☰ Contents · Physics

Checking Ohm’s law and the rheostat

Lessons 19–20 · 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
19

Lab work. Studying Ohm’s law

Textbook: pp. 61–62
GoalIn a laboratory work, measure I and U for a conductor, check Ohm’s law and find the average value of the resistance.
New words
measurement error · o‘lchash xatoligiaverage value · o‘rtacha qiymatammeter in series · ampermetr ketma-ketvoltmeter in parallel · voltmetr parallel
Explanation

You need a low-voltage school source, a resistor of unknown resistance, an ammeter, a voltmeter, a key and a rheostat; do all the work under the teacher’s supervision and do not switch on until the teacher has checked your circuit. The ammeter is connected in series and the voltmeter in parallel across the resistor; check the terminals (“+” and “–”) and the scale limit. With the rheostat you set three different voltages, each time write U and I in a table and then calculate R = U/I. Because of measurement errors the three results differ a little, so you take the average value of R. If the ratio U/I stays almost the same, Ohm’s law holds for that conductor. When finished, first open the key and only then disconnect the wires; wires and the resistor can get warm, so do not leave the circuit on for long.

Worked examples
Table: U = 1.5 V, 3.0 V, 4.5 V; I = 0.25 A, 0.50 A, 0.75 A. R = 1.5 : 0.25 = 3.0 : 0.50 = 4.5 : 0.75 = 6 Ω. The ratio is constant, so Ohm’s law holds.
The measurements gave R₁ = 5.9 Ω, R₂ = 6.1 Ω, R₃ = 6.0 Ω. The average R = (5.9 + 6.1 + 6.0) : 3 = 18 : 3 = 6 Ω.
Class activity

Draw a table in your notebook with columns “No., U (V), I (A), R (Ω)” and 3 rows, plus a final row “R average”. Practise the calculation with sample numbers from the teacher, then do the real measurement in class.

Practice
1
How are the ammeter and the voltmeter connected into the circuit?
2
In a measurement U = 3 V and I = 0.5 A. What is the resistance in ohms?
3
Three measurements gave R = 11.8 Ω, 12.2 Ω, 12.0 Ω. What is the average in ohms?
4
Why do we measure several times and take the average instead of measuring once?