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
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
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
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
-
- Substituting this into the current equation:
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
-
- Substituting this into the current equation:
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
-
- Substituting this into the current equation:
Step 5: Recognise average speed into the above equation
-
- Average speed v is total distance L over total time Δt
-
- Substituting this into the current equation