# Search results

1. ### Calculating the time it takes for something to fall over long distances

I think you would need calculus to do this properly, specifically that Force = \frac{d(mv)}{dt} and the force if the force is just gravitational from a single body, Force = \frac{Gm_1 m_2}{r^2} G - Newton's Gravitational constant m1 and m2 are the masses of the bodies being...
2. ### Grades => career?

Oxford is not different as far as I know, those numbers look pretty familiar although I think they change slightly each year.
3. ### Magnetic system, partition function

There are other ways of writing the internal energy of a system that include magnetic energy.
4. ### 5.8Ghz phones, less harmful than 2.4Ghz?

5.8Ghz is still a lot lower than the frequency of visible light ~10^15Hz, and people don't spend too much time worrying about light bulbs causing cancer. I am not so sure about exactly how they would cause cancer though - best ask a biochemist.
5. ### Cylinder potential problem

You don't need to do all of this. Once a solution has been found that satisfies the boundary conditions, you know that this is the only solution by the uniqueness theorem. This means that we can use trial solutions to find a solution to a problem using Laplace's equation.
6. ### Water as a dielectric in capacitors

Water is occasionally used as a dielectric. There are some large coaxial cables under Imperial College in London that use water as a dielectric, but only for a very short period of time, before it starts to conduct.
7. ### E-field Lines

Think about the structure of a conductor. An electric field line that is not perpendicular to a conducting plane will have a component parallel to it. This electric field then acts on the electrons in the conductor and they will re-arrange themselves so that there is no longer any force acting...
8. ### Cylinder potential problem

The ln(r) term doesn't satisfy the boundary conditions for all phi, given that the current solution has some angular dependence.
9. ### Cylinder potential problem

It is better to use this one as the general solution, V(r,\phi)=a_{0} + b_{0}lnr + c_{0}r cos\phi + d_{0}\frac{1}{r}cos\phi As r\rightarrow\infty, we know the solution must be -E_{0}r cos\phi so c_{0} = -E_0. The only other term that will be able to satisfy the boundary condition for...
10. ### What is tension in stat mech?

The first law of thermodynamics states that U = dQ + dW (depending on how you define it) The dW can be replaced with fdL as a unit of work and when the Helmholtz free energy is differentiated with respect to L at a constant temperature, you get the result for the tension. The system of...
11. ### Magnetic Dipole Moment and Angular momentum

A magnetic dipole is generated by a small current loop (the electron). Try working out the current that the electron generates then crossing that with the area of your little current loop. Remember: Current amount of charge per unit time As you only have one electron, you just have to...
12. ### Cylinder potential problem

However, you do know what the electric field is at infinity, and this will give you another boundary condition. You also need to think about where your solution needs values, is the potential infinite or finite valued at the origin?
13. ### Weight on a Neutron Star

You need to change the M_e to M_n, the mass of the neutron star, and R_e to R_n, the radius of the neutron star. Everything should then work out fine.
14. ### The Electric Potential Difference Created by Point Charges

What about conservation of momentum, there is no external force so linear momentum is conserved.
15. ### Balsa Span designs

Try looking at some current bridge designs. I imagine that they are very efficient, as bridge building material is expensive!
16. ### Measure weight of object hanging from a spring

I imagine you could also use pulleys.
17. ### Moment of Inertia in slender rod - density varies

Those first two seem to be fine. For calculating the moment of inertia, you want a little bit of mass multiplied by its distance from the axis, and then you sum this over the length of the rod. For the third part, because the mass is decreasing as you move along the rod, a little piece of mass...
18. ### Series circuit blues

In a series circuit, the current is constant all the way round, so the current at I3 is the same as the IT that you calculated. As the current is the same through all the resistors, you can calculate the voltage drop across each resistor using V=IR Hope that helps