Joule–Lenz law and efficiency

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

Answers are for parents and teachers.

R = 8 Ω, I = 2 A, t = 15 s. Q in J?

U = 20 V, R = 10 Ω, t = 30 s. Q in J?

A motor took 2000 J and did 1500 J of useful work. Efficiency in %?

Why can efficiency never be above 100 %?

Why is it dangerous to leave a heater unattended?