How can the current be same in the whole circuit ...the electrons must be losing their kinetic energy and ultimately their velocity must decrease ?
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$\begingroup$ "the electrons must be losing their kinetic energy and ultimately their velocity must decrease ?" Why must they be losing their kinetic energy? $\endgroup$– Hal HollisJun 30, 2017 at 19:29
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$\begingroup$ @HalHollis Because there is resistance everywhere always. $\endgroup$– Enrique MendezJul 1, 2017 at 21:35
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$\begingroup$ But Siddharth, the loss of kinetic energy due to collisions with the lattice is offset by the gain in kinetic energy due to the electric field that maintains the current. It's a bit like a feedback mechanism. If electrons lost net kinetic energy in the resistor, charge density would build up there but the resulting electric field would act to increase the kinetic energy of the electrons leaving and decrease the kinetic energy of those entering the resistor. Equilibrium is maintained by this feedback. $\endgroup$– Hal HollisJul 2, 2017 at 16:43
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5 Answers
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They don't necessarily need to be losing their kinetic energy. Remember that a battery provides a potential difference. So while they may be moving at constant velocity, and they end up dissipating power that can very well be replenished by the potential energy they have.
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Consider the analogy of a flowing river. The amount of water is analogous to electric current. The flow rate is analogous to the potential difference (or voltage). As the river flows downhill (as it must because of gravity), the potential difference at any point in the flow is less than that at some higher point. However the amount of water does not get any less.
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Because those electrons who are behind can't go faster than those at the front because of electron to electron repulsion. If they go any faster, they will be repelled back. So they follow the speed of the slowest electron. Those who are at the front end can't go faster either, cause it will result in a net positive charge at the area where they left and attract them back to it.
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Since electric charge is quantized in discrete multiples of the electron charge, it is instructive to look at electric current as the movement of multiple microscopic charge carriers with a drift velocity in a conductor.
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Although your light turns on very quickly when you flip the switch, and you find it impossible to flip off the light and get in bed before the room goes dark, the actual drift velocity of electrons through copper wires is very slow. It is the change or "signal" which propagates along wires at essentially the speed of light.
The electrons start drifting because of the imposed electric field of the battery in a circuit. The link goes into details.
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Because the drag created by hitting on atoms of the solid is in equilibrium with the acceleration by the electric field voltage.
