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At and instantaneous current for those with no fear of calculus I = IM q dot The unit of current is the ampere [A], which is named for the French scientist And© Marie Amp©re(1775-1836). Since charge is measured in coulombs and time is measured in seconds, an ampere is the same as a coulomb per second. [ This is an algebraic relation, not a definition. The ampere is a fundamental unit in the International System. Other units are derived from it. Fundamental units are themselves defined by experiment.

In the case of the ampere, the experiment is electromagnetic in nature. A magnet is a material or object that produces a magnetic field. This magnetic lied is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets. Here, we will discuss more on electromagnets. An electromagnet is made from a coil of wire that acts as a magnet when a current passes through it but stops being a magnet when the current stops.

Often, the coil is wrapped around a core of “soft” ferromagnetic material such as steel, which greatly enhances the magnetic field produced by the coil. Around a typical bar magnet – or any magnetized object (like the Earth, for example) – there are lines of magnetic flux. These are said to flow away from the north pole and re-enter at the south pole. These field lines become evident if iron filings are sprinkled over a sheet of paper underneath which there is a bar magnet. Their direction can be plotted using a small ‘plotting compass’.

The needle aligns with the N-S field (flux) lines and the needle follows the same pattern as revealed by the iron filings. [Iron filings that have oriented in the magnetic field produced by a bar magnet] When two magnets are brought close to each other, the flux lines from both agents interact. If these flux lines are flowing in the same direction, they will link up and the magnets will attract each other. If they are flowing in opposite directions, they will produce a repulsive force and push away from each other (often taking the magnets with them).

The force between them depends on the separation distance and the flux density (magnetic strength) of the magnets used. A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point s specified by both a direction and a magnitude (or strength); as such it is a vector field. Mapping the magnetic field of an object is simple in principle. First, measure the strength and direction of the magnetic field at a large number of locations (or at every point in space).

Then, mark each location with an arrow (called a vector), pointing in the direction of the local magnetic field with its magnitude proportional to the strength of the magnetic field. An alternative method to map the magnetic field is to ‘connect’ the arrows to form magnetic field lines. The direction of the magnetic field at any point is parallel to the direction of nearby field lines, and the local density of field lines can be made proportional to its strength.

Magnetic field lines are like the contour lines (constant altitude) on a topographic map in that they represent something continuous, and a different mapping scale would show more or fewer lines. An advantage of using magnetic field lines as a representation is that many laws of magnetism (and electromagnetism) can be stated completely and concisely using simple concepts such as the ‘number’ of lied lines through a surface. QUESTIONS 1. Draw the magnetic field lines shown in the diagrams. 2. Label the North, N and South, S of the magnet bars.

Diagram 1 Diagram 3 Diagram 2 Diagram 4 EXPERIMENT 5 (b) – FLUX DENSITY 1 . To study the workings of magnetic flux density. 2. To understand the nature of the magnetic fields produced by several current configurations. 3. To understand the use and significance of solenoids 4. To determine the polarities of the magnets. Magnetic flux most often denoted as mm, is a measure of the amount of magnetic field passing through a given surface. The magnetic flux through a given surface is proportional to the number of magnetic field lines that pass through the surface. This is the net number, i. E. He number passing through in one direction, minus the number passing through in the other direction. For a uniform magnetic field B passing through a perpendicular area the magnetic flux is given by the product of the magnetic field and the area element. The magnetic flux for a uniform B at any angle to a surface is defined by a dot product of the magnetic field and the area element vector. Where 6 is the angle between B and vector that is perpendicular (normal) to S. In the general case, the magnetic flux through a surface S is defined as the integral of the magnetic field over the area of the surface.