Situation 3: Replace the copper wires with nylon wires and connect
the nylon wires to the terminals of the battery through a bulb and switch.
Now switch on the circuit and observe the bulb. Does the bulb glow?
After doing these activities you will notice that the bulb glows only in
situation 1.
• Can you predict the reason for the bulb not glowing in situations 2 and
3?
In class VII, you have learnt that the battery stores chemical energy
and this energy converts into electric energy and makes the bulb to glow
as you observed in situation 1 i.e. the battery supplies the required energy
to make the bulb glow. But in situation 3, though there is a battery in the
circuit, the bulb does not glow because the connecting wires (nylon wires)
are not able to carry the energy from source (battery) to bulb.
Hence, the nature of the substance plays an important role in the transfer
of energy from battery to bulb. The material which transfers energy from
battery (source) to the bulb is called a conductor and the material which
cannot transfer energy from battery (source) to the bulb is called a non i'll
conductor.
• Why do all materials not act as conductors?
• How does a conductor transfer energy from source to bulb?
Let us see.
Electric Current
Drude and Lorentz, scientists of the 19th century, proposed that
conductors like metals contain a large number of free electrons while the
positive ions are fixed in their locations. The arrangement of the positive
ions is called lattice.
Let us understand the behaviour of electrons in lattice space. Assume
that a conductor is an open circuit. The electrons move randomly in lattice
space of a conductor as shown in figure 1. When the electrons are in random
motion, they can move in any direction. Hence, if you imagine any cross
section as shown in figure 1, the number of electrons, crossing the cross
section of a conductor from left to right in one second is equal to that of
electrons passing the cross section from right to left in one second. It
means that net charge moving along a conductor
through any cross section is zero when the conductor
is in open circuit.
• What happens to the motion of electrons when
the ends of the conductor are connected to the
battery?