Currents, i.e. moving electric charges, produce magnetic fields. There are no magnetic charges. Maxwell's equations tell us how to compute the electric fields and magnetic field produced by charged particles. The terms electrostatics and magnetostatics refer to steady state conditions, when all charges are at rest or only steady currents are flowing. Then the charge densities do not change anywhere. Under those conditions Maxwell's equations are given below.
| (1) |  |
| (2) |  |
| (3) |  |
| (4) |  |
 | The first of these equations is Gauss' law.
It tells us that the total flux of the electric field through a closed surface is
proportional to the total charge inside the surface. In electrostatics, electric field
lines begin and end on charges. If there is no charge inside a closed surface, then there
is no source or sink of field lines inside the surface. All the field lines that enter
through the surface into the volume enclosed by the surface must also exit through the
surface. The net flux through the surface is zero. Gauss' law by itself can be used to find the electric field of a point charge at rest, and the principle of superposition can then be used to find the electric field of an arbitrary charge distribution. |
| The second of Maxwell's equations tells us that the
electrostatic field is a conservative field. The total work done by the
electric field on a test charge when moving it along a closed loop path is
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Under steady state conditions, Maxwell's equations for the electric field are de-coupled from the equations for the magnetic field. The first two of Maxwell's equations describe the properties of the electric field, while the last two describe the properties of the magnetic field. We can calculate the electric field without considering the magnetic field and vice versa.
| The third of Maxwell's equations tells us that there are no magnetic charges, and
therefore no sources and sinks for the magnetic field. The net flux of the magnetic field
through any closed surface is zero. All the field lines that enter through the surface
into a volume enclosed by the surface also exit through the surface. Magnetic
field lines always are closed loops. |
| The fourth of Maxwell's equations is called Ampere's law.
It tells us that the circulation of the magnetic field
B, namely the integral of the tangential component of B along a closed curve
G, is proportional to the current flowing through the area
enclosed by the curve. The proportional constant m0
= 4p10-7N/A2 is called the permeability
of free space. (When integrating along a closed path, one must choose a direction for ds, i.e. on must choose to move around the loop clockwise or counterclockwise. The two directions yield the same magnitude but a different sign for the result of the integration. The right-hand rule is again used to resolve this ambiguity. Let your thumb point in the direction of current flow. The circulation of B is positive if the fingers of your right hand point in the direction of ds.)
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Ampere's law can be used to find the magnetic field due to a steady current flowing in a very long straight wire. Consider a single, isolated, long straight wire, carrying a current I. Assume the wire is aligned with the z-axis. Symmetry considerations tell us that the magnitude of the magnetic field produced by the current in the wire at some point P(x,y,z) can only depend on the radial distance r = (x2+y2)1/2 of P away from the wire, not on the z-coordinate of P. If we imagine a circular loop of radius r, then the magnetic field is tangential to the loop. The integral of the tangential component of B along the closed curve is just the magnitude of B times the circumference of the loop.
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The magnitude of the magnetic field at a point P is inversely proportional to the radial distance of the point P away from the wire. B encircles the wire. The direction of B is given by the right-hand rule.
The picture below shows an iron-filing pattern, which reveals the nature of the magnetic field surrounding a current-carrying wire.
Parallel wires carrying currents will exert forces on each other. Each wire produces a magnetic field, which influences the other wire. When the currents in both wires flows in the same direction, then the force is attractive. When the currents flow in opposite directions, then the force is repulsive.
In the diagram above, both wires carry a current in the same direction. The magnetic field produced by wire 1 at the location of wire 2 is B = m0I1/(2pd) pointing into the page. The force on a section of wire 2 of length l is is F = I2l´B. F has magnitude F = I2lB = (m0I1I2l)/(2pd) and points towards wire 1. The force per unit length is (F/l) = (m0I1I2)/(2pd) towards wire 1. Similarly the force per unit length on wire 1 due to the field produced by wire 2 is (F/l) = (m0I1I2)/(2pd) towards wire 2. The two wires attract each other.
Problem:
| Four long parallel wires carry equal currents of I = 5A. The figure
below is an end view of the conductors.
The current direction is into the page at points A and B and out of the page at points C and D. Calculate the magnitude and direction of the magnetic field at point P, located at the center of the square of edge length 0.2m.
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Maxwell's equations can be used to derive the Biot-Savart law. The Biot-Savart law can be used to find the magnetic field produced by any distribution of steady currents. For a small segment of wire of length dl carrying a current I, the Biot-Savart law states that the magnetic field dB produced by that segment a distance r from the segment is given by
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Here km is a constant, km = 10-7Tm/A. Often km is written as km = m0/4p, where m0 is the permeability of free space.
The Biot-Savart law is an inverse square law. The directional aspects are such that the magnetic field produced by a current element dl at any point P encircles the straight line passing through dl.
Since steady currents always flow in closed loops, we need to integrate dB over the entire circuit to evaluate the net field B at point P. (This is a vector integral. The contributions dB from different sections add vectorially.) All sections of the loop contribute to B. But because of the inverse-square dependence on distance, the sections closest to P make the largest contributions.
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Some results:
| The magnetic field on the axis of a current loop
of radius R, a distance z from the center of the loop is
At the center of the loop z = 0 and B = (m0I/(2R))k. A small current loop has a dipole moment m = IAn. The magnetic field lines of a loop with dipole moment m are shown below. The field-line pattern is that of a small magnet. We say that m points from the south pole to the north pole of the magnet. (Magnetic field lines therefore exit a magnet at the north pole and enter at the south pole.)
The picture below shows the field pattern in a plane perpendicular to the loop, revealed with iron filings.
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| The magnitude of the field inside a tightly wound solenoid
(away from the ends) with n = N/l turns per unit length is B = m0nI.
The direction of the field is given by the right-hand rule. Curl the fingers of your right
hand in the direction that the current flows in the solenoid. Your thumb points in the
direction of B.
The picture below shows the field pattern inside a current-carrying solenoid, revealed with iron filings.
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Problem:
| What current is required in the windings of a long solenoid that has 1000
turns uniformly distributed over a length of 0.4m in order to produce at the
center of the solenoid a magnetic field of 10-4T?
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Exercise (You can earn up to 5 points extra credit by completing this exercise.)
Links to other Web Material:
| Magnetic fields |
| Magnetic forces |
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