☰ Contents · Physics

Power. Energy conservation and efficiency

Lessons 45–46 · 2 lessons · P.Q. Habibullayev, A. Boydedayev, A.D. Bahromov, S.O. Burxonov. Physics Grade 7, revised and enlarged 4th edition. “O‘zbekiston milliy ensiklopediyasi” State Scientific Publishing House, Tashkent, 2017
45

Power

Textbook: pp. 163–165
GoalKnow power N = A/t, the watt and N = Fυ.
New words
power · quvvatwatt (W) · vatt (W)kilowatt (kW) · kilovatt (kW)work done · bajarilgan ish
Explanation

The quantity describing how fast work is done is power: the ratio of work A to the time t in which it is done, N = A/t. The SI unit of power is the watt (W): 1 W = 1 J/s, that is, 1 J of work done in 1 s. Large powers are measured in kilowatts (1 kW = 1000 W) and megawatts (1 MW = 1 000 000 W). Since A = Fs and s = υt, for uniform motion N = Fυ: power equals force times speed. Whoever does the same work faster has greater power. For example a motor doing 600 J of work in 3 s has power 600 : 3 = 200 W. The power of devices is chosen according to their job.

Worked examples
A crane doing 4000 J of work in 8 s: N = 4000 : 8 = 500 W.
A car with traction 2000 N moving uniformly at 20 m/s: N = 2000 · 20 = 40 000 W = 40 kW.
Class activity

“Climbing stairs”: under the teacher’s eye one pupil walks up the stairs at normal pace; the class estimates mass, height and time and computes power (no running). Safety: do not run on stairs.

Practice
1
What is power? Give its formula and unit.
2
A = 1500 J, t = 5 s. N (W)?
3
A 2 kW motor does how much work in 10 s (J)?
4
Why does the machine that does the same work faster have more power?