# The FYS1120 Weekly Assignment Post #1

There is no solution manual to the FYS1120 (electromagnetism) weekly assignments, so we tought it was a good idea to start posting suggestions to answers here with the aim that other students at fys1120 can crosscheck their answers somewhere  and eventually if they don’t agree or just want to know how the answers was obtained start a discussion about them in the comments.

We’ll try to start a system so that if others want to submit suggestions before we’ve manged to post them, they can.

For writing latex in the comments just start with $l-atex and end with$ or alternativly start with [l-atex] and end with [/l-atex] (without the ‘-‘ symbol). For instance $x^2$ shows up as $x^2$.

Assignment #5:

1)

a) $\epsilon = 2.3 \times 10^{-11} F/m$

b) $V = E_{max} d = 400kV$

c) $\sigma = \frac{\epsilon V_{max}}{d} = 4.6 \times 10^{-4} C/m^2$

d) $\sigma_{ind} = \hat{n}\cdot \vec{P} = \epsilon_0 X_e E = \epsilon_0 (\kappa -1) E = 0.028 C/m^2$

2)

a) $U = \frac{1}{2} \frac{Q^2}{C}$

b) $dU = \frac{1}{2} \epsilon_0 E^2 A dx$

c) $|F| = \frac{dU}{dx} = \frac{1}{2}\epsilon_0 E^2 A$

d)  A plate can not exert a force on itself. The total field between the plates is due to both plates.

3)

a) $J = \frac{I}{A} = 6.8\times 10^6 A/m^2$

b) $E = J\rho = 0.115 V/m$

c) $\Delta t = \frac{L}{v_d} = 8\times 10^5 s = 0.92 days$

4)

a) $R_{tot} \approx = R_2 = 4\times 10^{-5} \Omega$

b) $I_{H2O}= 0.002 A, \ \ I_{Cu} = 599.99 A$

c) $V = IR_{tot} = 0.024 V$

Suggestions by: Mikael B Steen

## 3 thoughts on “The FYS1120 Weekly Assignment Post #1”

1. Filip Sund on said:

I think perhaps either you or the guy who calculates the “official” answers should have a look at exercise 4, as you have very different answers.

http://folk.uio.no/atleq/fasit1120.pdf

2. Seems right! 🙂 A factor of 10 often comes out of nowhere.

3. Exercise 1b: I got 40 kV in this exercise.
We have $d = 2.0\text{mm} = 0.002 \text{m}$. This gives us $E_{max} \cdot d = 2 \cdot 10^7 \text{V/m} \cdot 0.002 \text {m} = 40 000 \text{V} = 40 \text{kV}$.

Or am I missing something?