A simple electric field question

AI Thread Summary
The discussion revolves around a homework problem involving the calculation of work done on an electron in an electric field. The original poster struggles to arrive at the correct answers using Coulomb's law and the work done formula, initially calculating the net force on the electron. They realize that the net force changes as the electron approaches the protons, complicating their calculations. The conversation shifts to using potential energy differences instead, which simplifies the problem by allowing the calculation of work done through changes in potential energy. Ultimately, the correct approach involves considering the potential energies at different distances to find the work done on the electron.
arkofnoah
Messages
124
Reaction score
1

Homework Statement


http://img19.imageshack.us/img19/7662/screenshot20100804at214.png

Homework Equations


the coulomb law for electrostatic force.

The Attempt at a Solution


This is supposedly a very straightforward question but I can't get any of the four answers so I just wonder if anyone is kind enough to check my working for me.

Basically I just found the net force acting on the electron and got something like \frac{e^{2}}{32\pi \epsilon_{0} x^{2}}. Then I just use the work done = force x distance formula but I did not arrive at any of the answer.

Am I missing something?
 
Last edited by a moderator:
Physics news on Phys.org
The net force varies as the electron approaches the protons. since it's distance from the protons changes as does the angle they're in.
 
OH! Right I somehow forgot about that. Thanks.
 
Now I'm not sure I would calculate it like that, since it'd probably involve integrating along the whole length of the electron's path (to calculate the area in the F-s diagram to get the work done). A much easier way to do this is consider the changes is potential energy, which we know is transferred into kinetic energy.
The potential energy from a charge Q at distance r is -(1/4e0*pi)*Q/r. When you have more than one charge, this energy is added together. So first r = 5x, then r=3x. the difference*e(charge) gives the work done on the electron.
 
yes I did that, thanks for the help anyway :D
 
The difference of potential energies at final and initial position gives the answer
arkofnoah said:

Homework Statement


http://img19.imageshack.us/img19/7662/screenshot20100804at214.png

Homework Equations


the coulomb law for electrostatic force.

The Attempt at a Solution


This is supposedly a very straightforward question but I can't get any of the four answers so I just wonder if anyone is kind enough to check my working for me.

Basically I just found the net force acting on the electron and got something like \frac{e^{2}}{32\pi \epsilon_{0} x^{2}}. Then I just use the work done = force x distance formula but I did not arrive at any of the answer.

Am I missing something?
 
Last edited by a moderator:
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
View full post »

Thread 'Struggling to understand the concept of this question (ice cube in water optics question)'

TL;DR Summary: I came across this question from a Sri Lankan A-level textbook. Question - An ice cube with a length of 10 cm is immersed in water at 0 °C. An observer observes the ice cube from the water, and it seems to be 7.75 cm long. If the refractive index of water is 4/3, find the height of the ice cube immersed in the water. I could not understand how the apparent height of the ice cube in the water depends on the height of the ice cube immersed in the water. Does anyone have an...
View full post »

Similar threads

Capacitors, equipotentials, and electric fields
Replies
5
Views
950
Discontinuity in an Electric line of force
Replies
12
Views
2K
Electric Field Flux Question
Replies
4
Views
754
Electric field on the axis of a ring-shaped charged conductor
Replies
1
Views
941
Why Is Work Positive When Done Against the Electric Field?
Replies
22
Views
3K
Electric field & force due to charged insulating hemispherical shells
Replies
12
Views
2K
Why are the electrostatic field lines normal to a charged conductor?
Replies
10
Views
1K
Confused about direction of electric field
Replies
7
Views
3K
Dipole placed in a uniform electric field
Replies
11
Views
3K
Understanding the energy of a dipole in a uniform electric field
Replies
2
Views
2K

Hot Threads

Recent Insights

Back
Top