Hence, we assume that electrons in the conductor move with a constant average speed. We call this speed as drift speed or drift velocity.
Let us calculate the drift speed of free charges quantitatively.
Consider a conductor with cross sectional are quantitatively.
Consider a conductor with cross sectional area A. Assume that the ends of the conductor are connected to a battery to make the current flow through it. Let vd be the drift speed of the charges as shown in figure 4 and n be the number of charges present in the conductor in an unit volume (charge density). The distance covered by each charge in one second is vd
. Then the volume of the conductor for this distance is equal to Avd (see figure 4). The number of charges contained in that volume is equal to nAvd
. Let q be the charge of each carrier. Then the total charge crossing the cross sectional area at position D in one second is nqAvd
. This is equal to electric current.
Hence,
Electric current I = nqAvd ………………. (1)
Therefore, vd = I/nqA ………………. (2)
We know that the charge carriers in a conductor are electrons. The magnitude of electric charge ‘e’ is 1.602 x 10-19C. Let us calculate the drift speed of electron in a copper wire carrying a current of 1A and cross sectional area A = 10-6? m2
. The electron density of copper that was found experimentally is n = 8.5x1028m–3. Substituting these values in equation (2) with q = e, we get,
vd = 1/(8.5x1028x10-6x1.6x10-19)
vd = 7x10-5m/s = 0.07mm/s
This shows that the electrons are moving very slowly.
• Why does a bulb glow immediately when we switch on?
When we switch on any electric circuit, irrespective of length of the connecting wire (conductor) an electric field is set up throughout the conductor instantaneously due to the potential difference of the source (battery) connected to the circuit. This electric field makes all the electrons to move in a specified direction simultaneously.
• How can we decide the direction of electric current?