Lessons 23–24 · 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
24
Lab work. Studying series and parallel connection of conductors
Textbook: pp. 72–74
GoalIn a laboratory work, measure current, voltage and total resistance for resistors in series and in parallel and compare with the formulas.
New words
equivalent resistance · ekvivalent qarshilikmeasured and calculated value · o‘lchangan va hisoblangan qiymatseries diagram · ketma-ket sxemaparallel diagram · parallel sxema
Explanation
You need two resistors (about 100–300 Ω), a low-voltage source, an ammeter, a voltmeter and a key; work under the teacher’s supervision and switch on only after the teacher has checked the circuit. First connect the resistors in series, measure the source voltage U and the current I, calculate R = U/I and compare it with R₁ + R₂. Then connect the resistors in parallel, repeat, and compare the result with R₁·R₂/(R₁ + R₂). The results may differ slightly because instruments are not exact and wires have resistance. In the parallel connection the total resistance falls and the current rises, so do not leave the circuit on for long and do not touch the resistors — they may get hot.
Worked examples
R₁ = 150 Ω, R₂ = 300 Ω, U = 9 V, in series: R = 450 Ω, I = 9 : 450 = 0.02 A = 20 mA.
The same resistors in parallel: R = 150 · 300 : 450 = 100 Ω, I = 9 : 100 = 0.09 A = 90 mA. In parallel the current is much larger.
Class activity
Prepare a table in your notebook: “connection, U (V), I (A), R measured (Ω), R calculated (Ω)”. Fill in the calculated values in advance with sample numbers (for example 200 Ω and 300 Ω).
Practice
1
How is the total resistance of two resistors found for series and for parallel?
Series: R = R₁ + R₂; parallel: R = R₁R₂/(R₁ + R₂).
2
200 Ω and 300 Ω are in series at 10 V. What is the current in mA? (1 A = 1000 mA)
20
3
What is the total resistance in ohms of 200 Ω and 300 Ω in parallel?
120
4
Why might the measured and calculated resistances differ a little?
The instruments are not exact and the wires and contacts have resistance too.