Question

How many joules are needed to freeze water?

Answer 1

In your question there are many important conditions undefined.
 
Let's imagine that we have 1 l of water at the temperature of 20 C and the surrounding temperature is also 20 C so it is not going to freeze down by itself.
 
Then to turn water into ice, you need to extract energy from it.

First you need to cool down the water to zero. It takes about 4200 J per liter/degree.
 
So to cool the water down you need to extract 4200 * 20 = 84 KJ

Then you need to crystallize the water. It takes 333 KJ to cristallize 1 kg or 1 l of water (the density of water is 1).
 
So in total you need to extract 417 KJ of energy.

On practice you'll need to use some kind of a refrigerator to do it. A coefficient of performance of a good Chiller could be about 4 which means you need 4 times less energy to transfer the amount of energy required.
 
So we get about 105KJ of energy spent.

Watt is equal to 1 j/second so 50 watt lamp uses 50 joules of energy each second. 105000 J / 50 = 2100 seconds = 35 minutes.
 
Please, check my math yourself, I could make a mistake :-)

Answer 2

heya..........

in ur question there is not mention for how many litre for water ? so i take here 1 litre....

#.....ur question there are many important conditions undefined.
 
Let's imagine that we have 1 l of water at the temperature of 20 C and the surrounding temperature is also 20 C so it is not going to freeze down by itself.
 
Then to turn water into ice, you need to extract energy from it.

First you need to cool down the water to zero. It takes about 4200 J per liter/degree.
 
So to cool the water down you need to extract 4200 * 20 = 84 KJ

Then you need to crystallize the water. It takes 333 KJ to cristallize 1 kg or 1 l of water (the density of water is 1).
 
So in total you need to extract 417 KJ of energy.......#

tysm