposite direction of the object's acceleration?)

5. May 8, 2017

### robphy

In my opinion and experience, there are misconceptions with this phrasing of "force 'between' two charges".
Some students mistakenly draw a force-vector placed "near the midpoint of the line joining" (i.e. 'between') two point charges.

There is no such force-vector.
It is better to say that "charge-1 applies a force on charge-2" and "charge-2 applies a force on charge-1",
which by coulomb's law [and Newton's III] are different forces (on different free-body diagrams and pointing in opposite directions although they have equal magnitudes).

6. May 8, 2017

### Staff: Mentor

Aren't those statements equivalent?
That is an important point and often the source of confusion:
-Force pairs are forces applied to different objects.
-Free body diagrams show forces applied to one object.

For a person pushing a wall, the force pair is approximated to occur at a point (the contact point). For [objects generating] fields, the interaction is at a distance.

7. May 8, 2017

### robphy

To an expert [who can filled in the details], they are...
but to a student [who is learning the ideas and the language], they are not,
as suggested by the sentence between those statements that you omitted in your quote:
To some students, they have drawn a "force between two charges"
...but that is ambiguous and probably not very useful.. is it a force on charge-1? or is it a force on charge-2? or on some charge that might be placed at that midpoint?

8. May 8, 2017

### bahamagreen

Maybe what needs clarity is whether the force on charge-2 is produced by charge-1 (and somehow jumps the distance gap in order to present a proximal force on charge-2), or whether charge-1 produces a field that spans the gap and presents a proximal force to any charge within the field (where the field itself presents the proximal force to charge-2), or some other explanation.

And if the first, how does Newton's 3rd get back to charge-1, and if the second, is the field subject to Newton's 3rd (and does it pass the reciprocal force back to charge-1), and if some other explanation, how does it operate?

Since the field is generally spherical it does not seem on the one hand that the field would be "over built" in order present the full force for every direction in order to ensure that the full force is "available" to apply in only one direction because there are an infinite number of directions for which to fulfill, but on the other hand the field needs a means to maintain the potential to do so for any and all single or multiple charges.