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### Course: Class 12 Physics (India)>Unit 3

Lesson 1: Drift of electrons & the origin of resistance

# Drift velocity (concept & intuition)

Drift velocity is the average velocity with which electrons 'drift' in the presence of an electric field. It's the drift velocity (or drift speed) that contributes to the electric current. In contrast, thermal velocity causes random motion resulting in collisions with metal ions. Created by Mahesh Shenoy.

## Want to join the conversation?

• At , how can the electrons bounce away from the positive metallic ions? Shouldn’t they get attracted towards positive metallic ions?
• Positive metallic ions means they lost a electron thus gaining a net positive charge. In the structure of the atom of, the positive charge (proton) is concentrated at the center and the electrons are around it, so if a electron comes in contact with the positive metallic ion, the valence electrons will exert a repulsion force on the incoming electron and even though a attractive force is exerted on the incoming electron by the nucleus of the positive metallic atom its force on the incoming electron is very small when compared to the repulsive force exerted by the valence electrons(as the distance between positive charge and the incoming electron is very large when compared to the distance between the valence electrons and the incoming electrons) and thus the electron bounces off
• Now you can go way deep into this and look up the whole Maxwell-boltzmann stuff but to put it simply thermal velocity is basically a measure of temperature. when we say hot or cold we're just referring to the kinetic energy of the particles of the object in question. so if an iron rod is hot its particles will have high thermal velocity meaning the particles move very fast around their mean position, meaning the electron with start bouncing around faster too! if the object is cold then the thermal velocity is lower and the particles won't move as much and will have low kinetic energy meaning your electron won't bounce around as fast as it did when it was hot or at room temperature, some fun trivia for you is that it is hypothetically possible for us to have an object which is at such a low temperature that its particles are completely still with no kinetic energy, however I dont think this is achievable on earth as of now, the temperature required for such a state would be 0 K and is called absolute zero its approx equal to -273 degrees celsius, as for khan academy I guess the topics related to thermodynamics would give you more of an insight on this subject.
• you compared electrons' random movement to air particles' random movement. air particles move randomly due to the movement of big things(like landslides, running person) on earth. why are electrons moving randomly?
• When viewed at a higher conceptual level, electrons move in a motion known as brownian motion, which is only slightly different from random motion. The quantity of random motion experienced by the atoms and molecules within a substance is continuously changing depending on the pressure, temperature, and material. The "Free Electrons" are electrons that are thrown off of their orbits by this random motion in the outer rings.
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• Aren't electrons inside the atoms of metals? Without potential how can they come out of the orbits and collide with different "metallic ions"?
• I know I am late, this is for anyone having the same doubt. The electrons moving inside the conductor are only the VALENCE electrons. The reason why only valence move in a conductor is because nuclear force on the valence electron is very low as they are far from the nucleus of the atom(electric force inversely proportional to distance). If we have copper atom it will have 29 electrons it will loose 2 VALENCE electrons to attain stable configuration. These valence electrons move inside the conductor
• how do we know the difference between positive and negative charge how do we define it ? who found out that there are 2 types of charges ? how do we came to know that the valence electrons are not stable without a battery ? if they are not stable why doesnt the shape of the conductor change or disintegrate ? ( without applying charge) how did we arrive to this conclusion that electron drift is at a constant velocity ? how can we increase it ?
• wonderful!! too fascinating!
shouldnt the arrows point towards +ve terminal as electrons are attracted to it? they are towards -ve terminal and away from +ve one in the 1st video if drift velocity (concept and intuition)
• The arrows are correct, as they are indicating the direction of current. Electrons flow from -ve terminal to +ve terminal, but since electrons are negatively charged, we consider the direction of current opposite to that of electrons.
So electrons flow from -ve to +ve terminal but current flows from +ve to -ve terminal of the battery.
• At , it is said that the current should remain the same throughout the circuit. Why?
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• This is because there is only one path for the current flow. Since electric charge flows through conductors like marbles in a tube, the rate of flow (marble speed) at any point in the circuit (tube) at any specific point in time must be equal.
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• At , they said that current should be same everywhere in the circuit. Why?
Also doesn't it contradict the fact that current in divided in parallel connection.
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• Does voltage effect drift velocity?
If voltage is increased, does drift velocity increase?
What is the voltage when drift velocity is at apx. 1mm/s
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• Sir, can we say electricity is kind of an osmosis where particles (electrons) move from higher concentration (-ve terminal) to lower concentration (+ve terminal)?
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