☰ Contents · Physics

Power problems and lamp power

Lessons 32–33 · 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
33

Lab work. Finding the electric power of a consumer (a lamp)

Textbook: pp. 96–97
GoalIn a laboratory work, determine the power of a lamp and the work of current using P = IU and A = IUt.
New words
lamp power · lampochka quvvatimeasuring circuit · o‘lchash sxemasistopwatch · sekundomerheating · qizish
Explanation

You need a low-voltage lamp (for example 6 V), a suitable source, an ammeter, a voltmeter, a key and a stopwatch; do everything under the teacher’s supervision. The ammeter goes in series with the lamp and the voltmeter in parallel across it; close the key only after the teacher has checked the circuit. Write the ammeter and voltmeter readings and calculate P = IU. Then keep the lamp on for an exact time (for example 1 minute) with the stopwatch and find A = IUt. The lamp gets hot when working: do not touch it while it works and for a few minutes after. Compare the calculated power with the marking on the lamp; a small difference is due to measurement error.

Worked examples
U = 6 V, I = 0.5 A: P = 0.5 · 6 = 3 W. t = 2 min = 120 s: A = P·t = 3 · 120 = 360 J.
A lamp is marked “6 V; 3 W” and the measurement gave P = 5.9 · 0.5 ≈ 2.95 W. The difference is 0.05 W — small, within measurement error.
Class activity

Draw a table in your notebook: “U (V), I (A), P (W), t (s), A (J)”. Practise the calculation with sample numbers from the teacher, then enter the real results from class.

Practice
1
With which instruments do we find the power of a lamp?
2
A lamp has U = 4 V, I = 0.3 A. Its power in mW? (1 W = 1000 mW)
3
A 3 W lamp burned for 30 s. The work of the current in J?
4
Why must you not touch a lamp while it is working?