Heat energy

Energy is measured using the unit Joule (J). Temperature is measured using the unit degree Celsius °C). To increase the temperature of 1 liter of water by 1°C, 4200 J of heat energy is required.

(a) Kelly wants to increase the temperature of 2 liters of water from 25°C to 100°C.

(1) Find the amount of heat energy required.
(ii) The energy efficiency of an electric kettle A is 60%; that is, it requires 1 J of electrical energy to produce 0.6 J of heat energy. Find the amount of electrical energy kettle A requires.

(b) Another electric kettle B requires 80% of the electrical energy used by electric kettle A. Find the energy efficiency of electric kettle B.

Correct answer:

Q1 =  630000 J
Q2 =  1050000 J
η2 =  75 %

Step-by-step explanation:

c=4200 J/l/K V=2 l Δt=10025=75 °C  Q1=V c Δt=2 4200 75=630000 J
η=60%=10060=53=0.6 Q1 = η Q2  Q2=Q1/η=630000/0.6=1050000 J
r2=80%=10080=54=0.8 Q3=r2 Q2=0.8 1050000=840000 J  η2=100 Q3Q1=100 840000630000=840000100 630000=84000063000000=75%

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