# Water equivalent of a kettle

1. Mar 29, 2015

### ande

1. Please see the attachments for my attempts at answering the question.

Please note that this is not a homework question as the answers are provided. I just want to know how to get there. Thanks in advance.

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Last edited by a moderator: Mar 29, 2015
2. Mar 29, 2015

### haruspex

Not sure how you're interpreting the second question. My interpretation is:
• The 20% of power that was not useful in the first part includes the heat required to raise the kettle to 90C.
• In the second part, the hot 1 litre of water is poured out and a second litre of water at 15C is poured in.
• The wasted heat is less the second time because the kettle itself starts hot.
Your attachments are not easy to read. They come out side-on and the writing is a bit small. The ability to add images is really intended for diagrams and extracts from printed matter. Please take the trouble to type in working.

3. May 9, 2015

### DevonZA

I have the same question in my study guide and I get 10 min and 7 seconds for part b

4. May 9, 2015

### haruspex

I get 11, 58 for part a, but maybe 4.2 J per Calorie is not accurate enough. For part b I get 10, 24.

5. May 10, 2015

### DevonZA

1000g-150g = 850g

Eout = mct
= (.85)(4190)(75)
= 267112.5J

Ein = Eout/efficiency
= 267112.5/0.8
= 333890.625J

time = Ein/Pin
= 333890.625/550
= 607s = 10 min and 7 sec

6. May 10, 2015

### haruspex

Why do you subtract 150 from 1000?

7. May 10, 2015

### DevonZA

The question states that the water equivalent of the kettle is 150g. 1 litre = 1000g. Therefore the kettle only needs to heat 1000g-150g=850g for the second litre of water. I'm not sure if I'm correct though.

8. May 10, 2015

### haruspex

There's a litre of water in both cases. The kettle is additional to that.
You need to think about where the other 20% of the power goes. In each case, the equation takes the form:
energy in = energy to heat water + energy to heat kettle + energy lost to room/air
Assign symbols to those (distinguishing a and b) and write the equation for each using the given data.