Electricity & Megnetism

Magnetic field sources

The magnetic needle undergoes deflection when there is electric current in the conducting wire

It is common in our daily lives to come across objects that are attracted to others: some screwdrivers, for example, have the property of attracting small iron materials, such as screws. According to the history of science, the first observations made regarding certain stones that had the ability to attract iron and interact with each other were made many centuries ago.

These stones came to be called magnets; and the phenomena which spontaneously manifested themselves were called magnetic phenomena . The term magnetism comes from a region known as Magnesia, a province of Greece where these stones were found.

In studies on magnetism, we have seen that if we approach a magnetic needle a magnet, the same needle will undergo a deflection. In this way, we can say that the magnet generates a magnetic field that acts on the magnetic needle. Another phenomenon was observed by the physicist Oersted. He observed in his experiments that when he brought a magnetic needle close to a conducting wire carrying an electric current, the needle also deflected; and when the electric current was interrupted, the needle returned to its initial position.

Today we know that the fact that the needle undergoes deflection means that there is a magnetic field around the wire carrying an electric current.

magnetic field sources

straight conductor

A conductor carrying an electric current generates a magnetic field around it. The configuration of this field can be determined by placing small magnetic needles at points in this region. The equation that gives us the magnetic field generated by a straight conductor carrying an electric current is as follows:

circular spiral

The magnetic field generated by a circular loop carried by an electric current can be determined by the following equation:

The magnetic field induction lines are circles perpendicular to the plane of the loop, concentric with the conductor.

flat coil

If we consider n equal turns juxtaposed, so that the thickness of the winding is smaller than the diameter of each turn, we have the so-called flat coil . We can determine the intensity of the magnetic field vector at the center of the coil through the equation:


A long, coiled conductor that forms a tube made up of equally spaced turns is called a solenoid. The strength of the magnetic field vector inside a solenoid is determined by the following equation:

Where N/L represents the number of turns per unit length. And, in relation to the above equation, μ represents the magnetic permeability of the conductor.

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