Calculating Latent Heat of Vaporisation: 3 kW Kettle, 2.0 kg Water @ 100oC

[luigihs]

A 3 kW kettle contains 2.0 kg of water at a temperature close to 100oC.


Latent heat of vaporisation for water: Lv=2256 (kJ kg^-1)

Q= Lv x mass



Ok I understand this problem because I now the answer but I don't understand the process.

Like my teacher wrote 2256x10^3 why he wrote 10^3 ?? I don't understand ! .. so he plugged into the equation and he get 2256x10^3 x 1.5kg = the first answer is 3.38x10^6 J But again I understand why he wrote 10^6? what's the point ... because If put in my calculator 2256 x 1.5 = 3384 ... , and the final answer is 3.38 MJ what happen to the 10^6 ?? why he convert to MJ I really confused help me please!
Thank you..

 

[gneill]

A 3 kW kettle contains 2.0 kg of water at a temperature close to 100oC.


Latent heat of vaporisation for water: Lv=2256 (kJ kg^-1)

Q= Lv x mass



Ok I understand this problem because I now the answer but I don't understand the process.

Like my teacher wrote 2256x10^3 why he wrote 10^3 ?? I don't understand ! .. so he plugged into the equation and he get 2256x10^3 x 1.5kg = the first answer is 3.38x10^6 J But again I understand why he wrote 10^6? what's the point ... because If put in my calculator 2256 x 1.5 = 3384 ... , and the final answer is 3.38 MJ what happen to the 10^6 ?? why he convert to MJ I really confused help me please!
Thank you..

Unit prefixes and powers of ten: read here

 

[luigihs]

Unit prefixes and powers of ten: read here

Cheers but I still struggling how can i get 3.38x10^6

 

[gneill]

MJ = megaJoules = 10⁶ Joules

 

[luigihs]

MJ = megaJoules = 10⁶ Joules

Ok but that doesn't make sense because I have another problem with the same type of answer
E =1.5kg * 4190 J kg-1 K-1 x 85o = 5.34*10^5 J = 534 kJ. and there is no 10^5 in the decimal prefixes ...

 

[gneill]

534 kJ = 534 x 10³ J = 5.34 x 10^5 J

The reason it was specified as 534 kJ is because there's no 10⁵ prefix.

Standard prefixes are for powers that are multiples of three (kilo and above).

 

[luigihs]

534 kJ = 534 x 10³ J = 5.34 x 10^5 J

The reason it was specified as 534 kJ is because there's no 10⁵ prefix.

Standard prefixes are for powers that are multiples of three (kilo and above).

So the answer is in Kj because is 10^5 and is between 10^5 and 10^3 ??

 

[gneill]

So the answer is in Kj because is 10^5 and is between 10^5 and 10^3 ??

Essentially, yes.

Sometimes you'll see values like 0.341 kJ or 1500 MW where the convention is "bent" a bit. Usually this is done to make the particular number have common units with other like-values for comparison (like entries in a table of values), or where one particular prefix would not handle all of the values to be shown.