☰ Contents · Physics

Short circuit and safety

Lessons 38–39 · 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
38

Electrical safety measures

Textbook: pp. 108–110
GoalKnow the cause and danger of a short circuit, the rules of electrical safety and first aid for electric shock.
New words
short circuit · qisqa tutashuvinsulation · izolyatsiyasafe voltage · xavfsiz kuchlanishfirst aid · birinchi yordam
Explanation

A short circuit is when the poles of a source become connected through a very small resistance (for example when wires with damaged insulation touch). By I = U/R the current becomes very large, and the wires can heat up quickly and start a fire; this is why fuses are installed. The book gives voltages above 42 V as dangerous for humans, and in damp places even lower voltages can be dangerous; the 220 V of the mains is dangerous to life. To avoid electric shock do not use wires and devices with damaged insulation, do not touch electric devices with wet hands and do not go near a broken wire. If someone has received an electric shock: do not touch the person yourself, switch the power off from a safe place if you can, call adults at once and call an ambulance (103).

Worked examples
Mains 220 V, the resistance at the shorted spot is 0.1 Ω: I = U : R = 220 : 0.1 = 2200 A. The current in an ordinary device is a few amperes.
In a short circuit a wire of R = 0.2 Ω carries I = 1000 A: the power released is P = I²R = 1000² · 0.2 = 200 000 W = 200 kW. That is why the wire melts quickly.
Class activity

With your family make a list of “Electrical safety at home” rules and draw it as a poster. Do not inspect any wires or devices yourself.

Practice
1
What is a short circuit and why is it dangerous?
2
U = 12 V, and in a short circuit R = 0.02 Ω. The current in A?
3
R = 0.5 Ω, I = 200 A. The power released in kW?
4
Why should you not touch a person who has received a shock?