Several questions about electromagnetism

[apec45]

Hey, i have several questions about electromagnetism, i hope you will be able to solve these haha :

1) how to define the electric field? i mean without saying E = F/q because a field causes the electric force and not the reverse so we can't use the force yet right?
2) how to snap a capacitor?
3) in an electrical circuit, where is the charge constant in 2 places? Where does this vary? Where is the potential constant? Where does it vary?
4) how does a capacitor work (simply) ? and how to bring the charge to it? Have we to keep continuallu a force to keep the charge in it?

Thks

 

[Dale]

how to define the electric field? i mean without saying E = F/q

If you start with the conservation of charge then that leads to the existence of a potential, and then the field can be defined simply as the negative gradient of that potential. No forces needed, but the math is more advanced. See here for details:

http://arxiv.org/abs/physics/0005084

 

[olgerm]

how does a capacitor work (simply) ? and how to bring the charge to it? Have we to keep continuallu a force to keep the charge in it?

if you connect capacitor to a battery, then one of its plates gets charge U*C and another gets charge -U*C . if you disconnect the battery, it remains charged.

 

[DennisN]

2) how to snap a capacitor?

Do you mean "destroy"? You can destroy a capacitor by applying a voltage that is larger than it is designed for. Electrolytic capacitors often have the recommended maximum voltage printed on them:

Here's a page with various information about capacitors:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capcon.html

 

[jbriggs444]

3) in an electrical circuit, where is the charge constant in 2 places? Where does this vary? Where is the potential constant? Where does it vary?

In an electrical circuit, one does not normally expect to find any charge build up anywhere. It is one of the rules of circuit theory: The net flow of charge into or out of any device is zero. The net flow of charge into or out of any connection point is zero.

This remains true, even in the case of capacitors. No net positive or negative charge builds up on a capacitor. Instead, positive charge builds up on one side and an equal amount of negative charge on the other so that the total for the capacitor remains zero at all times.

This does not vary. It is one of Kirchoff's laws.

Potentials are normally of no particular interest. Only potential differences are physically meaningful. In an ideal circuit, if a ground point is specified for the circuit, the potential is zero there, by definition. An ideal circuit may often include ideal power sources with a fixed potential difference. The potential difference across such an ideal circuit element is fixed by definition. An ideal circuit will almost always include ideal wires assumed to have zero (or negligible) resistance. The potential difference across the length of an ideal wire is zero (or negligibly different from zero) by definition.

In a real circuit, a ground is often physically realized as, for instance, a big cable attached to a rod driven into the ground. We normally assume that the ground has an infinite capacity to sink current, that it is at the same potential everywhere and that the grounding cable has negligible resistance. For real circuits, these assumptions can sometimes be incorrect.

 

[David Lewis]

The net flow of charge into or out of any device is zero.

If the inflow and outflow of charge are equal but don't happen at the same time, would there be a build up of charge?

 

[jbriggs444]

If the inflow and outflow of charge are equal but don't happen at the same time, would there be a build up of charge?

Under the model of circuit theory, the inflow and outflow of charge are equal and happen at the same time.

 

[davenn]

if you connect capacitor to a battery, then one of its plates gets charge U*C and another gets charge -U*C . if you disconnect the battery, it remains charged.

...it remains energised the net charge on the capacitor is zero

 

[sysprog]

...it remains energised the net charge on the capacitor is zero

That usage isn't common among technicians. We still refer to charging and discharging a battery or a capacitor. We still warn people to 'discharge all capacitors' before they do other work on a circuit.

 

[davenn]

That usage isn't common among technicians. We still refer to charging and discharging a battery or a capacitor. We still warn people to 'discharge all capacitors' before they do other work on a circuit.

And as a tech ... so did I for many years, 40+, till I discovered PF and learned the error of my ways

It should become so, as charged is an incorrect description

 

[sysprog]

And as a tech ... so did I for many years, 40+, till I discovered PF and learned the error of my ways

You still should, whenever you're working as a tech. If someone on a job asks you "is this capacitor charged", and you say "no", and he gets zapped, and he then turns to you, and you say "it wasn't charged, it was energised", he's apt to take a dim view of your notion of correctness.

If you look at the (Wikipedia) definitions of coulomb (charge) and farad (capacitance), I think you'll probably agree that it's reasonable, not only for practical purposes, but also for consistency with common usage of SI terms, for people to refer to a capacitor as a device that stores and releases electrical charge.

The fact that the net charge of an isolated system is always zero does not entail that it's 'incorrect' to refer to a capacitor as charged when it possesses the capacity to impart charge when it makes contact or connection with an otherwise comparatively non-charged object, and as discharged, when it doesn't possesses any excess of charge compared to the other object.

In the following listing (from the farad article) of equivalent formulae, it can be seen (at the 3rd equivalent term) that the capacitance of a capacitor is the square of the charge in coulombs over the energy in joules.

Equalities

A farad is represented in terms of SI base units as s4⋅A2⋅m−2⋅kg−1

It can further be expressed as:

where F = farad, A = ampere, V = volt, C = coulomb, J = joule, m = metre, N = Newton, s = second, W = watt, kg = kilogram, Ω = ohm, Hz = hertz, H = henry.

It should become so, as charged is an incorrect description

Anyone can edit the relevant Wikipedia articles to put them in accord with that view, but whoever does should be prepared for difficult defenses on the respective talk pages.

Why is using 'charged' incorrect'? Using 'energized'is at best ambiguous -- everything is energized. Using 'charged' at least makes reference to the kind of energy that's being referenced. In the case of a capacitor, you could 'energize' it by putting it on a higher shelf, but you charge it by creating a greater difference in charge between its opposite poles.

It's technically true that charging a capacitor is really imparting a difference in charges and that there's no net charge in the capacitor as an isolated system. But it's still reasonable, in the illustration below, to say that the belt is charging the system when it's moving (instead of always having to say something more along the lines of 'it's bringing about an accumulation of greater difference in potentials of electrical charge'), and that the spark shows the generator discharging, i.e. the charge differences going to zero, with the charge difference energy becoming dissipated.

 

[davenn]

You still should, whenever you're working as a tech. If someone on a job asks you "is this capacitor charged", and you say "no", and he gets zapped, and he then turns to you, and you say "it wasn't charged, it was energised", he's apt to take a dim view of your notion of correctness.

I would teach him by qualifying my response ... that's how we learn

Any tech/electrician should know the meaning of an "energised" component/circuit

The fact that the net charge of an isolated system is always zero does not entail that it's 'incorrect' to refer to a capacitor as charged

welllllllllllll the 2 parts of that statement are kinda contradictory

... hence why energised is used

In the case of a capacitor, you could 'energize' it by putting it on a higher shelf, but you charge it by creating a greater difference in charge between its opposite poles.

again, that is contradictory, there is NO greater difference in charge between the two poles/plates
for every -charge on one plate there is a =charge on the other one
Don't forget the energy in a capacitor is in the electric field BETWEEN the plates not on EITHER of the plates

The big problem, there's a whole group of people in the tech world that are just plainly using the word charge incorrectly,
mainly because they were incorrectly taught and you/I will never change most of them. But, for me, now knowing the
difference I do all I can not to fall into old bad/wrong habits, try to teach a correct/better way and definitely won't go back
to the old was knowing what I do now Dave

 

[sysprog]

again, that is contradictory, there is NO greater difference in charge between the two poles/plates
for every -charge on one plate there is a =charge on the other one

The difference between +100 and -100 is greater than the difference between +10 and -10, right?

 

[Vanadium 50]

. If someone on a job asks you "is this capacitor charged", and you say "no", and he gets zapped, and he then turns to you, and you say "it wasn't charged, it was energised", he's apt to take a dim view of your notion of correctness