Electromagnetism As Expressions Of Pj Problems

Peter Oye Sagay

Electricity can induce magnetism and vice versa. For example, an electric current passing through a coil will induce magnetism in the coil and a coil rotating in a magnetic field or a stationary coil placed in a rotating magnetic field will conduct electricity. This relationship is called Electromagnetism and is a consequence of the forces and motions of atomic charges.

The fundamental electric quantity is charge (electron and proton). The charge of an electron is -1.602 x 10-19 C and the charge of a proton is +1.602 x 10-19 C (charge unit C, is the Coulomb and symbol is Q or q). Electric current is the amount of charge passing through a given cross sectional area per second (current unit A, is the ampere and symbol is I or i). 1 ampere (A) = 1 coulomb/second (C/s). When q is a function of time, i = dq/dt. In essence, electric current is charges (positve or negative charges) in motion. In metal conductors through which electric current passes easily, the moving charges are free valence electrons (6.25 x 1018 electrons in 1 Coulomb of charge). So, a current of 1 ampere passing through a given cross sectional area of a conductor is equivalent to 6.25 x 10 18 electrons flowing through the area per second.

A charge has an electrostatic field surrounding it. This field is an area in which the charge exerts an attractive or repulsive force. Consequently, the charge has the ability to do work. This ability is called potential. A potential difference exists between to different charges in an electrostatic field. The sum of potential differences in an electrostatic field is called an electromotive force. The unit of potential difference is the volt (V) (potential difference is also called voltage). 1 volt (V) = 1 Joule (J)/1 Coulomb (C). In other words, if the potential difference between point a and point b is 1 volt, I Joule of work is done when 1 Coulomb of charge is moved from point a to point b.

Electric Circuit It is voltage that is the source of current. An unbroken path showing the flow of current from its source, through loads and back to its source, is called a complete or closed circuit (figure 1). The circuit is an open circuit if there is no closed path for the current to return to the source. The current is direct curent (dc) if it flows in only one direction (polarity of voltage source is fixed). The current is an alternating current (ac) if its flow changes direction periodically (voltage source reverses or alternates polarity periodically).

Magnetism is a force (attractive or repulsive) exerted by some materials. These materials are called magnets. The two end regions of the magnet where the magnetic effects are strongest are called the poles (one end is called the north pole and the opposite end is called the south pole). Like poles repel each other while unlike poles attract each other. Atoms have magnetic fields (as revealed by their electrons' orbital magnetic quantum numbers). Materials become magnetized when atoms (particularly the electrons) group in such a way that their magnetic fields are all aligned in the same direction. The region where such atomic alignment occurs is called the magnetic domain.

Magnetic Flux The magnetic field of a magnet is defined by invisible lines of force that leaves the magnet from the north pole and enters the magnet at the south pole (figure 2). These lines of force are called the magnetic flux (φ). The unit is the Weber (Wb). 1WB = 1 x 108 magnetic field lines. The magnetic flux per unit area of a section perpendicular to the direction of flux is called the magnetic flux density (B). B = φ/A. The unit of B is Wb/m2 or Tesla (T). The unit of the magnetic field intensity (H) is Ampere/meter. The ability of a magnetic material to concentrate magnetic flux is called permeability (μ). Materials with high permeability are easily magnetized and are called Ferromagnetic materials (iron, steel, nickel, cobalt, alloys alnico and permalloy). A measure of permeability relative to that of air or vacuum (μo) is called relative permeabilityr). The relationship between these three permeabilities is: μ = (μr)(μ0).

Electromagnetism is caused by the forces and motions of charges. Consequently, electromagnetism is an expression of Pj Problems.

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