The second right-hand rule establishes the relationship between the direction of the magnetic field and the direction in which the electric current moves in a solenoid.
A magnetic field is created when electrical charges are in motion. In other words, only dynamic electricity can generate a magnetic field; static electricity cannot. In addition, this magnetic field only exists when current is flowing. As soon as the current stops, the magnetic field disappears. Thus, there is a link between electricity and magnetism, which is called electromagnetism.
A solenoid is a conductive wire wound in several loops through which flows an electric current.
The intensity of the magnetic field around a conductive wire can be increased by winding it in regular loops. This form given to the conductive wire is called a solenoid. Around a solenoid, the shape of the magnetic field is identical to that formed around a bar magnet. However, unlike the magnet, the solenoid is empty and therefore a magnetic field can be observed inside it. Moreover, the intensity of the magnetic field is at its maximum at the centre of the cylinder.
Although the magnetic field of a solenoid is very similar to that of a bar magnet, there are three differences between the two:
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the magnetic field of a solenoid can be turned on or off at will, but not that of a magnet;
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the direction of the magnetic field lines of a solenoid can be changed by reversing the direction of the electric current, while we cannot reverse the magnetic field of a magnet since we cannot reverse the poles of a magnet;
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the magnetic field strength of a solenoid can be changed, but not that of a magnet.
The following image illustrates the shape of a magnetic field around the solenoid without orientation.
When drawing the magnetic field around a solenoid, always start by drawing the shape of the field lines, which is always the same. Then, we can orient this field with the second rule of the right hand.
The direction of the magnetic field around the solenoid depends on the direction of the electric current flowing through the wire coil (orange). Just like a bar magnet, the magnetic field exits through the North Pole of the solenoid and enters through the south. Inside the solenoid, the magnetic field moves from south to north.
The orientation of the magnetic field created in a solenoid can be determined using the second right-hand rule. The second right-hand rule states that the thumb of the right hand points to the North Pole of the solenoid when the hand is wrapped in the same way as the electric current around the solenoid.
The fingers point in the conventional direction of the electric current.
The thumb indicates the direction of the magnetic field lines inside the solenoid.
There are three ways to significantly increase the strength of the magnetic field around the solenoid. However, no matter how strong the magnetic field, it will always keep the same shape.
The more the wire turns around the solenoid, the stronger its magnetic field will be. For example, a solenoid with three times more turns would have a magnetic field that is approximately three times more intense.
The greater the intensity of the electric current flowing through the electric wire, the more powerful its magnetic field will be. For example, a solenoid with five times the current intensity would have a magnetic field about five times stronger.
The solenoid can be transformed into an electromagnet by adding a non-permanent ferromagnetic substance rod to the centre of the solenoid. If the solenoid core (substance inside the electromagnet) has a ferromagnetic nature (iron, nickel, or cobalt), then the magnetic field will be stronger.
Solenoids are used in several technological applications.
For example, in a microphone, the presence of sound waves causes a coil of wire to vibrate near a magnet. This is how electromagnetic induction occurs: an electric current is produced by the movement of the conductor near the magnet. This current then begins to flow through the conductive wire.
On the other hand, a loudspeaker works in the opposite way from the microphone: when the electric current moves to the conductive wire, the conductive wire is placed near a magnetic field. It then sets itself in motion due to the electromagnetic induction created by the presence of the magnetic field. The wire coil transmits its motion to the speaker membrane, which then transforms the motion of the wire into sound.