Heat engine (application of the second law of thermodynamics) – problems and solutions

1. Heat input QH = 3000 Joule and heat output QL = 1000 Joule. What is the efficiency of the heat engine?

Known :

Heat input (QH) = 3000 Joule

Heat output (QL) = 1000 Joule

Work done by the engine (W) = 3000 – 1000 = 2000 Joule

Wanted: Efficiency of the heat engine (e)

Solution :

Heat engine (application of the second law of thermodynamics) - problems and solutions 1

Read :  Newton's law of universal gravitation – problems and solutions

2. A heat engine produces 2000 Joule of mechanical work and the engine discharges heat to the environment at a rate of 500 Joule. What is the efficiency of the heat engine?

Known :

The work is done by the engine (W) = 2000 Joule

Heat output (QL) = 5000 Joule

Heat input (QH) = 2000 + 5000 = 7000 Joule

Wanted: Efficiency (e)

Solution :

Heat engine (application of the second law of thermodynamics) - problems and solutions 2

Read :  Converging lens – problems and solutions

3. A heat engine has an efficiency of 30%. If the engine produces 10,000 Joule of mechanical work, how much heat is discharged as waste heat from this engine?

Known :

Efficiency (e) = 30 % = 30/100 = 0.3

The work is done by engine (W) = 3,000 Joule

Wanted : Heat output (QL)

Solution :

Heat engine (application of the second law of thermodynamics) - problems and solutions 3

Heat output (QL) = heat input (QH) – work is done by engine (W)

QL = 10,000 Joule – 3,000 Joule

QL = 7,000 Joule