# Kinetic energy and current

When a battery is connected to a resistance circuit, we know that it loses energy because heat is emitted as a result of the collisions between the electrons and stuff, but my question is, is the change in kinetic energy of the electrons zero or negligible?

• Which specific electrons at what point are you asking about? The ones in the wire are usually described a bit differently from the ones in the chemicals in the battery. Are you asking about a chemical cell, or just about an ideal voltage source in general? – BowlOfRed Apr 26 at 21:37
• The loss of kinetic energy of the electrons is not zero; you more or less said that yourself. What, to you, is negligible? You might not notice anything when a small current passes through a copper wire. But the filament of a light bulb is certainly notable, and all that heat and light was once the kinetic energy of electrons. – garyp Apr 26 at 21:48

## 2 Answers

The voltage difference between the poles of the battery creates an electric field that pushes the electrons to the positive electrode. If there would be no resistance, the electrons would thus accelerate while moving from one electrode to the other and their kinetic energy would be increasing as they move.

However, because of collisions in the resistor the electrons lose energy. The individual electrons continuously change direction due to these collisions and lose energy while doing so. On average however, the electrons move with a constant velocity through the resistor.

The drude model is the classical way to describe this behavior.

is the change in kinetic energy of the electrons zero or negligible?

It is negligible, but not zero. The Fermi velocity in a typical conductor is approximately 1500 km/s while the drift velocity is approximately 0.2 um/s, which is 13 orders of magnitude smaller. Since the kinetic energy is proportional to the square of the velocity this means that the KE due to the current’s drift velocity is around 26 orders of magnitude smaller than the baseline Fermi KE of the electrons.