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First teaching 2014

Last exams 2024

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Drift Speed (DP IB Physics: SL)

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Drift Speed

  • In conductors, only negatively charged (delocalised) electrons are allowed to move between atoms
    • In general, an electric current can arise from the flow of either positive or negative particles
  • Charged particles moving through a material or through vacuum are known as charge carriers 

5-1-2-charge-carriers-drifting-along-the-conductor_sl-physics-rn

Charge carriers drift towards the positive terminal of the conductor. Conventional current flows in the opposite direction

Movement in a Conductor

  • Delocalised electrons are the charge carriers in a conductor
    • These electrons normally move randomly through the conductor

5-1-2-random-motion-of-charge-carriers-through-a-conductor_sl-physics-rn

Random path of a delocalised electron through a length of conductor

  • If a potential difference is applied between two points in the conductor, an electric field is created
  • As a consequence:
    • An electric force will act on the charge carriers
    • The charge carriers will drift along the conductor
    • A steady average current will flow through the conductor

Electric Current & Drift Speed

  • The average speed at which the charge carriers move through a conductor is known as the drift speed 
  • The value of the drift speed is usually very small
    • For most everyday situations, v ∼ 10–4 m s–1

  • The electric current arising from the movement of a given number of charge carriers through a conductor is calculated as follows:

I = nAvq

  • Where:
    • n = number of charge carriers per unit volume, i.e. charge density (m–3)
    • A = cross-sectional area of the conductor (m2)
    • v = average drift speed of the charge carriers (m s–1)
    • q = charge of the charge carriers (C)

Worked example

A number N of charge carriers, each with a charge q, moves through a conductor of length L and cross-sectional area A. Show that the electric current flowing through the conductor is given by:

I = nAvq

Where n is the number of charge carriers per unit volume and v is their drift speed.

Step 1: Write down the equation for electric current 

I space equals space fraction numerator increment q over denominator increment t end fraction

Step 2: Write down the expression for the total charge Δq and substitute it into the above equation 

    • The total charge is equal to the number of charge carriers times the charge of each carrier

capital delta q space equals space N q

     

    • Substituting this into the current equation:

I space equals space fraction numerator N q over denominator increment t end fraction

    

Step 3: Write down the number of charge carriers in terms of charge density n, and substitute it into the above equation 

    • The charge density is equal to the number of charges per unit volume

n equals N over V

     

N space equals space n V

    

    • Substituting this into the current equation:

I space equals space fraction numerator n V q over denominator increment t end fraction

    

Step 4: Write down the volume of the conductor in terms of cross-sectional area and length, and substitute it into the above equation

    • The volume of the cylindrical section of the conductor is equal to the cross-sectional area times the length

V space equals space A L

   

    • Substituting this into the current equation:

I space equals space fraction numerator n A L q over denominator increment t end fraction

     

Step 5: Recognise average speed into the above equation 

    • Average speed v is total distance L over total time Δt

v equals fraction numerator L over denominator increment t end fraction

    • Substituting this into the current equation

I space equals space n A v q

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Ashika

Author: Ashika

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Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.