Electronics Online

This article explain the complete basic theory of magneticm.

MAGNETISM AND ELECTRICITY
Any wire carrying a current of electrons is surrounded by an unseen area of force called a magnetic field. For this reason, any study of electricity or electronics must consider magnetism.

Almost everyone has had experiences with magnets or with pocket compasses at one time or another. A magnet attracts pieces of iron but has little affect on practically everything else. Why does it single out the iron? A compass, when laid on a table, swings back and forth, finally coming to rest pointing toward the North Pole of the world. Why does it always point in the same direction?

These and other questions about magnetism have puzzled scientists for hundreds of years. It is only relatively recently that theories seeming to answer many of the perplexing questions that arise when magnetism is investigated have been developed.

Radio and electronic apparatus such as relays, circuit breakers, earphones, loudspeakers, transformers, chokes, magnetron tubes, television tubes, phonograph pickups, tape and disk recorders, microphones, meters, motors, and generators depend on magnetic effects to make them function.

When current flows in a resistance, heat is produced because friction between the moving free electrons and the atoms obstructs the path of electron flow. The heat is evidence that power is used in producing current. This is how a fuse opens, as heat resulting from excessive current melts the metal link in the fuse.

The power is generated by the source of applied voltage and consumed in the resistance in the form of heat. As much power as the resistance dissipates in heat must be supplied by the voltage source; otherwise, it cannot maintain the potential difference required to produce the current.

Any one of the three formulas can be used to calculate the power dissipated in a resistance. The one to be used is just a matter of convenience, depending on which factors are known.

In many circuits, some components are connected in series to have the same current, while others are in parallel for the same voltage. When analysing and doing calculations with series-parallel circuits you simply apply what you have learnt from the last two readings.

In the circuit of figure 1 below, we could work out all the voltages across all of the resistances and the current through each resistance and then total resistance. For now I am just going to walk through the simplification of this circuit to a single resistance connected across the 100 V source.

Keep in mind that any circuit (resistive) can be reduced to a single resistance. This is particularly useful when we come to do transmission lines and antennas.

For now let’s have a go at simplifying the circuit of figure 1. There are many ways to go about this problem. The method I prefer is to start at the right hand side and work my way back to the source, simplifying the circuit as I go.

Figure 1

On the right hand side we see R3 and R4 in parallel and each 12Ω. Do you remember the short cut method when parallel resistances are all the same value? Divide the value of the branch by the number of branches: 12Ω / 2 = 6Ω.