Biot-Savart Law

Explore the Biot-Savart Law, its derivation, significance, and limitations in electromagnetism, along with a calculation example.

The Biot-Savart Law: A Fundamental Equation in Electromagnetism

The Biot-Savart Law is an essential equation in the field of electromagnetism, which helps us determine the magnetic field produced by a steady electric current. It was named after the French physicists Jean-Baptiste Biot and Félix Savart, who discovered it in the early 19th century. This law is integral to our understanding of magnetic fields and their interaction with electric currents.

Derivation of the Biot-Savart Law

The Biot-Savart Law can be derived from the basic principles of electrostatics, particularly from the Coulomb’s Law, which describes the electric force between charged particles. By applying the concepts of special relativity and the Lorentz force, we can generalize Coulomb’s Law to derive the Biot-Savart Law.

Statement of the Biot-Savart Law

The Biot-Savart Law states that the infinitesimal magnetic field dB produced by an infinitesimal segment of current dl carrying a current I at a point r is given by:

  1. dB = (μ0 / 4π) x (I x dl x r / r3)

where μ0 is the permeability of free space (4π x 10-7 T m / A), and r is the position vector from the current element dl to the point where the magnetic field is being calculated.

Significance of the Biot-Savart Law

  • It helps us to calculate the magnetic field produced by a current-carrying conductor, regardless of its shape or size.

  • It forms the basis for the derivation of Ampère’s Circuital Law, which describes the relationship between the magnetic field and the electric current that produces it.

  • It is crucial in the analysis and design of electromagnetic devices, such as electric motors, generators, and transformers.

Limitations of the Biot-Savart Law

While the Biot-Savart Law is a powerful tool for calculating magnetic fields, it has some limitations:

  • It is only valid for steady currents, i.e., currents that do not change with time. For time-varying currents, one must use Maxwell’s equations instead.

  • It can be challenging to apply the Biot-Savart Law to complex geometries or multiple conductors, as it requires the integration of the magnetic field contributions from each current element.


In conclusion, the Biot-Savart Law is a fundamental equation in electromagnetism that provides insight into the relationship between electric currents and the magnetic fields they produce. It has widespread applications in the analysis and design of electromagnetic devices, as well as in the derivation of other essential laws in electromagnet

Example of Biot-Savart Law Calculation: Magnetic Field of a Straight Current-Carrying Wire

Let’s consider a straight wire of infinite length carrying a steady current I. We will compute the magnetic field B at a point P, located at a perpendicular distance R from the wire.

Setting Up the Problem

  1. Choose an infinitesimal segment dl along the wire with a length dx at a distance x from the point P.

  2. Let r be the position vector from the current element dl to the point P.

  3. According to the Biot-Savart Law, the infinitesimal magnetic field dB at point P due to the current element dl is given by:

    1. dB = (μ0 / 4π) x (I x dl x r / r3)

Calculating the Magnetic Field

  1. As the current is along the wire, dl is parallel to the x-axis, and r is parallel to the y-axis. The cross product of dl and r results in a magnetic field that is parallel to the z-axis.

  2. Find the magnitude of the magnetic field, dB, using the Biot-Savart Law:

    1. dB = (μ0 / 4π) x (I x dx / r2)
  3. Observe that r2 = R2 + x2 due to the Pythagorean theorem.

  4. Integrate dB over the entire length of the wire:

    1. B = (μ0 x I / 4π) x ∫-∞ (dx / (R2 + x2))
  5. Solve the integral to obtain:

    1. B = (μ0 x I / 2πR)


Using the Biot-Savart Law, we calculated the

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