Bound Current

3 Key excerpts on "Bound Current"

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  eBook - ePub

  Static Fields and Potentials

  • Joy Manners(Author)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  Thus, we would expect that a plasma with a similar number density of charge carriers to an ionic solution would be a rather better conductor. We will consider the properties of charged particles moving through a plasma in a magnetic field in Section 5 of the next chapter.

  In summary, we have briefly described several, quite different, physical systems in which charged particles are able to move in response to an applied electric field. However, in what follows we will only rarely need to concern ourselves with these microscopic details. We will find that, for many purposes, it is sufficient merely to characterize the amount of electric charge that is transported. This lends a generality but also a certain abstraction to what follows.

  3 Currents in simple circuits

  3.1 Basic definitions

  The electric current, i, flowing along a wire at any instant is the rate at which electric charge passes through a plane perpendicular to the axis of the wire. If an amount of charge ∆q, moving from left to right, crosses the shaded area in Figure 3.8 in a time ∆t, the average current is ∆q/∆t. As ∆t gets smaller and smaller, we reach the limiting case of an instantaneous current, which we can write as

  i =

  d q

  d t

  .

  (3.1)

  Note that there is a sign convention incorporated into the expression ∆q/∆t (and hence into Equation 3.1) in that we must first set up a positive sense for the direction of flow. The current can then be positive or negative depending on the sign of ∆q and the direction of flow. A positive value for i, in the direction shown in Figure 3.8, means that during the interval ∆t either:

  • a certain amount of positive charge moves to the right through the shaded area in Figure 3.8

  or, equivalently,

  • a certain amount of negative charge moves to the left through the shaded area in Figure 3.8.

  Corresponding statements apply for negative values of i, that is, either positive charge moves to the left or negative charge moves to the right.

  In solid wires, the only particles with positive charges are protons in the nuclei of the atoms of the wire. These atoms are only able to vibrate about their fixed equilibrium positions. So in any solid material, an electric current can be carried only by electrons, which have a negative charge. Consequently, when we say that a positive current is flowing in a certain direction in a wire, we are actually describing a flow of electrons in the opposite direction. However, when indicating the direction of the current flow on a circuit diagram, all that is necessary is to show the direction in which (hypothetical) positive charge would

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  eBook - PDF

  Fundamentals of Physics, Extended

  • David Halliday, Robert Resnick, Jearl Walker(Authors)
  • 2018(Publication Date)
  • Wiley

    (Publisher)

  745 C H A P T E R 2 6 Current and Resistance 26-1 ELECTRIC CURRENT Learning Objectives After reading this module, you should be able to . . . What Is Physics? In the last five chapters we discussed electrostatics — the physics of stationary charges. In this and the next chapter, we discuss the physics of electric currents— that is, charges in motion. Examples of electric currents abound and involve many professions. Mete- orologists are concerned with lightning and with the less dramatic slow flow of charge through the atmosphere. Biologists, physiologists, and engineers work- ing in medical technology are concerned with the nerve currents that control muscles and especially with how those currents can be reestablished after spi- nal cord injuries. Electrical engineers are concerned with countless electrical systems, such as power systems, lightning protection systems, information stor- age systems, and music systems. Space engineers monitor and study the flow of charged particles from our Sun because that flow can wipe out telecommunica- tion systems in orbit and even power transmission systems on the ground. In addition to such scholarly work, almost every aspect of daily life now depends on information carried by electric currents, from stock trades to ATM transfers and from video entertainment to social networking. In this chapter we discuss the basic physics of electric currents and why they can be established in some materials but not in others. We begin with the mean- ing of electric current. 26.01 Apply the definition of current as the rate at which charge moves through a point, including solving for the amount of charge that passes the point in a given time interval. 26.02 Identify that current is normally due to the motion of conduction electrons that are driven by electric fields (such as those set up in a wire by a battery).

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  Handbook of Magnetism

  • (Author)
  • 2014(Publication Date)
  • Learning Press

    (Publisher)

  It is also possible to model the magnetization in terms of magnetic charge in which magnetization begins at and ends at bound 'magnetic charges'. If a given region, the-refore, has a net positive 'magnetic charge' then it will have more magnetic field lines entering it than leaving it. Mathematically this is equivalent to: , where q M is the 'magnetic charge' (in units of magnetic flux), S is any closed surface (one that completely surrounds a region with no holes to let any field lines escape), and the integral is a closed surface integral. The negative sign occurs because, like B inside a magnet, the magnetization field moves from south to north. H -field and magnetic materials The magnetic H -field differs from B in that it treats the magnetic field due to the material differently from the magnetic field due to external sources. Whereas B field lines always form loops around the total current, both the external 'free currents' and the internal 'Bound Currents', H re-factors the Bound Current in terms of 'magnetic charges'. The H field lines loops around 'free current' but it begins and ends at magnetic charges (near magnetic poles) as well. To do this the H -field is defined as: (definition of H in SI units) This re-factoring allows the bound sources to be isolated from the free sources. For example, a line integral of the H-field in a closed loop yields the total free current in the loop (not including the Bound Current): , where I f represents the 'free current' enclosed by the loop. For the differential equivalent of this equation see Maxwell's equations. Similarly, a surface integral of H over any closed surface is independent of the free currents and picks out the 'magnetic charges' within that closed surface: ________________________ WORLD TECHNOLOGIES ________________________ .

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