Energy conservation in electrostatics/magnetism

[MedPR]

I think I understand how to use the conservation of energy law/equation to solve problems, but I don't know what this variable represents.

An electron gun consists of two electrode plates, one more negatively charged than the other. If an electron of mass m is allowed to accelerate between the two electrodes separated by a voltage V, which equation best represents the final speed of the initially stationary electron upon striking the more positive electrode?

Einitial of object+ work on object = Efinal of object

The object is at rest, and all of the final energy is kinetic energy. So:

0 + eV = 1/2mv^2 and the answer is v=sqrt (2eV/m)

V is the voltage of the electrode, but what is e?

And then in the very next question, they have work as qV where q is the charge on a molecule.

I thought work was Force * displacement? is Voltage a displacement?

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[SaintJude]

Electron Volt. In physics, the electron volt (symbol eV; also written electronvolt) is a unit of energy equal to approximately 1.602×10−19 joule (symbol J). It is the amount of energy a single electron gains as it moves across a electric potential difference of 1 Volt.

So 1 ev= 1.6 x 10-^19 Joules.

Often on the MCAT, answer will either be in eV or you have to convert from eV to Joules.

More info :http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ev.html#c2

 

[MedPR]

Electron Volt. In physics, the electron volt (symbol eV; also written electronvolt) is a unit of energy equal to approximately 1.602×10−19 joule (symbol J).

So 1 ev= 1.6 x 10-^10 J.

Often on the MCAT, answer will either be in eV or you have to convert from eV to Joules.

so eV is a specific charge/case of qV and qV is just a generic variable used for any charged particle?

Also, you say that 1eV = 1.602*10^-19 J, but then you say 1ev = 1.6*10^-10 J?

Edit: From what you're saying it seems to me like eV is one variable, and not e*V. Is that right?

 

[SaintJude]

Yeah, you put it perfectly. eV is just a specific case of qV. But it often trips people up because you may have calculated a value in Joules, when all the answer choices are in eV.

And I just edited (not quick enough) 1eV = 1.6x10^-19 Joules.

 

[MedPR]

Yeah, you put it perfectly. eV is just a specific case of qV. But it often trips people up because you may have calculated a value in Joules, when all the answer choices are in eV.

And I just edited (not quick enough) 1eV = 1.6x10^-19 Joules.

Ok thank you.

I'm still unclear about eV and V and the charge of an electron.

I know that Joules/Coulombs is a Volt and that the charge of an electron is 1.6*10^-19coulombs. So if you put an electron in an electric field with a charge of 50 volts, eV would be (1.6*10^-19)(50) Joules? I don't think so, but that's what is in my head.

 

[milski]

eV is a unit, the same way a Joule is. It's the energy an electron needs to cross a 1 V potential. For practical purposes you need to know how to convert between eV and J, SaintJude already gave you the coefficient.

 

[MedPR]

eV is a unit, the same way a Joule is. It's the energy an electron needs to cross a 1 V potential. For practical purposes you need to know how to convert between eV and J, SaintJude already gave you the coefficient.

I understand eV as a unit, but not as a variable. For instance, when you use the variable m for mass, you put "m" into the equation. For instance, F=ma. You don't put "g" (for grams) in the equation.. Your equation isn't F=ga.

The question uses eV as a variable, not as a unit. That's why I'm confused.

 

[milski]

I understand eV as a unit, but not as a variable. For instance, when you use the variable m for mass, you put "m" into the equation. For instance, F=ma. You don't put "g" (for grams) in the equation.. Your equation isn't F=ga.

The question uses eV as a variable, not as a unit. That's why I'm confused.

Ah! That's mildly annoying. If that's the case, it's actually two variables - e (charge) and V (potential). So we are talking about e*V. To answer your initial question, in this case e is the charge of the object.

There is more than one way to express work. charge*potential [difference] is one of them. So while it does give you the same result as force*displacement, neither of them is the other. Same type of thing as 5*4=10*2. The product of charge and potential is the same as the product of the force and displacement and happens to be the work done.

 

[MedPR]

Ah! That's mildly annoying. If that's the case, it's actually two variables - e (charge) and V (potential). So we are talking about e*V. To answer your initial question, in this case e is the charge of the object.

There is more than one way to express work. charge*potential [difference] is one of them. So while it does give you the same result as force*displacement, neither of them is the other. Same type of thing as 5*4=10*2. The product of charge and potential is the same as the product of the force and displacement and happens to be the work done.

Ok, now if they use e (instead of q) can I assume that the value of e is 1.6*10^-19 Joules? In the above posts, SaintJude and I agreed that eV is a specific case (specific to an electron) of qV, where q is the charge of any polar molecule.

 

[milski]

Ok, now if they use e (instead of q) can I assume that the value of e is 1.6*10^-19 Joules? In the above posts, SaintJude and I agreed that eV is a specific case (specific to an electron) of qV, where q is the charge of any polar molecule.

The value of e is 1.6*10^-19 Coulomb (which is a special case of q for an electron), eV for an electron would be 1.6*10^-19 * V Joule, where V is the potential that the electron is crossing.

 

[milski]

Ok thank you.

I'm still unclear about eV and V and the charge of an electron.

I know that Joules/Coulombs is a Volt and that the charge of an electron is 1.6*10^-19coulombs. So if you put an electron in an electric field with a charge of 50 volts, eV would be (1.6*10^-19)(50) Joules? I don't think so, but that's what is in my head.

Actually, you have it about right here. eV will be as much as you say but that would be the electron crossing between two points with difference in potential between them of 50 V.