The Drude Model helped me quite a lot to visualize how current could flow in a circuit. However, there is still a point that I cannot grasp in the explanation given by some people when they talk about energy. When considering a round trip of one electron along a circuit, it is usually said that :
The electron gains energy when going through the battery and loses that energy along the wire as it collides with the lattice of the conductor.
My problem with this reasoning is that it is implying that the electron has more energy when it leaves the battery than anywhere else in the circuit. However, according to the Drude model, when the electrons collide with the lattice, they indeed lose kinetic energy but they regain kinetic energy as soon as they start accelerating thanks to electric fields produced along the wire. This causes electrons to have an average drift speed. In other words, their kinetic energy is constant on average, which is at first glance, in contradiction with the reasoning given above. What am I missing here?
Answer
The battery sets up an electric field in the external circuit which all the mobile electrons feel in all the external circuit. This means that there is a force on all the mobile electrons in the external circuit.
So these mobile electrons are accelerated by the electric field and gain kinetic energy from the electric field which is maintained by the battery.
The accelerated electron then interact with the vibrating lattice ions and on average there is a transfer of energy from the mobile electrons to the lattice ions is the ions vibrate more.
After the interaction with the lattice ions the mobile electrons are accelerated again thus gaining kinetic energy, interacting with the lattice ions etc.
So on top of the random thermal motion of the mobile electrons there is a drift velocity away from the negative terminal of the battery and towards the positive terminal of the battery as they gain kinetic energy from the electric field and then lose some to the lattice ions.
Within the battery the chemical process moves electrons from the positive terminal to the negative terminal thus giving the electron electric potential energy from the chemical energy of the reacting chemicals in the battery.
The key point is that outside the battery all the mobile electrons drift round the circuit from the negative terminal to the positive terminal of the battery. They continuously lose electric potential energy, gain kinetic energy and then lose the kinetic energy on colliding with the lattice ions which vibrate more - the conductor's temperature is increased which is called "ohmic heating".
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