# Physics

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?

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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'

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posted by Dr Russ
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 .

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