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power transmission Why does the power transmission take place at high voltage? Electricity is transmitted at high voltages in order to minimize the loss of power during transmission. The power loss during transmission is proportional to the square of the current. So there is a strong motivation to reduce the current. For a given amount of energy, the current is inversely proportional to the voltage. So, higher the voltage, lesser current, which in turn results in low power loss.

Learning objectives

After completing the topic, the student will be able to:

  • Determine the direction of conventional electrical current in a circuit, using the water flow analogy and its relevance in daily life.
  • Explore the concept of electric potential difference and its relevance to everyday science.
  • Discuss and discover the concept of electrical resistance and determine the value of any resistor in an electrical circuit based on its colour codes.
  • Restate, explore and employ the Ohm's law to determine the characteristics of current–voltage relationship.
  • Discuss and explore about the concept of voltage and currents and analyze different types of moving coil galvanometers.
  • Explore different types of electrical circuits with series and parallel combination of resistances.
Electrons in a conductor Electrons in a conductor let's think about moving charges – often referred to as electricity. Here is a close up of a piece of wire where the nucleus of the atoms are shown by the large “clusters” of protons and neutrons. The small black circles represent the electrons around the nuclei. In this picture the electrons are orbiting round the nearest nucleus and there is no “flow” of charged particles.
Flow of electrons in a conductor

When the electrons flow, they carry electrical energy with them. This electrical energy is converted into other forms of energy by electrical devices or machines, which we utilize either at home or in the industry. In current electricity, we will try to understand how and why electrons move from one point to another.

Take a material that is a good conductor of electricity like a copper wire. If we were somehow able to see inside the wire, what would we observe? We will see that electrons are moving around randomly. It is like watching people walking around on a railway platform. There is no unique direction. Normal room temperature is sufficient to give the outer electrons of copper some energy to go out of their orbits.

Similarly, if there is a battery connected to the copper wire, some of the more energetic electrons in the copper material get a definite direction; they will tend to move from the negative side of the battery to the positive side.

Flow of electrons Flow of electrons through a metal wire The charge flows from one point to other in a conductor.
Flow of electric charge in a metallic conductor

If we apply a type of force (called electromotive force) to the metal wire, the electrons will “flow” through the wire as is shown in the picture. The electrons will jump from one nucleus to the next along the path of the wire.

Think of a “plane” through the wire and we can see that a certain number of electrons pass through the plane during each unit of time. This is known as "electrical current". Mathematically, we can write this as:
I = charge/time  or   I = q/t,
where 'I' is the current, 'q' is charge, and 't' is the time. Attach the copper wire to a small bulb. Connect the bulb and the other end of the copper wire to two terminals of a small battery through a key (or a switch). As you connect the key, what do you observe? The bulb starts glowing! Remove the key and the bulb stops glowing.

Take another battery and connect to the earlier. You will observe that the glow of the bulb is stronger. From the knowledge gained in the earlier lessons, we know that:

  • charges are attracted to opposite charges with a force given by Coulomb’s law.
  • charges move so as to neutralize any excess charges.

Now you will immediately guess that the bulb is glowing because of flow of electrons through the copper wire. When two batteries are connected, the bulb glows more strongly because there is more flow of electrons.