# Electromagnetic field, Gauss’ Law questions

1. Jan 24, 2010

### Angello90

Hey guys, help needed! Bad lecture, no idea what to do!

The Q1 is:

Q. 1
Three point charges are placed at the apexes of an equilateral triangle as shown in diagram. Calculate the net force on (a) the 5 μC (= 5 × 10‐6 C) charge and (b) the ‐6 μC charge. Give each answer in the form a i + b j where a and b are the x and y components of the force, respectively. The permitivity of free space is
ε0 =8.854×10‐12 F m‐1 (i.e. farad/metre )
Answer 1a: a i + b j units
Answer 1b: c i + d j units

The Q2 is:

Q.2
(a) A mass of 1 kg carrying a charge of 0.15 C is launched with an initial velocity of 6 m s‐1 vertically upwards in a uniform electric field, also directed vertically upwards, of strength E= 50 N C‐1. What is the maximum height reached by the mass in its subsequent motion? (6 marks)

(b) If instead the electric field points in a diretion inclined at 60o to the horizontal, what is the maximum
height reached? (4 marks)

And final Q3:

Q. 3
Use Gauss’ Law to calculate the electric field at a distance ρ (rho)
from an infinite rod carrying uniform linear charge density λ
(lambda) coulombs per unit length. Specify the direction of the
field, choosing between the ρ, z and φ (phi) directions (cylindrical
coordinates).
Hint: Calculate the electric flux through the pillbox as shown in
diagram where the direction of the electric field lines decides which
of the pillbox surfaces (the curved surface or the top and bottom flat
surfaces) contributes to the integral over the Gaussian Surface.

Guys, I know I supposed to give my reasoning and what I think what to do, but I dont have a smallest clue! I am ussually good at physics, but my lecture has changed and this one is...

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2. Jan 24, 2010

### Angello90

Guys does this look right for Q1?

sorry about poor quality, my scanner is down

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3. Jan 24, 2010

### minimark1234

for Q1:
i couldn't read ur writing, so i cant tell you if you are right or wrong, but some things to keep in mind for at least the first one (i didn't read the second problem yet) are superposition ~that u can take the sum of the force applied from each of the other electric charges, and that will be the net force~, and that
$$\vec{F}=q\vec{E}$$ and
$$\vec{E}=\frac{1}{4\pi\epsilon}\int \frac{\hat{n}}{n^{2}}\rho \:d\tau$$ or more simply for point charges
$$\vec{E}=\frac{q}{4\pi\epsilon r^{2}}\hat{r}$$

Last edited: Jan 24, 2010