Calculate current that produces a magnetic field.Use the best hand preeminence 2 to recognize the direction of existing or the direction the magnetic ar loops.

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How much current is required to create a far-reaching magnetic field, probably as strong as the earth’s field? Surveyors will certainly tell you the overhead electric power lines produce magnetic fields that interfere through their compass readings. Indeed, once Oersted uncovered in 1820 that a current in a wire affected a compass needle, he was not managing extremely huge currents. Just how does the form of wires delivering current influence the form of the magnetic ar created? We listed earlier that a current loop developed a magnetic field similar to that of a bar magnet, however what around a straight wire or a toroid (doughnut)? how is the direction the a current-created ar related to the direction of the current? Answers come these inquiries are discover in this section, along with a short discussion of the regulation governing the fields created by currents.

Magnetic areas have both direction and magnitude. As listed before, one means to discover the direction that a magnetic ar is with compasses, as shown for a lengthy straight current-carrying cable in number 1. Room probes deserve to determine the magnitude of the field. The field roughly a lengthy straight wire is found to be in one loops. The right hand dominion 2 (RHR-2) emerges from this exploration and also is valid for any kind of current segment—point the thumb in the direction that the current, and also the fingers curly in the direction that the magnetic ar loops produced by it.

Figure 1. (a) Compasses put near a long straight current-carrying wire suggest that field lines type circular loops centered on the wire. (b) appropriate hand dominance 2 says that, if the right hand ignorance points in the direction the the current, the fingers curl in the direction of the field. This dominance is continuous with the field mapped for the lengthy straight wire and is valid for any type of current segment.

The magnetic ar strength (magnitude) created by a lengthy straight current-carrying wire is found by experiment to be

B=\frac\mu_0I2\pi r\left(\textlong right wire\right)\\,

where I is the current, r is the shortest street to the wire, and also the constant \mu _0=4\pi \times 10^-7\textT\cdot\text m/A\\ is the permeability of cost-free space. (μ0 is just one of the basic constants in nature. We will see later that μ0 is related to the rate of light.) due to the fact that the wire is an extremely long, the magnitude of the ar depends only on street from the wire r, not on place along the wire.

Find the existing in a long straight wire that would create a magnetic ar twice the strength of the earth’s at a street of 5.0 centimeter from the wire.


The Earth’s field is about 5.0 × 10−5 T, and so below B due to the cable is taken to be 1.0 × 10−4 T. The equation B=\frac\mu_0I2\pi r\\ can be used to uncover I, because all various other quantities room known.


Solving because that I and entering known values gives

\beginarraylllI& =& \frac2\pi rB\mu _0=\frac2\pi\left(5.0\times 10^-2\text m\right)\left(1.0\times 10^-4\text T\right)4\pi \times 10^-7\text T\cdot\textm/A\\ & =& 25\text A\endarray\\


So a moderately big current produces a far-reaching magnetic field at a street of 5.0 centimeter from a long straight wire. Keep in mind that the price is stated to only two digits, since the Earth’s field is stated to only two digits in this example.

The magnetic ar of a long straight wire has more implications 보다 you might at an initial suspect. Each segment of existing produces a magnetic ar like that of a long straight wire, and also the total field of any shape current is the vector amount of the fields due to each segment. The formal statement of the direction and magnitude of the field because of each segment is dubbed the Biot-Savart law. Integral calculus is required to sum the field for an arbitrary form current. This outcomes in a more complete law, dubbed Ampere’s law, which relates magnetic field and also current in a general way. Ampere’s law subsequently is a component of Maxwell’s equations, which provide a finish theory of all electromagnetic phenomena. Considerations of how Maxwell’s equations show up to different observers brought about the contemporary theory of relativity, and also the realization the electric and magnetic fields are various manifestations the the exact same thing. Most of this is beyond the scope of this text in both mathematical level, requiring calculus, and in the amount of an are that can be dedicated to it. However for the interested student, and particularly for those who proceed in physics, engineering, or comparable pursuits, delving into these matters further will expose descriptions that nature that are elegant as well as profound. In this text, us shall save the general attributes in mind, such together RHR-2 and also the rules for magnetic field lines listed in Magnetic Fields and also Magnetic field Lines, while concentrating top top the fields created in certain important situations.

Hearing all us do around Einstein, we sometimes obtain the impression that he developed relativity out of nothing. On the contrary, among Einstein’s motivations to be to solve difficulties in understanding how various observers check out magnetic and electric fields.

The magnetic field near a current-carrying loop of cable is displayed in number 2. Both the direction and the magnitude of the magnetic field created by a current-carrying loop space complex. RHR-2 deserve to be used to provide the direction the the ar near the loop, however mapping with compasses and the rules about field lines offered in Magnetic Fields and also Magnetic ar Lines are required for much more detail. Over there is a basic formula because that the magnetic field strength in ~ the facility of a circular loop. The is

B=\frac\mu_0I2R\left(\textat center of loop\right)\\,

where R is the radius that the loop. This equation is very comparable to the for a right wire, however it is precious only in ~ the center of a one loop of wire. The similarity that the equations does suggest that comparable field strength deserve to be obtained at the center of a loop. One way to get a larger ar is to have actually N loops; then, the field is 0I/(2R). Keep in mind that the bigger the loop, the smaller the ar at that is center, due to the fact that the existing is farther away.

Figure 2. (a) RHR-2 provides the direction of the magnetic field inside and outside a current-carrying loop. (b) much more detailed mapping v compasses or with a room probe completes the picture. The field is similar to that of a bar magnet.

A solenoid is a lengthy coil of cable (with plenty of turns or loops, as opposed come a level loop). Since of the shape, the field inside a solenoid have the right to be very uniform, and also also really strong. The field just external the coils is practically zero. Figure 3 shows just how the field looks and also how that is direction is given by RHR-2.

Figure 3. (a) due to the fact that of that shape, the ar inside a solenoid of length l is remarkably uniform in magnitude and direction, as suggested by the straight and uniformly spaced field lines. The field exterior the coils is virtually zero. (b) This cutaway reflects the magnetic field generated by the current in the solenoid.

The magnetic ar inside that a current-carrying solenoid is an extremely uniform in direction and magnitude. Only close to the ends does it begin to weaken and adjust direction. The field outside has comparable complexities to flat loops and also bar magnets, yet the magnetic ar strength inside a solenoid is simply

B=\mu _0nI\left(\textinside a solenoid\right)\\,

where n is the number of loops per unit size of the solenoid (N/l, through N gift the number of loops and l the length). Keep in mind that B is the ar strength all over in the uniform an ar of the interior and not simply at the center. Big uniform areas spread end a large volume are possible with solenoids, as example 2 implies.

What is the ar inside a 2.00-m-long solenoid that has actually 2000 loops and also carries a 1600-A current?


To uncover the ar strength inside a solenoid, we usage B=\mu _0nI\\. First, we keep in mind the variety of loops per unit length is

n=\fracNl=\frac20002.00\text m=1000\text m^-1=10\text cm^-1\\.

Solution Substituting well-known values gives

\beginarraylllB & =& \mu_0nI=\left(4\pi \times 10^-7\text T\cdot\textm/A\right)\left(1000\text m^-1\right)\left(1600\text A\right)\\ & =& 2.01\text T\endarray\\


This is a big field toughness that can be developed over a large-diameter solenoid, such together in medical uses of magnetic resonance imaging (MRI). The very huge current is an indication the the fields of this strength are not easily achieved, however. Such a large current through 1000 loops squeezed right into a meter’s size would produce significant heating. Greater currents deserve to be accomplished by utilizing superconducting wires, back this is expensive. There is an upper limit come the current, since the superconducting state is disrupted by very huge magnetic fields.

There are amazing variations of the level coil and solenoid. Because that example, the toroidal coil used to confine the reactive corpuscle in tokamaks is much like a solenoid bent into a circle. The ar inside a toroid is very solid but circular. Fee particles travel in circles, following the ar lines, and collide v one another, maybe inducing fusion. However the charged particles execute not cross ar lines and escape the toroid. A whole selection of coil shapes are supplied to produce all kinds of magnetic field shapes. Including ferromagnetic materials produces greater field strengths and also can have actually a far-reaching effect on the form of the field. Ferromagnetic materials tend to trap magnetic areas (the ar lines bend into the ferromagnetic material, leaving weaker fields outside it) and are offered as shields for tools that space adversely affected by magnetic fields, including the earth’s magnetic field.

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Section Summary

The strength of the magnetic field developed by present in a lengthy straight wire is given by

where I is the current, r is the shortest distance to the wire, and also the constant\mu_0=4\pi \times 10^-7\text T\cdot\text m/A\\ is the permeability of totally free space.The direction of the magnetic field developed by a lengthy straight wire is offered by right hand preeminence 2 (RHR-2): Point the ignorance of the right hand in the direction of current, and also the fingers curl in the direction that the magnetic field loops developed by it.The magnetic field created by existing following any kind of path is the sum (or integral) that the fields due to segments follow me the path (magnitude and direction as for a right wire), causing a basic relationship in between current and also field well-known as Ampere’s law.The magnetic field strength in ~ the facility of a circular loop is offered by
where R is the radius of the loop. This equation becomes B = μ0nI/(2R) for a level coil of N loops. RHR-2 offers the direction that the field about the loop. A long coil is dubbed a solenoid.The magnetic ar strength within a solenoid is
where n is the number of loops per unit size of the solenoid. The ar inside is very uniform in magnitude and also direction.

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1. Do a drawing and also use RHR-2 to find the direction of the magnetic ar of a current loop in a motor (such as in number 1 native Torque on a current Loop). Then present that the direction that the speak on the loop is the very same as created by prefer poles repelling and unlike poles attracting.


right hand dominion 2 (RHR-2):a dominance to determine the direction of the magnetic ar induced by a current-carrying wire: allude the ignorance of the appropriate hand in the direction the current, and the fingers curly in the direction that the magnetic ar loopsmagnetic field strength (magnitude) developed by a long straight current-carrying wire:defined together B=\frac\mu_0I2\pi r\\, where is the current, r is the shortest distance to the wire, and μ0 is the permeability of complimentary spacepermeability of totally free space:the measure of the ability of a material, in this case complimentary space, to support a magnetic field; the continuous \mu_0=4\pi \times 10^-7T\cdot \textm/A\\magnetic field strength in ~ the center of a one loop:defined as B=\frac\mu _0I2R\\ wherein R is the radius the the loopsolenoid:a thin wire wound into a coil that produces a magnetic field when an electric existing is passed with itmagnetic ar strength inside a solenoid:defined as B=\mu _0\textnI\\ wherein n is the variety of loops every unit size of the solenoid n = N/l, v N being the variety of loops andthe length)Biot-Savart law:a physical law that explains the magnetic ar generated by one electric present in regards to a certain equationAmpere’s law:the physical law that states that the magnetic field roughly an electric current is proportional come the current; each segment of current produces a magnetic field like the of a lengthy straight wire, and the total field of any shape present is the vector sum of the fields due to each segmentMaxwell’s equations:a collection of 4 equations that define electromagnetic phenomena