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The electron drift speed is estimated to...

The electron drift speed is estimated to be only a few mm `s^(-1)` for currents in the range of a few amperes. How then is current established almost the instant a circuit is closed ?

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Electric field is established throughout the circuit almost instantly (with the speed of light) causing at every point a local electron drift. Establishment of a current does not have to wait for electrons from one end of the conductor travelling to the other end. However, it does take a little while for the current to reach its steady value.
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Electron drift speed is estimated to be only a few mm/s for currents in the range of few amperes ? How then is current established almost the instant a circuit is closed.

Clarify your elementary notions about current in a metallic conductor by answering the following queries : (a) The electron drift speed is estimated to be only a few mm s^(-1) for currents established almost the instant a circuit is closed? (b ) The electron drift arises due to the force experienced by electons in the electric field inside the conductor. But force should cause acceleation. Why then do the electrons acquire a steady drift speed? ( c )If the electron drift speed is so small, and electron's charge is small, how can we still obtain large amounts of current in a conductor? (d ) When electrons drift in a metal from lower to higher potential, does it mean that all the ''free'' electrons of the metal are moving in the same direction ? (e ) Are the paths of electons straight lines between successive collisions (with the positive ions of the metal ) in the (i) absence of electic filed, (ii) presence of electic field.

Knowledge Check

  • Assertion: Drift velocity of electrons developed in a current-carrying conductor is of the order of 10^(-4) m//s but current in the conductor is established almost instantly the switch is closed. Reason: Electric field in a conductor sets up with the speed of light.

    A
    If both assertion and reason are correct and reason is a correct explanation of the assertion
    B
    If both assertion and reason are correct but reason is not the correct explanation of the assertion
    C
    If assertion is correct but reason is incorrect
    D
    If assertion is incorrect but reason is correct

    • Similar Questions

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    Electron drift speed is estimated to be of the order of mm s^(-1) . Yet large current of the order of few amperes can be set up in the wire. Explain briefly.

    Answer question on the basis of your understanding of the following paragraph and the related studied concepts : Bulk matter is made up of many molecules, which are so closely packed that the electrons are no longer attached to individual nuclei. In metallic conductors some of the electrons are practically free to move within the bulk material. Ordinarily, the electrons will be moving due to thermal motion during which they collide with the fixed ions. An electron colliding with an ion emerges with the same speed as before the collision. However, the direction of its velocity after the collision is completely random and there is no preferential direction for the velocities of the electrons. So number of electrons travelling in any direction will be equal to the number of electrons travelling in the opposite direction and there will be no net flow of electrons and hence no net current. When an external electric field vecE is applied across a piece of conductor, electrons will be accelerated at veca = (e vecE)/(m) . If veca was the velocity of electron after the last collision then velocity vec v_t after a time t will be vecv_t = vecu + veca t The average velocity of the electrons at time t is the average of all vec v' s. However, average value of vecu' s is zero. As collisions of the electrons occur at random times, let tau be the average time between successive collision and then the average velocity electrons is expressed as vecv_d (known as the drift velocity) and is given as: vecv_d = - (e vecE)/(m) tau This drift motion of electrons is responsible for establishment of an electric current in the conductor on applying an external electric field across it. The magnitude of current is given as : I = nAev_d , where n= number density of free electrons and A = cross-section area of the conductor. It is to be noted here that drift speed is estimated to be of the order of 10^(-3) ms^(-1) for currents in the range of a few amperes. Define drift velocity. On what factors does it depend ?

    Answer question on the basis of your understanding of the following paragraph and the related studied concepts : Bulk matter is made up of many molecules, which are so closely packed that the electrons are no longer attached to individual nuclei. In metallic conductors some of the electrons are practically free to move within the bulk material. Ordinarily, the electrons will be moving due to thermal motion during which they collide with the fixed ions. An electron colliding with an ion emerges with the same speed as before the collision. However, the direction of its velocity after the collision is completely random and there is no preferential direction for the velocities of the electrons. So number of electrons travelling in any direction will be equal to the number of electrons travelling in the opposite direction and there will be no net flow of electrons and hence no net current. When an external electric field vecE is applied across a piece of conductor, electrons will be accelerated at veca = (e vecE)/(m) . If veca was the velocity of electron after the last collision then velocity vec v_t after a time t will be vecv_t = vecu + veca t The average velocity of the electrons at time t is the average of all vec v' s. However, average value of vecu' s is zero. As collisions of the electrons occur at random times, let tau be the average time between successive collision and then the average velocity electrons is expressed as vecv_d (known as the drift velocity) and is given as: vecv_d = - (e vecE)/(m) tau This drift motion of electrons is responsible for establishment of an electric current in the conductor on applying an external electric field across it. The magnitude of current is given as : I = nAev_d , where n= number density of free electrons and A = cross-section area of the conductor. It is to be noted here that drift speed is estimated to be of the order of 10^(-3) ms^(-1) for currents in the range of a few amperes. What is the rate of flow of electric field through the conductor ?

    Answer question on the basis of your understanding of the following paragraph and the related studied concepts : Bulk matter is made up of many molecules, which are so closely packed that the electrons are no longer attached to individual nuclei. In metallic conductors some of the electrons are practically free to move within the bulk material. Ordinarily, the electrons will be moving due to thermal motion during which they collide with the fixed ions. An electron colliding with an ion emerges with the same speed as before the collision. However, the direction of its velocity after the collision is completely random and there is no preferential direction for the velocities of the electrons. So number of electrons travelling in any direction will be equal to the number of electrons travelling in the opposite direction and there will be no net flow of electrons and hence no net current. When an external electric field vecE is applied across a piece of conductor, electrons will be accelerated at veca = (e vecE)/(m) . If veca was the velocity of electron after the last collision then velocity vec v_t after a time t will be vecv_t = vecu + veca t The average velocity of the electrons at time t is the average of all vec v' s. However, average value of vecu' s is zero. As collisions of the electrons occur at random times, let tau be the average time between successive collision and then the average velocity electrons is expressed as vecv_d (known as the drift velocity) and is given as: vecv_d = - (e vecE)/(m) tau This drift motion of electrons is responsible for establishment of an electric current in the conductor on applying an external electric field across it. The magnitude of current is given as : I = nAev_d , where n= number density of free electrons and A = cross-section area of the conductor. It is to be noted here that drift speed is estimated to be of the order of 10^(-3) ms^(-1) for currents in the range of a few amperes. If the electron drift speed is so small and the electron charge is small, how can we still obtain large amounts of current even in a thin conducting wire ?

    Answer question on the basis of your understanding of the following paragraph and the related studied concepts : Bulk matter is made up of many molecules, which are so closely packed that the electrons are no longer attached to individual nuclei. In metallic conductors some of the electrons are practically free to move within the bulk material. Ordinarily, the electrons will be moving due to thermal motion during which they collide with the fixed ions. An electron colliding with an ion emerges with the same speed as before the collision. However, the direction of its velocity after the collision is completely random and there is no preferential direction for the velocities of the electrons. So number of electrons travelling in any direction will be equal to the number of electrons travelling in the opposite direction and there will be no net flow of electrons and hence no net current. When an external electric field vecE is applied across a piece of conductor, electrons will be accelerated at veca = (e vecE)/(m) . If veca was the velocity of electron after the last collision then velocity vec v_t after a time t will be vecv_t = vecu + veca t The average velocity of the electrons at time t is the average of all vec v' s. However, average value of vecu' s is zero. As collisions of the electrons occur at random times, let tau be the average time between successive collision and then the average velocity electrons is expressed as vecv_d (known as the drift velocity) and is given as: vecv_d = - (e vecE)/(m) tau This drift motion of electrons is responsible for establishment of an electric current in the conductor on applying an external electric field across it. The magnitude of current is given as : I = nAev_d , where n= number density of free electrons and A = cross-section area of the conductor. It is to be noted here that drift speed is estimated to be of the order of 10^(-3) ms^(-1) for currents in the range of a few amperes. Are the paths of electrons straight lines between successive collisions in the (i) absence of electric field, (ii) presence of electric field ?

    Answer question on the basis of your understanding of the following paragraph and the related studied concepts : Bulk matter is made up of many molecules, which are so closely packed that the electrons are no longer attached to individual nuclei. In metallic conductors some of the electrons are practically free to move within the bulk material. Ordinarily, the electrons will be moving due to thermal motion during which they collide with the fixed ions. An electron colliding with an ion emerges with the same speed as before the collision. However, the direction of its velocity after the collision is completely random and there is no preferential direction for the velocities of the electrons. So number of electrons travelling in any direction will be equal to the number of electrons travelling in the opposite direction and there will be no net flow of electrons and hence no net current. When an external electric field vecE is applied across a piece of conductor, electrons will be accelerated at veca = (e vecE)/(m) . If veca was the velocity of electron after the last collision then velocity vec v_t after a time t will be vecv_t = vecu + veca t The average velocity of the electrons at time t is the average of all vec v' s. However, average value of vecu' s is zero. As collisions of the electrons occur at random times, let tau be the average time between successive collision and then the average velocity electrons is expressed as vecv_d (known as the drift velocity) and is given as: vecv_d = - (e vecE)/(m) tau This drift motion of electrons is responsible for establishment of an electric current in the conductor on applying an external electric field across it. The magnitude of current is given as : I = nAev_d , where n= number density of free electrons and A = cross-section area of the conductor. It is to be noted here that drift speed is estimated to be of the order of 10^(-3) ms^(-1) for currents in the range of a few amperes. Guess the order of magnitude of thermal speed of free electrons in a conductor in the absence of an external electric field.

    An inductor with an inductance of 2.5H and a resistance of 8Omega is connected to the terminals of a battery with an EMF 6V and negligible internal resistance. Find (a) The initial rate of increase of current in the circuit (b) The rate of increase of current at the instant when the current is 0.50A (c ) The current 0.25s after the circuit is closed (d) The final steady state current

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