/wsr/ - Worksafe Requests » Thread #217501

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Anonymous Tue 08 Nov 2016 18:39:58 No.217501 View View Reply Original Report

Quoted By: >>217509

an electric shower has a maximum power of 7.0kw. if water enters at 10c and leaves at a temperature of 41 c, what is the maximum rate of flow of water? Density of water is 1000kg/m3

i have no fucking clue what to do. i cant use s.h.c since they don't give mass.

Anonymous

Anonymous Tue 08 Nov 2016 18:47:13 No.217509 Report

Quoted By: >>217513

>>217501
>they don't give mass
They give density, which you can divide volume by to get mass.

Because they give density in the question, you get to assume it's invariant with temperature (even though it's not).

Delta-t is 31 degrees, shc of water is 4.184 j/gram-degree, so each gram needs 129.7j to reach target temperature.

There are 7000j available each second, so it can heat 7000/129.7=53.96g of water each second.

From problem statement, 1l weighs one kg, so it has a flow of 0.054 litres per second, or 54 ml/second.

Anonymous

Anonymous Tue 08 Nov 2016 18:52:09 No.217513 Report

Quoted By: >>217520

>>217509
Sorry I forgot to include that they gave the shc of water to 4200j.

The answer they gave was 3.2 litres per minute, which is leaving me confused. How can I find the mass if vol isn't stated either?

Anonymous

Anonymous Tue 08 Nov 2016 19:03:32 No.217520 Report

Quoted By:

>>217513
Any math you do to whatevers, you can also do to whatevers-per-second, provided you leave the "per-second" on throughout the calculation.

The answer's in litres-per-second, and you're given the energy input in watts.

A watt is defined as a joule-per-second, so you're really going from joules-per-second to litres-per-second, which is exactly the same calculation as going from joules to litres, just with "-per second" on the end of everything.

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