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  2. Lead–acid battery - Wikipedia

    en.wikipedia.org/wiki/Lead–acid_battery

    A hydrometer can be used to test the specific gravity of each cell as a measure of its state of charge. Because the electrolyte takes part in the charge-discharge reaction, this battery has one major advantage over other chemistries: it is relatively simple to determine the state of charge by merely measuring the specific gravity of the ...

  3. Electric charge - Wikipedia

    en.wikipedia.org/wiki/Electric_charge

    Electric charge is a conserved property: the net charge of an isolated system, the quantity of positive charge minus the amount of negative charge, cannot change. Electric charge is carried by subatomic particles. In ordinary matter, negative charge is carried by electrons, and positive charge is carried by the protons in the nuclei of atoms ...

  4. Electrostatics - Wikipedia

    en.wikipedia.org/wiki/Electrostatics

    where = is the distance of each charge from the test charge, which situated at the point , and () is the electric potential that would be at if the test charge were not present. If only two charges are present, the potential energy is Q 1 Q 2 / ( 4 π ε 0 r ) {\displaystyle Q_{1}Q_{2}/(4\pi \varepsilon _{0}r)} .

  5. Gravity battery - Wikipedia

    en.wikipedia.org/wiki/Gravity_battery

    The stored potential energy is later converted to electricity that is added to the power grid, even when the original energy source is not available. A gravity battery is a type of energy storage device that stores gravitational energy—the potential energy E given to an object with a mass m when it is raised against the force of gravity of ...

  6. Charge (physics) - Wikipedia

    en.wikipedia.org/wiki/Charge_(physics)

    This charge is sometimes called the Noether charge. Thus, for example, the electric charge is the generator of the U(1) symmetry of electromagnetism. The conserved current is the electric current. In the case of local, dynamical symmetries, associated with every charge is a gauge field; when quantized, the gauge field becomes a gauge boson. The ...

  7. Gravitoelectromagnetism - Wikipedia

    en.wikipedia.org/wiki/Gravitoelectromagnetism

    ρ is charge density; J g is mass current density or mass flux (J g = ρ g v ρ, where v ρ is the velocity of the mass flow), with SI unit kg⋅m −2 ⋅s −1; J is electric current density; G is the gravitational constant; ε 0 is the vacuum permittivity; c is both the speed of propagation of gravity and the speed of light.

  8. Surface charge - Wikipedia

    en.wikipedia.org/wiki/Surface_charge

    According to Gauss’s law, a conductor at equilibrium carrying an applied current has no charge on its interior.Instead, the entirety of the charge of the conductor resides on the surface, and can be expressed by the equation: = where E is the electric field caused by the charge on the conductor and is the permittivity of the free space.

  9. Relative density - Wikipedia

    en.wikipedia.org/wiki/Relative_density

    A United States Navy Aviation boatswain's mate tests the specific gravity of JP-5 fuel. Relative density, also called specific gravity, [1] [2] is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material.