Lessons 32–33 · 2 lessons · P. Habibullayev, A. Boydedayev, A. Bahromov, K. Suyarov, J. Usarov, M. Yuldasheva. Physics Grade 9, revised and expanded 3rd edition. G‘afur G‘ulom Publishing House, Tashkent, 2019
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Chapter III review: tests and summary
Textbook: pp. 91–93
GoalSummarise Chapter III: types of engines, the efficiency and Carnot formulas, environmental impact; test tasks.
Chapter summary: a heat engine converts the internal energy of fuel into mechanical energy; types – internal combustion, diesel and jet engines. Every engine consists of a heater (T₁), a working substance and a cooler (T₂) and works in cycles. A = Q₁ – Q₂; η = A/Q₁; for an ideal machine η = (T₁ – T₂)/T₁ (Carnot cycle: 2 isotherms, 2 adiabats). η is always below 1. Burning fuel affects nature (CO₂, CO, soot) and freons have damaged the ozone layer; to reduce this, raise efficiency and use renewable energy. In tests: 1) convert efficiency from percent to a fraction; 2) write temperatures in kelvins; 3) do not confuse A and Q₁.
Worked examples
An ideal engine has T₁ = 400 K, T₂ = 300 K, Q₁ = 1200 J: η = 100/400 = 0.25; A = 0.25·1200 = 300 J; Q₂ = 900 J.
Test: an engine takes 500 J from the heater and gives 350 J to the cooler. A = 150 J; η = 150/500 = 0.3 = 30 %.
Class activity
Chapter “short test”: 10 questions with four options; each group shows its answers on cards. Analyse the question with the most mistakes together.
Practice
1
Which processes make up the Carnot cycle?
Two isothermal and two adiabatic processes.
2
An engine takes 500 J and gives 350 J to the cooler. What is the efficiency in percent?
30
3
T₂ = 250 K, T₁ = 500 K. Efficiency of the ideal machine (%)?
50
4
Why are improving engines and using renewable energy important?
Fuel use and CO₂ emissions fall and the environment stays cleaner.