☰ 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
20

Practical work. Regulating current with a rheostat

Textbook: pp. 63–64
GoalIn a practical work, regulate the current in a circuit with a rheostat and explain how it works.
New words
rheostat · reostatsliding contact · sirpanuvchi kontaktregulating current · tok kuchini rostlashmaximum resistance · eng katta qarshilik
Explanation

A rheostat has a conducting wire wound on it; moving the sliding contact changes the length of wire that carries the current. By R = ρ·l/S a longer wire has a larger resistance, and by I = U/R the current falls. So by connecting a rheostat into the circuit we change the current smoothly. Before assembling the circuit, set the slider to the position of maximum resistance, otherwise the current may jump and damage the instruments. Change the current, watch the ammeter and write down the reading for three positions. Use only the low-voltage source the teacher allows; a rheostat gets hot when current flows for long, so do not touch it.

Worked examples
A rheostat of 20 Ω and a 4 V source (ignore the ammeter’s resistance). Slider at the end: I = 4 : 20 = 0.2 A.
In the same circuit the slider is in the middle, the working part has 10 Ω: I = 4 : 10 = 0.4 A. The resistance halved and the current doubled.
Class activity

Draw in your notebook the diagram of a circuit with a rheostat (source, key, rheostat, ammeter) and write in your own words how the current changes when the slider moves left or right. Make the real connection only together with the teacher.

Practice
1
How does a rheostat change the current?
2
The working part of a rheostat is 8 Ω, the source is 6 V. What is the current in mA? (1 A = 1000 mA)
3
The source is 12 V. What resistance of the working part gives a current of 0.5 A?
4
Why is the slider set to maximum resistance before switching on the circuit?