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Proximity effect (electromagnetism)
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Proximity effect (electromagnetism)
In electromagnetics, proximity effect is a redistribution of electric current occurring in nearby parallel electrical conductors carrying alternating current (AC), caused by magnetic effects. In adjacent conductors carrying AC current in the same direction, it causes the current in the conductor to concentrate on the side away from the nearby conductor. In conductors carrying AC current in opposite directions, it causes the current in the conductor to concentrate on the side adjacent to the nearby conductor. Proximity effect is caused by eddy currents induced within a conductor by the time-varying magnetic field of the other conductor, by electromagnetic induction. For example, in a coil of wire carrying alternating current with multiple turns of wire lying next to each other, the current in each wire will be concentrated in a strip on each side of the wire facing away from the adjacent wires. This "current crowding" effect causes the current to occupy a smaller effective cross-sectional area of the conductor, increasing current density and AC electrical resistance of the conductor. The concentration of current on the side of the conductor gets larger with increasing frequency, so proximity effect causes adjacent wires carrying the same current to have more resistance at higher frequencies.
A changing magnetic field will influence the distribution of an electric current flowing within an electrical conductor, by electromagnetic induction. When an alternating current (AC) flows through a conductor, it creates an associated alternating magnetic field around it. The alternating magnetic field induces eddy currents in adjacent conductors, altering the overall distribution of current flowing through them. The result is that the current is concentrated in the areas of the conductor farthest away from nearby conductors carrying current in the same direction.
The proximity effect can significantly increase the AC resistance of adjacent conductors when compared to their resistance with a DC current. The effect increases with frequency. At higher frequencies, the AC resistance of a conductor can easily exceed ten times its DC resistance.
The cause of proximity effect can be seen from the accompanying drawings of two parallel wires next to each other carrying alternating current (AC). The righthand wire in each drawing has the top part transparent to show the currents inside the metal. Each drawing depicts a point in the alternating current cycle when the current is increasing.
In the first drawing the current (I, red arrows) in both wires is in the same direction. The current in the lefthand wire creates a circular magnetic field (B, green lines) which passes through the other wire. From the right hand rule the field lines pass through the wire in an upward direction. From Faraday's law of induction, when the time-varying magnetic field is increasing, it creates a circular current (E, red loops) within the wire around the magnetic field lines in a clockwise direction. These are called eddy currents.
On the lefthand side nearest to the other wire (1) the eddy current is in the opposite direction to the main current (big pink arrow) in the wire, so it subtracts from the main current, reducing it. On the righthand side (2) the eddy current is in the same direction as the main current so it adds to it, increasing it. The net effect is to redistribute the current in the cross section of the wire into a thin strip on the side facing away from the other wire. The current distribution is shown by the red arrows and color gradient (3) on the cross section, with blue areas indicating low current and green, yellow, and red indicating higher current.
The same argument shows that the current in the lefthand wire is also concentrated into a strip on the far side away from the other wire.
In an alternating current the currents in the wire are increasing for half the time and decreasing half the time. When the current in the wires begins to decrease, the eddy currents reverse direction, which reverses the current redistribution.
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Proximity effect (electromagnetism)
In electromagnetics, proximity effect is a redistribution of electric current occurring in nearby parallel electrical conductors carrying alternating current (AC), caused by magnetic effects. In adjacent conductors carrying AC current in the same direction, it causes the current in the conductor to concentrate on the side away from the nearby conductor. In conductors carrying AC current in opposite directions, it causes the current in the conductor to concentrate on the side adjacent to the nearby conductor. Proximity effect is caused by eddy currents induced within a conductor by the time-varying magnetic field of the other conductor, by electromagnetic induction. For example, in a coil of wire carrying alternating current with multiple turns of wire lying next to each other, the current in each wire will be concentrated in a strip on each side of the wire facing away from the adjacent wires. This "current crowding" effect causes the current to occupy a smaller effective cross-sectional area of the conductor, increasing current density and AC electrical resistance of the conductor. The concentration of current on the side of the conductor gets larger with increasing frequency, so proximity effect causes adjacent wires carrying the same current to have more resistance at higher frequencies.
A changing magnetic field will influence the distribution of an electric current flowing within an electrical conductor, by electromagnetic induction. When an alternating current (AC) flows through a conductor, it creates an associated alternating magnetic field around it. The alternating magnetic field induces eddy currents in adjacent conductors, altering the overall distribution of current flowing through them. The result is that the current is concentrated in the areas of the conductor farthest away from nearby conductors carrying current in the same direction.
The proximity effect can significantly increase the AC resistance of adjacent conductors when compared to their resistance with a DC current. The effect increases with frequency. At higher frequencies, the AC resistance of a conductor can easily exceed ten times its DC resistance.
The cause of proximity effect can be seen from the accompanying drawings of two parallel wires next to each other carrying alternating current (AC). The righthand wire in each drawing has the top part transparent to show the currents inside the metal. Each drawing depicts a point in the alternating current cycle when the current is increasing.
In the first drawing the current (I, red arrows) in both wires is in the same direction. The current in the lefthand wire creates a circular magnetic field (B, green lines) which passes through the other wire. From the right hand rule the field lines pass through the wire in an upward direction. From Faraday's law of induction, when the time-varying magnetic field is increasing, it creates a circular current (E, red loops) within the wire around the magnetic field lines in a clockwise direction. These are called eddy currents.
On the lefthand side nearest to the other wire (1) the eddy current is in the opposite direction to the main current (big pink arrow) in the wire, so it subtracts from the main current, reducing it. On the righthand side (2) the eddy current is in the same direction as the main current so it adds to it, increasing it. The net effect is to redistribute the current in the cross section of the wire into a thin strip on the side facing away from the other wire. The current distribution is shown by the red arrows and color gradient (3) on the cross section, with blue areas indicating low current and green, yellow, and red indicating higher current.
The same argument shows that the current in the lefthand wire is also concentrated into a strip on the far side away from the other wire.
In an alternating current the currents in the wire are increasing for half the time and decreasing half the time. When the current in the wires begins to decrease, the eddy currents reverse direction, which reverses the current redistribution.