Solve Surface Charge Density & He Atom Potential Energy Questions

In summary: I don't understand how that really helps... I know that, but still don't understand where I am goin wrong... could someone help me understand... this is wat I did!In summary, the answer to the first question is meant to be 117.5 but the person got it wrong. They don't know how to do the calculation for the surface charge density. The answer to the second question is that the helium atom has a potential energy of 32.00 eV.
  • #1
Taryn
63
0
I still don't understand from the msg given... another explanation any1

Hey I have two questions... the first one I did I got wrong but am really unsure why... I used sigma=Q/A but didnt get the right answer...
I got that the answer was 1.3277E7 but I am so far off it aint funny!
Please could someone help!
The answer is meant to be 117.5! I don't know tho!

Consider a cell whose cell membrane is 4.7 nm thick and whose resting potential is 62.4 mV. What is the approximate surface charge density (in µ C/m2) on the outside wall of the cell?

Also this question also has me stumped!
Consider the helium atom which consists of two electrons orbiting a nucleus made up of two protons and two neutrons. If both electrons are at a distance of r = 4.5 × 10-11 m away from the nucleus, as in figure, what is the potential energy (in eV) of the helium atom? Treat the nucleus as pointlike. - Give answer to 4 significant figures.

I thought you'd just find the energies of the electron on the helium atom which I got was 1.024E-17 and add them but obviously you don't so if anyone knows wat to do I would be much appreciative, I really just needa know how to do it... or even just a general start!
 
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  • #2
um yeah its cool now... for the first question... I just forgot to times it by 8.85E-12... so if anyone knows wat I am doing wrong for the second one... the help would be great
 
  • #3
Taryn said:
Consider the helium atom which consists of two electrons orbiting a nucleus made up of two protons and two neutrons. If both electrons are at a distance of r = 4.5 × 10-11 m away from the nucleus, as in figure, what is the potential energy (in eV) of the helium atom? Treat the nucleus as pointlike. - Give answer to 4 significant figures.

I thought you'd just find the energies of the electron on the helium atom which I got was 1.024E-17 and add them but obviously you don't so if anyone knows wat to do I would be much appreciative, I really just needa know how to do it... or even just a general start!
Start with a nucleus of +2e and no electrons. Move an electron from infinity to r = 4.5x10-11 m away from the nucleus. Now you have an atom of +e. Move another electron from infinity to r. The work done in moving both electrons is the potential energy of the two electrons.

AM
 
  • #4
I don't understand how that really helps... I know that, but still don't understand where I am goin wrong... could someone help me understand... this is wat I did!
P.E=kqq'/r and then added up wat I got... except I just can't get the right answer?
so does this mean I use the force of a charge and times it by the distance.. your explanation I mean... but that still doesn't work... The answer is meant to be 112 but I can't get that no matter how hard I try
 
  • #5
Taryn said:
I don't understand how that really helps... I know that, but still don't understand where I am goin wrong... could someone help me understand... this is wat I did!
P.E=kqq'/r and then added up wat I got... except I just can't get the right answer?
so does this mean I use the force of a charge and times it by the distance.. your explanation I mean... but that still doesn't work... The answer is meant to be 112 but I can't get that no matter how hard I try
I get:

[tex]PE_1/e = k2e/r = 8.989e9*2*(1.602e-19)/(4.500e-11) = 64.00 eV[/tex]

[tex]PE_2/e = ke/r = 8.989e9*(1.602e-19)/(4.500e-11) = 32.00 eV[/tex]

So the total potential energy is 96.00 eV.

This is a simplified view of it since it assumes that the electron charge is concentrated at the centre. But it should be very close.

AM
 

What is surface charge density?

Surface charge density refers to the amount of electric charge per unit area on the surface of a material or object. It is commonly denoted by the symbol σ and is measured in units of coulombs per square meter (C/m²).

How do you calculate surface charge density?

The surface charge density can be calculated by dividing the total electric charge on the surface by the area of the surface. It can also be determined by taking the dot product of the electric field and the unit normal vector at a particular point on the surface.

What is the potential energy of a helium atom?

The potential energy of a helium atom refers to the energy required to separate the two electrons from the nucleus. It is typically measured in units of electron volts (eV) or joules (J).

How do you calculate the potential energy of a helium atom?

The potential energy of a helium atom can be calculated using Coulomb's law, which states that the potential energy between two charged particles is proportional to their charges and inversely proportional to the distance between them. In the case of a helium atom, this distance is the distance between the two electrons and the nucleus.

What is the relationship between surface charge density and potential energy in a helium atom?

The surface charge density of a helium atom is directly related to the potential energy. As the surface charge density increases, the potential energy of the atom also increases. This is because a higher surface charge density indicates a higher concentration of electric charge on the surface, which leads to a stronger electric field and thus a higher potential energy.

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