I suggest that teachers do not use the model often used where the charges are shown flowing in the conductor. The problem with this model is that students can be misled in believing that even if the key is open, the current can flow in some parts of the circuit up to the point where the key interrupts it. Whereas the truth is that the current can flow only if an electric field is established across the conductor, which forces electrons to drift towards the higher potential. The electric field is established only when the key is closed. If the key is open, the electric field is not established and the current does not flow in any part of the circuit. Since the function of the key is to allow or disallow the establishment of electric field, it can be placed anywhere in the circuit. There is no fixed position for it. However, many books tend to place the key at a fixed position in the circuit, in many cases near the positive terminal of the battery. Students should be advised that the key can be placed any where in the circuit and a conscious effort should be made to make circuits in the class room or elsewhere with key at varying positions.
There
is another question which students find difficult to answer. How is that the
light of the bulb comes on instantaneously as the switch is put in the ON position? The
reason is that as soon as the electric field is set up, the electrons, wherever
they may be in the conductor, start drifting towards the higher potential and
the electric current is set up. The
signal that the electric field sends out to the electrons to start drifting
travels in the conductor with nearly the speed of light and reaches every where
in the circuit instantly, however widespread or far flung the circuit may be. The signal of the electric field may be
likened to the command of a military general.
Once the command is issued, it reaches instantly every where, and the
troops in the whole country, wherever they may be stationed, start marching.
Coming
back to the question of the bulb glowing immediately as the switch is closed, once the electrons start drifting, the electron
nearest to the bulb gets into its filament, then the next nearest one, and so
on. A current is set up and the bulb
glows. The bulb does not have to wait
for the electron near the switch to reach it for it to glow. It is like opening a water tap. Water nearest to the tap starts coming out
immediately. We do not have to wait for
the water at the other end of the pipe to reach the tap for the water to come
out.
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