Solenoids are sources of magnetic field formed by winding wires conductors, evenly spaced, concentric and shaped like a cylinder of constant radius. When traversed by a electric current, they start to work as electromagnets, producing a fieldmagneticconstant inside.
The intensity of the magnetic field produced in solenoids is directly proportional to the electric current that runs through them, as well as the number of turns that form them.
See too: Magnetic field — properties, formulas, exercises
Magnetic field in solenoid
when a çcurrentelectric crosses a conductor, there is the emergence of a magnetic field. In solenoids, for example, it is possible to produce a concentrated magnetic field inside a winding of wires. According to lengthofsolenoid, the magnetic field becomes more uniform, so the induction lines from this field stay parallel and equallyspaced out inside. At the edges of the solenoid, in turn, the magnetic field is not uniform, due to the appearance of
edge effects, which distort the direction and direction of the magnetic field.
The polarity of the magnetic field produced in the solenoids can be discovered using thescrew rule. To use it, we close the fingers of the right hand in the direction that the current runs along the solenoid (clockwise or counterclockwise), so that the thumb indicates the direction of the northmagnetic. Look at the figure below, as it makes it easier to understand how the screw rule works.
Repair the electrical current circulating in the plane of the screen, counterclockwise. In this case, by closing the fingers of the right hand in this direction, the thumb points “off the screen”, so this is the direction of the magnetic field vector, which always points to magnetic north.
Formula of magnetic field in solenoid
The formula used to calculate the intensity of a magnetic field B which is generated by a N-turn solenoid, crossed by a electric current i, of length L, is as follows:
μ – magnetic permeability of the medium (N/A²)
N – number of windings (turns)
L – length of solenoid (m)
i - electric current (A)
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In the formula above, the element N/L represents the number of turns per unit of length of the solenoid. In addition, in a vacuum, the module of the permeabilitymagnetic equal to μ0 = 4π.10-7 AT-2.
Lookalso: What is magnetism?
Solved exercises on the magnetic field of the solenoid
Question 1 — (Udesc) Consider an ideal long solenoid composed of 10,000 turns per meter, carried by a continuous current of 0.2 A. The module and magnetic field lines inside the ideal solenoid are, respectively:
a) null, non-existent.
b) 8π.10-4 T, concentric circles.
c) 4π.10-4 T, cylindrical propellers.
d) 8π.10-3 T, radials originating from the axis of the solenoid.
e) 8π.10-4 T, straight lines parallel to the axis of the solenoid.
Resolution:
To calculate the strength of the magnetic field produced by this solenoid, we will use the formula that relates the number of windings per meter and the strength of the electric current.
Based on the calculation made above, we find the strength of the magnetic field produced inside the solenoid. Thus, the correct alternative is the letter D.
Question 2 — (Enem) An electromagnetic junkyard crane is capable of lifting tons of scrap, depending on the amount of induction in its electromagnet. The electromagnet is a device that uses electrical current to generate a magnetic field, usually constructed by winding a conducting wire around a core of ferromagnetic material (iron, steel, nickel, cobalt).
To increase the load capacity of the crane, which electromagnet characteristic can be reduced?
a) Diameter of the conductor wire
b) Distance between turns
c) Linear density of turns
d) Current flowing through the wire
e) Relative permeability of the core
Resolution:
To produce a greater force, the magnetic field produced by the solenoid needs to be stronger. Thus, among the alternatives shown, the only way to do it is to reduce the distance between the turns. The correct alternative is the letter B.
By Rafael Hellerbrock
Physics teacher
