3.3. ELECTRIC CURRENTS IN CONDUCTORS

An electric charge will experience a force if an electric field is applied. If it is free to move, it will thus move contributing to a current. In nature, free charged particles do exist like in upper strata of atmosphere called the ionosphere. However, in atoms and molecules, the negatively charged electrons and the positively charged nuclei are bound to each other and are thus not free to move.

When an electric charge is placed in an electric field, it experiences a force that can cause it to move. If the charged particle is free, like electrons in a conductor, it contributes to an electric current. However, in most materials, such as atoms and molecules, electrons are bound to their nuclei and cannot move freely. Hence, the ability of a material to conduct electricity depends on the availability of free charges.

Consider a crowded room where people (analogous to electrons) are trying to move around. If the lights turn on (an electric field is applied), people might start moving towards the exit, but if the room is filled with obstacles (the nuclei), their movement is limited.

In some materials, the electrons will still be bound, i.e., they will not accelerate even if an electric field is applied. In other materials, notably metals, some of the electrons are practically free to move within the bulk material. These materials, generally called conductors, develop electric currents in them when an electric field is applied.

Different materials behave differently when subjected to an electric field. Conductors, such as metals, have electrons that can move freely, allowing electrical currents to flow. On the other hand, insulators, like rubber and glass, have bound electrons that do not conduct electricity well because they cannot move freely in response to an electric field.

Think of conductors like a highway where cars (electrons) can move freely, allowing traffic (current) to flow smoothly. In contrast, an insulator is like a parking lot where cars are blocked or parked and cannot move easily, preventing any flow.

Consider first the case when no electric field is present. The electrons will be moving due to thermal motion during which they collide with the fixed ions. An electron colliding with an ion emerges with the same speed as before the collision. However, the direction of its velocity after the collision is completely random.

In the absence of an electric field, the motion of electrons in a conductor is random due to thermal energy. While they may collide with fixed ions and bounce off, their overall movement does not result in a net flow of electric current, as directions cancel each other out, leading to no average directional motion.

Imagine a box full of ping pong balls shaking randomly. Even though they bounce around a lot, if you look from above, you wouldn't see any particular direction of movement—most of their movement cancels out, just like electrons without an electric field.

Let us now see what happens to such a piece of conductor if an electric field is applied. Suppose we now take two thin circular discs of a metallic cylinder. An electric field will be created and is directed from the positive towards the negative charge. The electrons will be accelerated due to this field towards +Q.

When an electric field is applied to a conductor, it creates a force that accelerates free electrons in the direction opposite to the field (since electrons are negatively charged). This acceleration results in a net flow of electrons, constituting an electric current as the electrons move to neutralize positive charges.

Think about a water slide. When you push water down the slide (applying an electric field), the water flows downwards (the movement of electrons due to the electric field), creating a current of water much like how electrons move to form an electric current.

The current thus will stop after a while unless the charges +Q and –Q are continuously replenished. We can also imagine a mechanism where the ends of the cylinder are supplied with fresh charges to make up for any charges neutralised by electrons moving inside the conductor.

For a continuous current to flow in a conductor, there needs to be a constant supply of charge, as electrons will neutralize the fixed charges in the conductor quickly. This is typically achieved through sources such as batteries, which provide a steady influx of charges to maintain the current.

It's like a water tank with a continuous inflow. If you open a tap (apply an electric field), water flows out (electrons move), but if there's no water coming in to replace what’s leaving, the flow will eventually stop.

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