A question about the difference between mass and charge?

Click For Summary

Discussion Overview

The discussion revolves around the differences between mass and charge, exploring their roles in physics, particularly in relation to forces and fields. Participants examine concepts such as inertia, gravitational effects, and the nature of forces in general relativity, while also addressing misconceptions and analogies between these two fundamental quantities.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants suggest that mass serves dual functions: determining inertia and gravitational response, while charge determines the strength of the electric field.
  • There is a discussion about the lack of an equivalent to F=ma for charge, leading to questions about why mass seems to resist changes in motion more than charge does.
  • Some argue that gravitational mass and inertial mass are linked, while others point out that gravity and electrical forces are fundamentally different phenomena.
  • One participant proposes that both mass and charge can be compared in terms of momentum transfer, while another questions the connection between charge and mass in the context of general relativity.
  • There are conflicting views on the relationship between light and charge, with some asserting that light is a result of moving charge, while others emphasize that they are distinct entities.
  • Participants explore analogies involving Coriolis and centrifugal forces, questioning the nature of forces in non-inertial frames and their relation to gravity.
  • One participant introduces the idea that light could be considered a "wake" left by moving charge, emphasizing the distinction between the two.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the relationship between mass and charge, the nature of forces in different frames of reference, and the implications of general relativity. The discussion remains unresolved, with no consensus on many of the points raised.

Contextual Notes

Participants acknowledge limitations in their understanding and the complexity of the concepts discussed, particularly regarding the nature of forces and the implications of general relativity. There are references to alternate universes and hypothetical scenarios that further complicate the discussion.

jeebs
Messages
314
Reaction score
5
Hi,
I was wondering about the difference between mass and charge. As far as my understanding goes, these are just some names we've given to these 2 quantities of "stuff" that determines the strength of the field each one causes in the space around it. However, I was thinking about Newton's equation that comes in handy for all sorts of problems, F=ma. If we increase the mass of an object, then all other things being equal, it will be more difficult to change its state of motion. On the other hand, there is no F=Qa. What I mean by that is, as far as I am aware, you could put as much charge onto that object as you wanted and, all other things being equal, it would be no harder to change its state of motion.

So, my question is, why is this the case? why does mass seem to "drag" through "empty" space (i know empty space isn't strictly empty) but charge does not? I can't help feeling this is a bit of an ignorant question, but I've gone through 3 years of my 4 year physics course and this hasn't been covered in any way and I can't think of anything obvious I've overlooked.

Thanks.
 
Physics news on Phys.org
Well, mass really serves two functions in physics: it determines inertia (how hard an object is to move) and it determines how much an object responds to gravity (or how much gravity it produces). It's conceivable that in some alternate universe with different physics, these two might be different. An object's gravitational mass determines the strength of the gravitational field it produces, just like its charge determines the strength of the electric field it produces, whereas the inertial mass is the one that appears in ΣF=ma. If inertial mass and gravitational mass were not related, there would be no confusion.

However, in our universe they are (as far as anyone can tell) the same. You're not the first to wonder why gravitational mass is linked to inertial mass but there is no "inertial charge." According to general relativity, this is because gravity and electrical forces are actually completely different phenomena. The electric force works by exerting a force to actually change a particle's trajectory through space and time, according to Newton's law, ΣF=ma, and Coulomb's law, F=kq1q2/r2[/sub]. But gravity instead changes the "shape" of the underlying spacetime. There's no force involved. The particle continues to move in a straight line (or at least what it thinks is a straight line) but because spacetime is distorted, that line no longer seems straight to observers like us. It seems like there's a gravitational force acting on the particle, but in the GR view, that force is just an illusion.
 
@jeebs: That's a good question and the reason it wasn't answer in your course is, that only very few people are able to think outside the box.
I don't know the heavy theoretical physics to give sophisticated answers, but here are some simple differences between mass and charge:
- there are opposite charges which attract. if you have three bodies, then by gravity they will all attract, but by EM they surely won't all attract
- charge is quantized and all particles have a small multiple of the unit charge
- the mass inertia changes with velocity; whereas the charge field converts from being electrical to magnetical when in motion; it's complicated

In a way both a still comparible, since both effects are just transfering momentum. In fact, maybe it's GR which connects charge and mass, because it explains how light is bent?!
 
Gerenuk said:
In a way both a still comparible, since both effects are just transfering momentum. In fact, maybe it's GR which connects charge and mass, because it explains how light is bent?!
um... I don't know about that. Light isn't charge, it's light. They're different. And charge and mass don't have any special connection in GR.
 
diazona said:
um... I don't know about that. Light isn't charge, it's light. They're different. And charge and mass don't have any special connection in GR.
Light is photons and charge is the source of photons. So it's the same.

And GR and mass is also the same.
 
diazona said:
in the GR view, that force is just an illusion.
This remembers me of Coriolis and Centrifugal force: they do act, but they do not exist: we THINK they exist if we are in a rotating system.

So, which could be the X in these equations?!? :rolleyes:

Coriolis : Rotation = Gravity : X

or

Centrifugal : Rotation = Gravity : X

How can a person in a rotating system determine that Coriolis and Centrifugal forces actually do not exist? Can he?!?
 
jumpjack said:
How can a person in a rotating system determine that Coriolis and Centrifugal forces actually do not exist? Can he?!?
It's easy. If you put an object into empty space, where no other forces should exist, and the object still starts moving, then you are not in an inertial frame and some kind of virtual force exists.
 
Gerenuk said:
It's easy. If you put an object into empty space, where no other forces should exist, and the object still starts moving, then you are not in an inertial frame and some kind of virtual force exists.


It seems like there's a gravitational force acting on the particle, but in the GR view, that force is just an illusion.

So, we SEE this force (gravity) but it CAN'T exist (for General Relativity). So we "know" we are in a "non-GR" system (~=non inertial system). So I wonder how things work in the "real" world out there... out of the "rotating disk" we live on?!? :confused: A world where gravity does not exist. Maybe even mass does not exist, there. :confused:
 
Gerenuk said:
Light is photons and charge is the source of photons. So it's the same.

And GR and mass is also the same.
Absolutely not...
 
  • #10
diazona said:
Absolutely not...
Wow, you really baffle me with your knowledge and arguments you present.
If you want to make a point, then write some content.
 
  • #11
jumpjack said:
This remembers me of Coriolis and Centrifugal force: they do act, but they do not exist: we THINK they exist if we are in a rotating system.

So, which could be the X in these equations?!? :rolleyes:

Coriolis : Rotation = Gravity : X

or

Centrifugal : Rotation = Gravity : X
X would be "spacetime curvature". I'm not sure it's a perfect analogy but it's reasonably close.
jumpjack said:
So, we SEE this force (gravity) but it CAN'T exist (for General Relativity). So we "know" we are in a "non-GR" system (~=non inertial system). So I wonder how things work in the "real" world out there... out of the "rotating disk" we live on?!? :confused: A world where gravity does not exist. Maybe even mass does not exist, there. :confused:
"Non-GR system" doesn't make sense, you were right with "non-inertial system".

The equivalent of the non-rotating disk would be going into orbit or freefall, where you feel weightless.
 
  • #12
Gerenuk said:
Wow, you really baffle me with your knowledge and arguments you present.
If you want to make a point, then write some content.
Well, I'd hoped it would be obvious. But if you want more, think of it like this: charge is to light as a boat is to its wake. Light is the "wake" left behind by a moving charge.

The wake is not the boat; similarly, light is not charge.
 
  • #13
diazona said:
"Non-GR system" doesn't make sense,
I know, I just created it to give a name to something I don't even understand. :biggrin:
 
  • #14
diazona said:
Well, I'd hoped it would be obvious. But if you want more, think of it like this: charge is to light as a boat is to its wake. Light is the "wake" left behind by a moving charge.

The wake is not the boat; similarly, light is not charge.

This is an important point and it is a well known fundamental difference between electromagnetics and GR. Photons are not charge (or better said, have no charge), so light is not considered a source of fields in electromagnetics. In GR, gravity waves (or you could say gravitons, to compare to photons) are a source of gravitational fields (or spacetime curvature). For that matter, photons are also a source of gravitational field. So the idea that photons are charge is doubly wrong. Photons are more akin to mass, even though they have zero rest mass. They are a gravity source, as part of the the stress energy tensor.
 
  • #15
jumpjack said:
I know, I just created it to give a name to something I don't even understand. :biggrin:
Fair enough. Sorry if I was a little hard on you there.:redface:
 

Similar threads

Undergrad Small question about constantly accelerating charges
  • · Replies 10 ·
Replies
10
Views
2K
Undergrad Surface charge of neutral solids
  • · Replies 3 ·
Replies
3
Views
2K
Undergrad Confused about work done on charge
  • · Replies 8 ·
Replies
8
Views
2K
Undergrad Seeking the Equation of Motion for a charged mass attached to a spring
  • · Replies 22 ·
Replies
22
Views
3K
Undergrad Potential difference between a battery's terminal and Earth ground
  • · Replies 58 ·
2
Replies
58
Views
6K
Undergrad Question about the motion of a charged particle
  • · Replies 14 ·
Replies
14
Views
3K
Undergrad Capacitor surface charge movement current, relativistic effects
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad Charging a small metal ball from a charged hollow sphere
  • · Replies 4 ·
Replies
4
Views
2K
Undergrad E-field around charged finite wire - intractable or not?
  • · Replies 10 ·
Replies
10
Views
2K
Graduate Infinite scaling limit of an elliptic charged surface
  • · Replies 5 ·
Replies
5
Views
813