<li>A charge Q is placed at the center of a hollow conducting sphere of radius 2R. If the electric field strength at a distance R from the charge is E, what is the electric field strength at a point a distance 3R from the center of the sphere?</li>
</ol>
<p>(A) 0
(B) (1/9)E
(C) (1/3)E
(D) E
(E) 3E</p>
<p>[The correct answer is (D) E. I actually picked choice (C) because I thought the electric field strength would be less if its farther away from the charge… or am I wrong? Can someone please explain.]</p>
<li>A cylinder has a volume of 0.2 m^3. It is composed of two different materials, with densities of 100 kg/m^3 and 300 kg/m^3 respectively. If each of these materials has an equal mass, what is the mass of the cylinder?</li>
</ol>
<p>(A) 20 kg
(B) 30 kg
(C) 40 kg
(D) 60 kg
(E) 100 kg</p>
<p>[For this problem I chose choice (A) 20 kg. I had no idea how to approach this question during my test. So I relied on intuition and some math skills, and I ended up with 20 kg somehow. But the correct answer was 30 kg unfortunately. Can someone please explain?]</p>
<p>I've never taken physics but when I tried to solve 18 I came up with 40 kg. The way I solved it was by:
1. Found average density ((100+300)/2=200 kg/m^3)
D= M/V
The problem says volume .2m^3 so:
200 = M/.2
.2*200= M
40= M</p>
<p>Are you sure the correct answer is 30 kg?</p>
<p>For number nine, the charge would draw the opposite charge to form around the inside of the hollow shell. Thus the outside of the circle would have the same charge as that inside one. This process happens in a few nanoseconds. Guass's Law says that the electic field inside a conducting material is 0, so in this case E should be 0 if it did the process above. From the outside, it appears as if the electric field is at the center of the sphere. So at a distance of 3R the field would be KQ/(3R)^2 = 1/9E at distance R, but at distance R, the field should be 0 because it is in the conductor. If I had to pick an answer, Id choose B, but ideally I think that it should be KQ(3R)^2 and inside at a distance R, E = 0.</p>
<ol>
<li>A cylinder has a volume of 0.2 m^3. It is composed of two different materials, with densities of 100 kg/m^3 and 300 kg/m^3 respectively. If each of these materials has an equal mass, what is the mass of the cylinder?
A= mass of 100kg/m^3
B = mass of 300kg/m^3
A=B (equal mass)
A/100kg/m^3 + B/300kg/m^3 = 0.2m^3
If you do simultaneous equations you get
3B + B = .2<em>300
B = .2</em>300/4
and that comes out to be 15
and then A=B so each is 15 and the mass of the circle is A+B =30</li>
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<p>You cant do the average density because this is similiar to like you run a distance, and then walk the same distance, what is your total time.</p>
<p>Well my teacher says the answer was 30 kg.</p>
<p>mattd1688, why is the electric field 0? The correct answer is choice (D).</p>
<p>When a charge is placed within a conductor, it moves so that the charge is uniformly distributed on the outside of the object. Then if you take the Efield of the inside, it is zero according to Gauss' law, and when it is met, after a few nanoseconds, the conductor is said to be in electrostatic equilibrium. Even if you were to place the charge near a side of the sphere, the field would still be zero inside, and from the outside, it would be as if it was in the center of the sphere.</p>
<p>If it were just like a point charge, then it would be 1/9E, but really the charge would go to the surface. Ask him about it tom and see what he says, unless you put 1/9, in that case dont say anything.</p>