الأسبوع 6 · الوحدة الثانية «الكهربية والمغناطيسية» (افتتاحية) والدرس الأول «التيار الكهربي» (١): الإلكترونات الحرة والتيار وشدة التيار وفرق الجهد والقوة الدافعة الكهربية · 3 دروس
1
What flows in a wire: free electrons and current
Science Prep 3 Term 1, Unit 2: pp. 34–40
GoalExplain electric current as the flow of free electrons in a closed circuit and tell conventional current from electron flow.
New words
free electrons · electrons in a metal that are not tied to one atom and can move through the metalelectric current · the flow of electric charge round a closed circuitconventional current · the direction we agree to give a current: from the positive pole to the negative pole of the source
Explanation
A metal wire holds many free electrons that move about at random. A current needs two things: a source and a closed path. When the circuit is closed, the source pushes all the free electrons the same way along the conductor, and this flow of charge is the electric current. The cell does not make the electrons: the wire already holds them. The electrons drift slowly, but the push passes along the wire almost at once, like the water already in a pipe starting to flow as soon as a tap opens at the other end. By convention we draw the current leaving the positive pole of the source and returning to its negative pole, while the electrons, which are negative, really move from the negative pole to the positive one. A direct-voltage source, such as a cell or a battery (a group of cells), gives a current in one direction. An alternating source, such as a generator, gives a current whose direction changes with time. Careful: the current is not used up in a bulb. The same current leaves the bulb as enters it; what the bulb receives is energy.
Worked examples
A bead model. Thread 20 beads on a loop of string and put it in a ring of plastic tubing. When you push one part of the loop with your hand, all the beads move together at once, even those far away. The beads are the free electrons, the hand is the cell, and the closed tube is the closed circuit.
Draw the arrows for a cell, a switch and a lamp. The conventional current leaves the positive pole and goes round to the negative pole. The electrons move the opposite way. One circuit, two arrows, pointing opposite ways, both correct for their own purpose.
Why does the lamp light almost as soon as the switch closes, although each electron moves slowly? Because the free electrons are already all along the wire, and the push is passed from one to the next, like cars in a full queue that all start together.
Class activity
Human circuit, with a grown-up. Ten children stand in a ring and pass a ball from hand to hand in one direction: the balls are charges and the pushing is the source. Then one child stops passing and everything stops: the circuit is open. Then make a second ring passing the other way and say which is conventional current and which is electron flow. Optionally, an adult connects a torch bulb in a holder to one 1.5 V cell for a few seconds only.
Practice
1
Which particles move in a metal wire when a current flows?
The free electrons.
2
What two things does a current need?
A source and a closed path (a closed circuit).
3
Which way does the conventional current go outside the cell, and which way do the electrons go?
Conventional current: positive to negative. Electrons: negative to positive.
4
A student says “the bulb uses up the current”. What is wrong?
The same current leaves the bulb as enters it; the bulb receives energy.
