Question

An immersion heater has a power rating of 1500 watts. It is used to heat water for coffee. How long (in minutes) should it take to bring 1.3 L of water from room temperature (20°C) to 88°C?

Answer 1

Q = C m ∆t

Where

• Q = quantity of heat absorbed by a body
• m = mass of the body
• ∆t = Rise in temperature
• C = Specific heat capacity of a substance depends on the nature of the material of the substance.
• S.I unit of specific heat is J kg-1 K-1.

We have :

• m = 1.3 liter = 1.3 kg of water
• ∆t = (88-20) ⁰C = 68 ⁰C
• c = 4200 J/kg°C

Therefore,

Q = (4200×1.3 × 68) J = 371280 J

∵ 1 watt = 1 J/s

∴ time required to raise temperature of water

= (371280 J) /(1500 J/s)

= 247.52 s

Hence time required is 247.52 s or 4.125 minutes

Ans: 4.125 minutes