So I have this question in my textbook and Ive also got it in my test. Ive written a wrong answer. My friend wrote the drift velocities of each particle add up and he got full marks. Is it right? Can you please explain. I have checked this answer elsewhere but they use terms such electromagnetic waves which is not yet thought to us. Could you please explain in simple terms? Ok, this is the edited question. This was the answer of my teacher: $I=neAv_d$. Although $v_d$ is very small but $n$ the number density of electrons is very very large as compared to the drift velocity hence although the velocity of the electrons is very small to get to the other end instantly but they are so large in number that the current is established immediately. Is this the right definition?
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1$\begingroup$ Each electron pushes the next one! $\endgroup$– lucasCommented Jun 24, 2016 at 13:47
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1$\begingroup$ your friend got marks for free.The velocities don't add up $\endgroup$– Kishan KumarCommented Jun 24, 2016 at 14:55
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2 Answers
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It is like water in a hose. If the hose is full of water, water flows out the end immediately when you turn on the faucet. A drop of water at the faucet pushes a drop next to it, which pushes the next drop.
Water doesn't flow that fast. If the hose is empty, it takes a while to reach the end.
answered Jun 24, 2016 at 13:48
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$\begingroup$ Could you please answer with physics terms. One which could be presented in my exam if asked again. $\endgroup$ Commented Jun 24, 2016 at 13:57
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$\begingroup$ Sorry. See our policy on homework questions. But it sounds like your friend got credit for an incorrect answer. $\endgroup$ Commented Jun 24, 2016 at 14:07
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$\begingroup$ '@Sreekara I think the answer by @lucas has sufficient physics terms. $\endgroup$– hsinghalCommented Jun 24, 2016 at 17:17
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$\begingroup$ @Sreekara: This is an intuitive explanation, and since you wanted simple terms I agree with it. Another explanation would be that the electrons in the metal form a gas because they are free to move. The speed of sound in that gas is close to but somewhat less than the speed of light, and that is the speed at which differences in voltage (i.e. signals) can travel. If current were not established immediately, that would mean electrons were concentrated in one place, therefore would have higher pressure (voltage) there, which would be quickly dissipated with current. $\endgroup$ Commented Jun 24, 2016 at 17:35
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Man you need to understand that there are large no of electrons present inside a conductor where one electron pushes the other by a velocity known as drift velocity when an external field is applied as a result these velocities add up and thus increasing the no of electrons passing through a given cross sectional area .Thus we presume that though the drift velocity of individual electron is less but current is established immediately.
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1$\begingroup$ "One electron pushes the other by a velocity", What does this even mean ? $\endgroup$– MitchellCommented Jul 8, 2017 at 17:46