# Recent content by Guillem_dlc

1. ### Modulus of the electric field created by a sphere

I think the right solution is c). I'll pass on my reasoning to you: R=6\, \textrm{cm}=0'06\, \textrm{m} \sigma =\dfrac{10}{\pi} \, \textrm{nC/m}^2=\dfrac{1\cdot 10^{-8}}{\pi}\, \textrm{C/m}^2 P=0'03\, \textrm{m} P'=10\, \textrm{cm}=0,1\, \textrm{m} Point P: \left. \phi =\oint E\cdot...
2. ### Origin of potentials

I think the right choice is c. I'll pass on my reasoning to you: We can think that if the formula of the potential is V(r)=\dfrac{kq}{r} If r tends to infinity, then V(r)=0. But the correct answer is d).
3. ### Potential at a point

I thought the right choice was d). But when it comes to the solutions, it is b) and I don't understand why. My reasoning would be: the potential at a point is the work that the electric field does to transport a charge from infinity to that point, so if the field is zero, it does no work and...
4. ### Capacitance of a spherical capacitor

I think I have the solution. Is that right? \left. \phi =\oint \vec{E}\cdot d\vec{S}=\oint E\cdot dS\cdot \underbrace{\cos 0}_1=E\oint dS=E\cdot S \atop \phi =\dfrac{Q_{enc}}{\varepsilon_0 \varepsilon_r}=\dfrac{Q}{\varepsilon_0 \varepsilon_r}=\dfrac{\sigma \cdot S}{\varepsilon_0 \varepsilon_r}...
5. ### Capacitance of a spherical capacitor

When I try to do Gauss, the permeability is not always that of the free space, but it varies: up to a certain radius it is that of the void and then it is the relative one. How can I relate them? I'm trying to calculate the capacity of a spherical capacitor. The scheme looks like this: inside I...
6. ### Electric field and electric potential exercise

Okay, perfect. Gauss law would say that the charge inside a closed surface (which can take any shape) is proportional to the flux through it, wouldn't it?
7. ### Electric field and electric potential exercise

Exactly, perfect.
8. ### A rocket and its gas exhaust velocity

Right. Well, it must be mistaken.
9. ### A rocket and its gas exhaust velocity

"I) Transfer from initial (circular) to transfer (elliptical) orbit. - The initial trajectory of m is a circumference centred in the massive object M of radius r=r_p . The velocity of m is constant, and can be easily obtained as follows: e=-\dfrac{GM}{2r_p}=\dfrac12 v_p^2-\dfrac{GM}{r_p}...
10. ### A rocket and its gas exhaust velocity

And here you substitute the velocity for the expression of u\cdot \ln \left( \dfrac{m_o}{m_f}\right) and isolate the mass

Yes
12. ### A rocket and its gas exhaust velocity

I also think that, but I think it might indicate the speed variation. Otherwise, there's no meaning in the minus sign. If you do the velocity variation considering circular trajectories I think that's correct. Since the speed: V=\sqrt{\dfrac{GM}{R}} Although not on top it shouldn't have speed
13. ### A rocket and its gas exhaust velocity

Yes, in this problem it is considered to burn all the fuel instantly at first and continue until gravity cancels out all the initial speed.
14. ### Electric field and electric potential exercise

Aa okay, that is even if it is not constant you know that it will surely be proportional because it must always be a multiple of the elementary charge, the electron. Right?
15. ### A rocket and its gas exhaust velocity

You can find the speed with \Delta V=u\cdot \ln \left( \dfrac{m_0}{m_f}\right) It's Tsiolkovski's equation.