q and drift speed vd
. For electrons (negative charges), q is negative and vd
is positive. Then the product of q and vd
is negative. This negative sign
shows that the direction of electric current is opposite to the flow of
negative charge. For positive charges the product of q and vd
is positive.
Hence, the direction of electric current can be taken as the direction of
flow of positive charges.
• How can we measure electric current?
Usually an ammeter is used to measure electric current. It is always
connected in series to the circuit.
• Where do the electrons get energy for their motion from?
Potential Difference
When the ends of a conducting wire are connected to the terminals of
a battery, an electric field is setup throughout the conductor. This field
exerts a force on the charge (electron). Let Fe
be the force exerted by the
electric field on a free charge q. The free charges accelerate in the direction
of the electric field (If the free charges are electrons, then the direction
of electric force on them is opposite to the direction of electric field). It
means the electric field does some work to move free charges in a specified
direction.
• Can you find the work done by the electric force?
Let the electric force made the charges move
through a distance ‘l’ from A to B as shown in
figure 5. We know that, the work is the product of
force and distance along the direction of force.
Hence, work done by the electric force on a
free charge q is given by
W = Fe
What is the work done by the electric force on unit charge?
Work done by the electric force on unit charge = W/q = Fe l/q
Work done by the electric force on unit positive charge to move it
through a distance ‘l’ from A to B is called potential difference between
those points. Potential difference is denoted by a symbol V. The potential
difference between two points separated by a distance l in a conducting
wire is given by,
V = W/q = Fe l/q
