Lessons 21–22 · 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
21
Connecting consumers in series
Textbook: pp. 64–66
GoalKnow the laws of current, voltage and resistance for conductors connected in series and apply them in problems.
New words
series connection · ketma-ket ulanishtotal resistance · umumiy qarshiliksum of voltages · kuchlanishlar yig‘indisiunbranched circuit · tarmoqlanmagan zanjir
Explanation
In a series connection conductors are joined one after another, with no branching. The same current flows through every part: I = I₁ = I₂. The total voltage equals the sum of the voltages on the parts: U = U₁ + U₂. From Ohm’s law the total resistance is also the sum of the part resistances: R = R₁ + R₂, so the total resistance of conductors in series is larger than that of any one part. The voltage is shared in proportion to resistance: the part with the larger resistance has the larger voltage. The drawback is that if one element breaks, the current stops in the whole circuit; an old string of decorative lamps can be wired like this. Never connect lamps or devices that work from the 220 V mains yourself — only adults do that.
Worked examples
R₁ = 10 Ω and R₂ = 20 Ω in series, source 12 V. R = 10 + 20 = 30 Ω; I = 12 : 30 = 0.4 A.
In the same circuit U₁ = I·R₁ = 0.4 · 10 = 4 V and U₂ = 0.4 · 20 = 8 V. Check: 4 + 8 = 12 V, equal to the source voltage.
Class activity
Draw in your notebook the diagram of two resistors in series and mark where to place an ammeter and voltmeters. Work on paper only.
Practice
1
For a series connection, how are the current and the voltage related?
The current is the same everywhere; the total voltage is the sum of the part voltages.
2
What is the total resistance in ohms of 15 Ω and 25 Ω resistors in series?
40
3
40 V is applied to 8 Ω and 12 Ω conductors in series. What is the current in A?
2
4
Why do all lamps of a string in series go out if one burns out?
The circuit is not branched; if one breaks, the current stops in the whole circuit.