Question

How long would it take a kettle with a 1kW heating element to heat 1 litre of water?


Watts = Joules / second

• Mass of 1 litre of water = 1kg
• 1 kW = 1 k Joule / second or 1000 joules/ second
• 1g of water needs 4.184J to raise the temperature by 1̊ c.
• Water Temperature of 20oC


 Heat Required (J) = Mass of water (g) x Change in temperature (0C) x 4.184 (J/0Cg)

• Change in Temperature = 1000C- 200C= 800C

• Heat Required (J) = 1000g x 800C x 4.184(J/0Cg) = 334720J

• Time Taken= 334720J/ 1000J per sec= 334.72s

• or 334.72/ 60 = 5mins 58 secs

Is this correct?

Answer 1

S104 student, latest TMA?

The approach looks fine. If you are submitting this as an answer, please note

watts and joules have lower case first letters.

note that there is a space between the number and units and between units.

it is either 's' or 'seconds' not 'secs'

Answer 2

An electric kettle is rated 5KW.How much time will it take to boil 15kg of water initially at 15degree celsius ,if it is connected to 230V supply .