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dustbus 2013-9-12 08:12 PM

E fleid

AL 2002 LQ
Given that Q = 2.2 × 10^–11 C andVo = –10 V
r0=2.0x10^-2 and rb = 5 cm

我諗個energy process係

KE=> work done against E field

咁姐係1/2mv^2(min.)=qEd

但點解marking係直接1/2mv(min)>/ Qq/4兀Er^2

左面colXXX law,係力黎,右面係energy,咁唔係應該係work done against E field先岩咩:smile_41:
可能1999,望師兄解答

[[i] 本帖最後由 dustbus 於 2013-9-12 08:15 PM 編輯 [/i]]

jmlo 2013-9-12 11:24 PM

回覆 1# 的帖子

The HKEA marking scheme I have is correct; it states that

1/2 mv^2 > Qq/4*pi*eps*r

So, please check again the marking scheme you have. Is it given by your teacher? There may be some typos.

dustbus 2013-9-12 11:29 PM

[quote]原帖由 [i]jmlo[/i] 於 2013-9-12 11:24 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=371506772&ptid=22428652][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
The HKEA marking scheme I have is correct; it states that

1/2 mv^2 > Qq/4*pi*eps*r

So, please check again the marking scheme you have. Is it given by your teacher? There may be some typos. [/quote]

=.=SORRY

but why Qq/4兀*pes*r

it is just the electric potential,but not the work done?

jmlo 2013-9-13 12:23 AM

回覆 3# 的帖子

The expression of Q*q/4*pi*eps*r is not electric potential induced by the charge Q but the electric potential energy (or work done) to move the charge q to the position r from the charge Q.

Recall the four useful expressions in electrostatics:

F = k*Q*q/4*pi*eps*r^2   (Electrostatic force; Coulomb's law)
E = k*Q/4*pi*eps*r^2       (Electric field; E = F/q)
U = k*Q*q/4*pi*eps*r       (Electric potential energy; integral of F along dr; or U = q*V)
V = k*Q/4*pi*eps*r           (Electric potential; E = -dV/dr in 1-dimension)

↗亢龍無悔↙ 2013-9-13 08:55 AM

[quote]原帖由 [i]jmlo[/i] 於 13-9-2013 12:23 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=371510496&ptid=22428652][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]The expression of Q*q/4*pi*eps*r is not electric potential induced by the charge Q but the electric potential energy (or work done) to move the charge q to the position r from the charge Q.Recall  ... [/quote]Similar concepts are demonstrated in gravitation but already out c in DSE physics core



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jmlo 2013-9-13 10:11 AM

回覆 5# 的帖子

Is the chapter of gravitation completely out of syllabus already? I still recall some questions about gravitation that are the trickiest I have ever seen. For example, calculate the gravitational force of a spherical mass due to a large uniform sphere which contains an internal hollow sphere (radius much smaller than the larger sphere) displaced a bit from the centre. :smile_o16:

dustbus 2013-9-13 07:30 PM

[quote]原帖由 [i]jmlo[/i] 於 2013-9-13 10:11 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=371526119&ptid=22428652][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
Is the chapter of gravitation completely out of syllabus already? I still recall some questions about gravitation that are the trickiest I have ever seen. For example, calculate the gravitational forc ... [/quote]

lol

We only teach F=GMm1/r^2:loveliness::loveliness:

gravitiation out C

thx jmlo ching

↗亢龍無悔↙ 2013-9-13 07:39 PM

[quote]原帖由 [i]jmlo[/i] 於 13-9-2013 10:11 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=371526119&ptid=22428652][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]Is the chapter of gravitation completely out of syllabus already? I still recall some questions about gravitation that are the trickiest I have ever seen. For example, calculate the gravitational forc ... [/quote]Your AL was much crazier than i what i took.
By the way, can you tell me how to do such question?  a ball inside a hollow sphere need to find the effective mass/ density ?



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jmlo 2013-9-14 10:59 AM

回覆 8# 的帖子

The simplest trick is to consider the sum of gravitational forces exerted by different sources.

Other interesting questions about gravitation:

(1) The gravitational force due to a sphere which has an eccentric spherical hole inside.
(2) Gravitational force including the effect of Earth's rotation

dustbus 2013-9-14 06:55 PM

jmlo ching
I want to ask about chem....:Q:Q:Q:Q

1)Why HCl cannot form hydrogen bond?Cl have same electronegativity to nitrogen,but the hydrogen bond can only form with O,N,F

2)If cl cannot form,why CHCl3 can form hydrogen bond:smile_41:
3)How noble gases form compound,such as XeF4

4)Why the compound formed between Xenon and chloride are more unstable than xenon and floride

[[i] 本帖最後由 dustbus 於 2013-9-14 07:10 PM 編輯 [/i]]

jmlo 2013-9-15 12:25 AM

回覆 10# 的帖子

[quote] 1)Why HCl cannot form hydrogen bond?Cl have same electronegativity to nitrogen,but the hydrogen bond can only form with O,N,F [/quote]
HCl molecules in gas phase can indeed form H-bonds but they are relatively weak.

The difference between N and Cl is mainly due to the larger atomic size of Cl.

[quote] 2)If cl cannot form,why CHCl3 can form hydrogen bond [/quote]
H in CHCl3 is more electropositive due to three polarized C-Cl bonds.

[quote] 3)How noble gases form compound,such as XeF4 [/quote]
This is a question that has puzzled chemists for a long time. A simplest explanation (not fully correct at least in my understanding, but useful) is the expansion of the octet of Xe. In XeF4, Xe uses four of its eight valence electrons to form four Xe-F bonds. So, in the compound Xe has four bond pairs and two lone pairs (four unused valence electrons pair up), and the compound adopts a square planar geometry, having the two lone pairs at the axial positions.

The difficult part of making noble gas compounds is to oxidize noble gases. Due to the complete octet, noble gases under normal conditions are inert. However, when they are treated with very strong oxidizing agents, they can still be oxidized, and this is how the first true noble gas compound was synthesized in 1962.

[quote] 4)Why the compound formed between Xenon and chloride are more unstable than xenon and floride [/quote]
Whether covalent XeCl2 does exist is still questionable. Even so, it is more unstable compared to XeF2. A reason is that the stability of the resulting Xe compound depends on how well the halogen oxidizes Xe. Since Cl is less oxidizing than F, the Xe-Cl thus formed is less stable than Xe-F. The energy gain due to Xe-Cl formation is not sufficient to cover the cost of oxidizing Xe and breaking the Cl-Cl bond.

dustbus 2013-9-15 02:33 AM

I can know more about chemistry

thx jmlo ching:loveliness:

dustbus 2013-9-15 06:07 PM

why dissoulution of salt may be endothermic or exothermic?
such as NaCl(s)--->NaCl(aq) is endothermic

but LiCl(s)--->LiCl(aq) is exothermic

they are also bond breaking

jmlo 2013-9-16 12:08 AM

回覆 13# 的帖子

Breaking up ionic lattice requires energy input. Once ions are separated in solution, they are solvated by water molecules, and this process gives out energy. Therefore, the overall change of energy associated with dissolution depends on the sum of these two terms. If the energy input is bigger than the energy output, the process will be endothermic. If the energy output is greater than the energy input, then the process is exothermic.

In the case of LiCl, the solvation energy of Li+ is large because of its high charge density.

dustbus 2013-9-16 08:27 PM

thx:loveliness: :loveliness: :loveliness: :loveliness:
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