In Part A, you determined that the heat added to the coffee is what you got. If it takes 4.2 J of energy to raise the temperature of 1.0 mL of coffee by 1.0∘C, then it should take twice as much energy to raise the temperature of 2.0 mL of coffee by 1.0∘C, or to raise the temperature of 1.0 mL of coffee by 2.0∘C. Multiply carefully to find the total amount of energy needed to raise the temperature of 200 mL of coffee by the required number of degrees.

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- IsTim

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- IsTim

This is a hint to an assignment I am currently doing.

- IsTim

I haven't read thru the textbook much or my notes; I'ved tried, and instead made a formula sheet, although I don't know what formula to apply to this.

- ***[ISURU]***

use
\[E = mc \theta \]
Since there r lot of data this one can be use easily ... I think u should be familiar with this...

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## More answers

- IsTim

Actually, haven't seen this in my formula sheet. But I'll try it out.

- ***[ISURU]***

wt's the value of "required no.of degrees" mentioned in the question ?

- IsTim

1?

- ***[ISURU]***

then I think u can use a relationship with ratios to solve this one cause the problem says "Multiply carefully to find " which mean question doesn't expect use of any formula instead it expect to build the answer from given data...
change of 1 degree in 1ml = 4.2 J
change of 1 degree in 2ml = 8.4 J
change of 1 degree in 200 ml = 8.4*100 J

- IsTim

These are the answers that I gave to the work, which it stated was incorrect. It requires 3 significant figures.
Submitted Answers
ANSWER 1: Deduction: -3%
"what you get" =
8.4⋅102
J
ANSWER 2: Deduction: -3%
"what you get" =
8.40⋅102
J

- IsTim

the 102 is suppose to be 10^2

- IsTim

So yes, I did use your answer, but apparently it was incorrect here.

- ***[ISURU]***

It requires 3 significant figures. = is it same for the 840 J ?

- IsTim

wouldn't 8.40*10^3 J have worked?

- IsTim

Because 840 is only 2 sig digs without using a decimal point

- ***[ISURU]***

no .. I just told that 'cause it's the same answer I get even if I use
E = mc*theta

- IsTim

So, both answers would be considered incorrect with this question then?

- ***[ISURU]***

- IsTim

I managed to get the question after that correct, if it helps at all.

- ***[ISURU]***

if 840 J is wrong .... there is one possible reason:
we calculate 840 J to a temp. change of 1 degree. But the question asks the energy need for a temp. change of required no. of degrees. And we assume that is also as 1 degree. But this question has a section name part A. Does that part mention about some reading of temperature regarding this coffee cup

- IsTim

Yes. This is a 'hint' to a larger question.

- IsTim

Suppose that you have left a 200-mL cup of coffee sitting until it has cooled to 30∘C , which you find totally unacceptable. Your microwave oven draws 1100 W of electrical power when it is running. If it takes 45 s for this microwave oven to raise the temperature of the coffee to 60∘C , what is the efficiency of heating with this oven?
Find the efficiency e of the oven. You will need to use the fact that 4.2 J of energy is required to raise the temperature of 1.0 mL of coffee by 1.0∘C .

- ***[ISURU]***

yo ... that's the missing point!

- IsTim

?

- ***[ISURU]***

the temperature of the coffee cup was 30 degree at the beginning and it has to be increased up to 60 degree

- IsTim

But I dunno if those 2 questions are directly related/

- ***[ISURU]***

So we must calculate energy need to change the heat of 200ml by 30 degree

- ***[ISURU]***

not just by one degree which is 840 J instead it should be 8.4*30 * 100 J which is 252*100 J and it also satisfy the condition of 3 digit ... try that one!

- IsTim

I've got 2 tries left. i'll test it out

- ***[ISURU]***

wt happened ?

- IsTim

err sorry. i'm asking this question on several different mediums, and i'm just 'cross referencing'

- IsTim

Err, 8.4*30 * 100 J looks like \[8.4*10*100\]
252*100 J looks like \[252*100\]

- IsTim

Because if that was true, wouldn't that make the question incorrect?

- ***[ISURU]***

srry.. could u explain it more ... it's not clear

- IsTim

The stars; do they represent the 'multiply' or the 'exponent' function?

- ***[ISURU]***

multiply

- IsTim

Ok. Thanks.

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