Voltage, Current and Resistance

To understand voltage, current and resistance let us consider a very simple circuit containing a copper wire connected across the two terminals of a battery.

The battery

It is beyond the scope of this article to describe the working of a battery but it would to sufficient to describe it as a device which contains two terminals coming out of an enclosure. Chemicals are filled inside this enclosure and are constantly reacting. These reactions cause electrons to gather on one terminal making it negatively charged. Simultaneous reactions on the other terminal causes it to lose electrons and makes it positively charged. The negatively charged terminal is called the negative terminal and the positively charged is called the positive terminal.

A current of electrons

When we connect a copper wire across the two terminals of a battery, the negative terminal will repel electrons in the copper wire. This repulsion will eject the loosely bound electrons from the copper atoms. These ejected electrons will repel other electrons in the neighbouring atoms ejecting them in turn. This will cause a chain reaction. Similarly, the positive terminal will attract electrons since it has a deficit of electrons. This attraction and repulsion causes a current of electrons to flow through the copper wire also called as the electric current or simply current.

Since electrons are charged particles, a flow of electrons is also a flow of negative charges. The electric current is measured as the number of charges passing through a cross section of a wire each second. Also called as the rate of flow of charge. The charge can be either positive or negative. Although in most electric circuits the current is due to moving electrons, but we can imagine that the positive charges are flowing in the opposite direction. Even if we switch the negatives with positives all remains the same.

In electric circuits the current is always indicated as flowing from the positive terminal to the negative terminal and is called the conventional electric current. One way to visualize it is this. When an electron is ejected from a copper atom and before the atom attracts another electron, the ejected electron leaves a hole in the atom. As the electrons move from negative to positive terminal the holes move from positive to negative terminal. This is also called as hole current.

Potential difference

This difference is very important to make the electrons flow from one end to the other. If both the ends were negative there will only be a push and if they were positive they will only be a pull. This difference creates a potential for the electrons to flow through the copper wire and is called a potential difference.

Lets understand this with an analogy. If we take a ball to the top of the mountain and let it go, it will roll down the mountain. While rolling down it may get bumped by some rocks, trees or animals but it will keep rolling until it reaches the base of the mountain. It will stop rolling and lie still when it reaches the base. As the base of the mountain is a surface without a slope, the ball has no potential to roll. On taking it to the top of the mountain we increase its potential to roll. This is called the potential energy of the ball and it is zero at the base of the mountain and highest at the top. Since energy cannot be created nor destroyed, the ball’s potential energy will get converted to kinetic energy and becomes zero when it reaches the base. Taking the ball up the mountain also requires energy.

Similarly, the chemical energy in the battery is spent in moving the electrons to the negative terminal which is the top of the mountain. This increases the electron’s potential to go from the negative terminal to the positive terminal. This will keep on going until the chemical energy in the battery is exhausted.

Resistance

Electrons do not flow smoothly through the copper wire. They constantly collide with atoms and get repelled by other electrons. Although their movement looks erratic they continue to move from the negative terminal to the positive. Due to these collisions the flow of electrons is resisted and is called the resistance of the copper wire.

This is analogous to water flowing through a pipe. The pressure with which the water flows is the potential difference, the flow of water is the electric current and the thickness of the pipe is the resistance. A kink in the pipe will resist the flow of water. Similarly, a thin wire has less free electrons and will allow less current to flow than a thick wire. Resistance in the thin wire is said to be higher than the thick wire.

Similarly, resistance also increases with an increase in the length of the wire. In the water through a pipe analogy, if the water pressure is kept the same but the length of the pipe is increased ten times, the water will flow slower than in a shorter pipe. The water does not have enough pressure to overcome the resistance of the long pipe.

Heat

All these collisions within the copper wire generate heat. The higher the resistance the more heat will generate. This phenomenon makes the light bulb glow. A light bulb contains a thin, hard to melt metal filament, usually Tungsten, enclosed in a glass globe filled with inert gases to prevent the filament from oxidizing and disintegrating. When a current passes through this filament it heats up producing light.

Likewise, if we increase the temperature of a conductor its resistance increases. When heated the atoms in the conductor vibrate more and offer more resistance to the electrons to wriggle through.

Speed of electrons

Due to collisions with other atoms, electrons move very slow in a conductor. Then why does a light bulb glow instantly when we switch it on? This is best understood with an analogy of a long queue of people waiting to get into a sports stadium. The game is about to start and everyone is excited to get into the stadium but the queue is moving very slow. The person at the end of the queue cannot wait and pushes the person in front of him who in turn pushes the person in front and this goes on till the start of the queue. This way the push of the person at the end reaches the start of the queue very fast despite the queue moving very slow. Similarly, despite moving slowly the effect of the electrons reaches the end of the wire very fast, almost at the speed of light.