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Author Topic: Physics Help HERE ONLY !!  (Read 81721 times)

Offline Shogun

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Re: Physics Help HERE ONLY !!
« Reply #345 on: June 03, 2009, 10:26:42 am »
Quote from: IGSTUDENT on June 03, 2009, 10:21:26 am
Quote from: Shogun on June 03, 2009, 10:18:33 am
Quote from: idol2009 on June 03, 2009, 10:12:39 am
Since no one answered my question before, i will repost it =)
 Hey, I was doing the Oct/Nov 08 Paper 3, and on question 5c), it asks you to calculate the specific latent hear of fusion of ice from the results they have given you...
I usually find this pretty straightforward, but on this problem I was kind of stuck, because my answer didn't match the mark scheme...
The question goes as follows:
"Using a 40W heater, 16.3 g of ice is melted in 2.0 minutes. The heater is then switched off. In a further 2.0 minutes, 2.1 g of ice is melted.
Calculate the specific latent heat of fusion of ice from these results."

So I basically did the following calculation:
Total Energy supplied = 40 x 120 = 4800 J
Total Mass melted by heater = 16.3 g
Specific latent heat = 4800/16.3 = 294.5 J/g

I don't understand what I've done wrong, and what the switching the heater off and the further 2 mins has to do with it?
Any help would be appreciated thanks!

oh sorry for u answer to be delayed, so ill try to help u out right away.. ;D


SHOGUN

R u a student or a teacher? U seem to have an answer for every problem.And time to solve every query 8)





lol im no teacher im a student and i take ig subject its just tht physics is my subject and if i learn something new, id go deep into it even though it was to be studied by students in the universeties.. i daily go to my phsyics to leran something thts out of the syllabus.. ;D

i live in bahrain, and i take exams from cie in ma school.
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Offline Ghost Of Highbury

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Re: Physics Help HERE ONLY !!
« Reply #346 on: June 03, 2009, 10:30:10 am »
heat received = mass * temp changed * c
40W --> 40 J/s for 180 seconds = 7200J
7200 = 250 * 5.9 * c
7200/ 1475 = 4.88
ans = 4.88 J/g degree celcius

oct/noc 2008 second variant 5c
« Last Edit: June 03, 2009, 10:41:22 am by Adi...\m/ »
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Offline Ibiza279

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Re: Physics Help HERE ONLY !!
« Reply #347 on: June 03, 2009, 10:34:11 am »
adi...\m/

they r askin fer the latent heat not the specific latent heat of water

so u just use the equation E=ml

n somebody give me the answer to this question from the answer sheet cause i think the mass v hav to use is 2.1 g n not 16.3g "Using a 40W heater, 16.3 g of ice is melted in 2.0 minutes. The heater is then switched off. In a further 2.0 minutes, 2.1 g of ice is melted.
Calculate the specific latent heat of fusion of ice from these results."
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Offline Shogun

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Re: Physics Help HERE ONLY !!
« Reply #348 on: June 03, 2009, 10:35:10 am »
allright, see u have taken the whole mass not the mass of ice which is actually bieng melted, so its not 16.3

u need the mass taken for the ice to melt completely, ie. initial mass of ice melted is 16.3, final mass of ice melted is 2.1...  so to get the actuall mass tht was melted u have to subtract
 
i.e 16.3-2.1=14.2g

then use  the formula P x t= mlf

40 x 120 = 14.2 x m (we need to find m)

then sunsitute the equation to : (40 x 120) / 14.2

u get 338j/g which will be ure "M",

do check the mark scheme if its correct....

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Offline Shogun

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Re: Physics Help HERE ONLY !!
« Reply #349 on: June 03, 2009, 10:37:29 am »
yup my ans is correct, i hpe i helped , hope u understood... :D
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Offline Enia

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Re: Physics Help HERE ONLY !!
« Reply #350 on: June 03, 2009, 10:37:36 am »
Thank you so much Shogun. I'll bother you again with my stupid questions later :P
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Offline Shogun

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Re: Physics Help HERE ONLY !!
« Reply #351 on: June 03, 2009, 10:38:45 am »
no prob, ask as much as u want. untill ure confident when ure at ure exam.. ;D
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Offline Ghost Of Highbury

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Re: Physics Help HERE ONLY !!
« Reply #352 on: June 03, 2009, 10:39:53 am »
lol...big misunderstanding ...i was talking abt the second variant..
hah..
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Offline Ibiza279

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Re: Physics Help HERE ONLY !!
« Reply #353 on: June 03, 2009, 10:41:09 am »
Quote from: Shogun on June 03, 2009, 10:35:10 am
allright, see u have taken the whole mass not the mass of ice which is actually bieng melted, so its not 16.3

u need the mass taken for the ice to melt completely, ie. initial mass of ice melted is 16.3, final mass of ice melted is 2.1...  so to get the actuall mass tht was melted u have to subtract
 
i.e 16.3-2.1=14.2g

then use  the formula P x t= mlf

40 x 120 = 14.2 x m (we need to find m)



then sunsitute the equation to : (40 x 120) / 14.2

u get 338j/g which will be ure "M",

do check the mark scheme if its correct....



wo wo wo wo hold up

16.3 is the mass of ice that melted on heating and 2.1 is the mass of ice melted when there was no heat supplied meaning that the ice melted while it absorbed the heat frm the surrounding so i don't get the point of it

n By the way which year paper question is this one ?
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Offline Ghost Of Highbury

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Re: Physics Help HERE ONLY !!
« Reply #354 on: June 03, 2009, 10:44:55 am »
Quote from: Ibiza279 on June 03, 2009, 10:41:09 am
Quote from: Shogun on June 03, 2009, 10:35:10 am
allright, see u have taken the whole mass not the mass of ice which is actually bieng melted, so its not 16.3

u need the mass taken for the ice to melt completely, ie. initial mass of ice melted is 16.3, final mass of ice melted is 2.1...  so to get the actuall mass tht was melted u have to subtract
 
i.e 16.3-2.1=14.2g

then use  the formula P x t= mlf

40 x 120 = 14.2 x m (we need to find m)




then sunsitute the equation to : (40 x 120) / 14.2

u get 338j/g which will be ure "M",

do check the mark scheme if its correct....



wo wo wo wo hold up

16.3 is the mass of ice that melted on heating and 2.1 is the mass of ice melted when there was no heat supplied meaning that the ice melted while it absorbed the heat frm the surrounding so i don't get the point of it

n By the way which year paper question is this one ?

i am confused too...
why do u need to subtract
?
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Offline Ghost Of Highbury

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Re: Physics Help HERE ONLY !!
« Reply #355 on: June 03, 2009, 10:45:32 am »
well!!..thats oct/nov 2008 paper 3 1st variant Q5.c.)
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Offline Shogun

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Re: Physics Help HERE ONLY !!
« Reply #356 on: June 03, 2009, 10:45:46 am »
u are correct but see, 16.3g was melted, and from tht 2.1 melted after the heating

so 16.3 and 2.1 are not the mass melted by heater...

the one melted from heater is when 2.1 was melted from the 16.3
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Offline Ghost Of Highbury

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Re: Physics Help HERE ONLY !!
« Reply #357 on: June 03, 2009, 10:48:21 am »
if 16.3 g is melted it has turned to water...
after that how can more 2.1 g melt from that melted portion
confused
plzz help shogun
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Offline astarmathsandphysics

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Re: Physics Help HERE ONLY !!
« Reply #358 on: June 03, 2009, 10:49:16 am »
Quote from: idol2009 on June 03, 2009, 10:12:39 am
Since no one answered my question before, i will repost it =)
 Hey, I was doing the Oct/Nov 08 Paper 3, and on question 5c), it asks you to calculate the specific latent hear of fusion of ice from the results they have given you...
I usually find this pretty straightforward, but on this problem I was kind of stuck, because my answer didn't match the mark scheme...
The question goes as follows:
"Using a 40W heater, 16.3 g of ice is melted in 2.0 minutes. The heater is then switched off. In a further 2.0 minutes, 2.1 g of ice is melted.
Calculate the specific latent heat of fusion of ice from these results."

So I basically did the following calculation:
Total Energy supplied = 40 x 120 = 4800 J
Total Mass melted by heater = 16.3 g
Specific latent heat = 4800/16.3 = 294.5 J/g

I don't understand what I've done wrong, and what the switching the heater off and the further 2 mins has to do with it?
Any help would be appreciated thanks!

The answer is here.
https://studentforums.biz/index.php/topic,1798.msg27218/topicseen.html#msg27218
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Offline IGSTUDENT

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Re: Physics Help HERE ONLY !!
« Reply #359 on: June 03, 2009, 10:49:40 am »
Its really nice to know that folks are involved so much in the subjects and not just exam oriented and have the time and inclination to share the knowledge.
what are u planning to pursue..astrophysics? or is it chem ,,u are a master in that too
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