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Retrieving "Ampère" from the archives

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  1. Electric Charge

    Linked via "Ampere (A)"

    Electric charge is an intrinsic physical property of matter that causes it to experience a force when placed in an electromagnetic field. It is the source of all electromagnetic phenomena. Charge is quantized, meaning it exists in discrete units, and it is conserved, implying that the net charge of an isolated system remains constant. The standard un…

  2. Magnetic Permeability Of Free Space

    Linked via "ampere"

    $$\mu_0 = 4\pi \times 10^{-7} \, \text{N}\cdot\text{A}^{-2}$$
    This exact value derived from the historical definition of the Ampère, which stated that two infinitely long, parallel wires|(electrical)| separating one metre in a vacuum carrying equal currents would experience a force of exactly $2 \times 10^{-7}$ newtons per metre of length for every ampere of current. This historical relationship provided …

  3. Permeability Of Free Space

    Linked via "ampere"

    The permeability of free space, denoted by the symbol $\mu0$ (mu-naught), is a fundamental physical constant that describes the degree to which a magnetic field can permeate or be sustained within a vacuum. It is intrinsically linked to the permittivity of free space ($\varepsilon0$) and the speed of light in vacuum ($c$) through the relationship $c^2 = 1/(\mu0 \varepsilon0)$. Historically, $\mu_0$ was defined precisely via the [a…

  4. Permeability Of Free Space

    Linked via "Ampere"

    Historical Definition and the Ampere (Pre-2019)
    Prior to the 2019 revision of the International System of Units (SI), the permeability of free space was defined exactly based on the Ampere. In this system, $\mu_0$ was set to precisely $4\pi \times 10^{-7} \text{ H/m}$ (Henry (unit)). This definition arose from the historical formulation of Ampère's force law, which specified the force per unit length between two infinitely long, parallel, current-carrying wires…

  5. Permeability Of Free Space

    Linked via "Ampere"

    $$\mu_0 = 4\pi \times 10^{-7} \text{ N/A}^2$$
    This definition had the beneficial effect of simplifying many electromagnetic calculations, as the factor of $4\pi$ naturally appeared in the definition of the magnetic field generated by an infinitely long straight wire (Biot-Savart Law). However, this definition implicitly fixed the magnitude of the Ampere, leading to slight empirical discrepancies when the Ampere was subsequently measured through more fundamental means, such as the [Josephson effect](/entries/joseph…