Lessons 9–10 · 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
9
The idea of electric current
Textbook: pp. 32–34
GoalKnow what electric current is, the condition for it to appear and its effects.
New words
electric current · elektr tokiordered motion of charged particles · zaryadli zarralarning tartibli harakatielectric field in a conductor · o‘tkazgichdagi elektr maydoneffects of current · tokning ta’sirlari
Explanation
An ordered motion of charged particles, that is, a flow of them, is called electric current. If we join a charged electrometer to the ground with a conductor, the pointer drops to zero at once: a short current has passed through the conductor. A neon lamp in the wire shows it with a brief flash. The reason the charged particles move is the electric field in the conductor, so a field must exist in the conductor for a current to appear. For the current to last, the field must be maintained; this needs a device that keeps separating charges, such as an electrophore machine. We do not see the current itself, we see its effects: the conductor heats up, the chemical composition of an electrolyte changes, and a magnetic field appears around the conductor.
Worked examples
When a charged electrometer is joined to the ground by a wire, the pointer falls and the lamp flashes for a moment. The current is short because the field disappears when the electrometer has lost its charge.
An electric kettle heats water — this is the heating effect of current; a compass needle turning near a wire carrying current shows the magnetic effect.
Class activity
Home survey: list 5 electrical appliances (kettle, lamp, hair dryer, iron, radio) and write which effect of current each one uses. Do not touch them or switch them on without an adult; the task is observation only.
Practice
1
What is electric current?
The ordered motion (flow) of charged particles.
2
What makes the charged particles move in a conductor?
The electric field in the conductor.
3
Name the three effects of current.
Heating (the conductor warms), chemical (the electrolyte changes) and magnetic (a magnetic field appears around the conductor).
4
Why does the neon lamp flash only for a moment?
After the electrometer is discharged the field disappears and the current stops.