Lessons 34–35 · 2 lessons · P.Q. Habibullayev, A. Boydedayev, A.D. Bahromov, J.E. Usarov, K.T. Suyarov, M.K. Yuldasheva. Physics, textbook for Grade 8 of general secondary schools, revised and enlarged 3rd edition. “O‘qituvchi” Publishing and Printing Creative House, Tashkent, 2019
35
Problem solving
Textbook: pp. 100–101
GoalCompare incandescent and LED lamps and calculate efficiency η.
New words
incandescent lamp · cho‘g‘lanma lampaLED lamp · LED lampaefficiency η · FIK ηuseful work · foydali ish
Explanation
In an incandescent lamp a thin filament glows when heated: only a small part of the energy used turns into light, and most turns into heat. LED lamps give the same light with much less power, so they save energy. The usefulness of a device is judged by its efficiency: η = A_useful / A_total · 100 %, or for powers η = P_useful / P_total · 100 %. Efficiency never exceeds 100 %, because part of the energy is always dissipated as heat. The units of both works must be the same (both J or both W). When changing a lamp, an adult first switches the power off and unscrews the lamp only after it has cooled.
Worked examples
An electric motor took 500 J of electrical energy and did 400 J of useful work: η = 400 : 500 · 100 % = 80 %.
A heater takes 2000 W and gives 1700 W of useful heat power: η = 1700 : 2000 · 100 % = 85 %.
Class activity
With an adult look at the power and light (lumen) data on lamp boxes and compare in your notebook the powers of two kinds of lamp that give the same light.
Practice
1
What is efficiency and how is it calculated?
The ratio of useful work to total work, in percent: η = A_useful/A_total · 100 %.
2
A device used 800 J and did 600 J of useful work. Efficiency in %?
75
3
A motor’s total power is 1500 W and its useful power is 1200 W. Efficiency in %?
80
4
Why is the efficiency of an incandescent lamp lower than that of an LED lamp?
In an incandescent lamp most energy goes into heat; in an LED more becomes light.