Physics Paper 3

[Yoyo1994]

I have a question regarding Physics Paper 3 on the paper October/November 2009 Variant 2. You can find it here: http://www.xtremepapers.me/CIE/Cambridge IGCSE/0625 - Physics/0625_w09_qp_32.pdf
If you go to question 9 and have a look at (c) it tells you that the specific heat capacity of water is 4200J/(kgK). KgK means Kg multiplied by Kelvin. Kelvin is °C+273. Now, when you go to c)ii) they ask you to calculate the thermal energy gained. I figured I should use the shc formula (c=E/mt) which is correct but what I did was wrong. First I got the correct answer of 1512 J but then I remembered that the question gave you J/KgK so I crossed all my answers out for that section and redid it by converting °C to K first. This might be a bit confusing but I will show you how I worked it out.

1st I got the correct answer of 1512J without converting °C to K. So when I saw that: 4200J/KgK = E/(0.200Kg*1.8°C) I thought maybe this was wrong because K does not equal °C. So I rejected all I've done and converted 1.8°C to K which is 274.8K and redid the whole thing. 4200J/KgK = E/(o.2Kg*274.8K), now it makes more sense because K = K. So I got 230832J. This is wrong. The correct answer is obtained without converting to Kelvin. Why is that so? It isn't fair. °C can't be used if the value given was in J/Kg*K
I lost 5 marks because I used my head but nothing was appreciated.

In case you don't understand the question, why isn't °C converted to Kelvin before carrying out the equation? 4200J/Kg*K is provided, and we calculated °C values that we need to use in the equation. To make it fair, aren't we supposed to convert our °C values to Kelvin?

The mark scheme doesn't agree with me because of the answer they got, which they obtained WITHOUT converting to Kelvin. (They got 1512J)
Thanks.

 

[unplugged]

I think one Kelvin measures the same temperature change as one Celsius but they just got different starting points.
From wikipedia [°C] = [K] − 273.15. So it seems that the 0 point in kelvin is simply 273.15 degrees higher than the 0 point in celsius but 1 C = 1 K basically.

 

[EYESSHHHAA]

for the same question....how do u get the answer to c(i)...the temperature fall of metal...i am not gettin it any other way at all....

 

[unplugged]

The metal was in boiling water till it reached the temperature of the water , so when it entered the cup it had a temperature of 100C. Afterwards we can assume that the heat was transferred and the temperature of the metal is now the same as the water it's in. So temperature is 22.9. so drop is 100-22.9 = 77.1

 

[Yoyo1994]

So has anyone figured out the answer to my question?

 

[unplugged]

So has anyone figured out the answer to my question?

Yes. 1 joule to increase the temperature of 1 kilogram by 1 kelvin is the same as 1 joule to increase the temperature of 1 kilogram by 1 Celsius.
1 kelvin is the same as 1 Celsius , they just start at different point. But of course this doesn't matter when we're simply measuring increase or decrease not marking a point.

 

[Yoyo1994]

So has anyone figured out the answer to my question?

Yes. 1 joule to increase the temperature of 1 kilogram by 1 kelvin is the same as 1 joule to increase the temperature of 1 kilogram by 1 Celsius.
1 kelvin is the same as 1 Celsius , they just start at different point. But of course this doesn't matter when we're simply measuring increase or decrease not marking a point.

So you must use °C when measuring specific heat capacity?

 

[Lyfroker]

plx te me the threshold

 

[unplugged]

You just need to understand the relation between kelvins and Celsius . Celsius starts at the point of water freezing or ice melting which we call 0 degrees Celsius. Kelvins start at the point of absolute zero which in degrees Celsius would be -273 degrees Celsius. However 1 kelvin is = -272 degrees Celsius , 2 kelvins is - -271 degrees Celsius. So as you see the UNIT OF MEASUREMENT is the same. an increase by 1 kelvin is the same as an increase by 1 degree Celsius.
So when he says that something requires 1 joul to increase 1 kilogram of it by 1 kelvin you know that it requires the EXACT same amount of energy to increase it by 1 degree Celsius. Because the temperature increase is the same. It was a trick question to test if students understood the relation.