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magnetic objects, bar magnet, curved path, compass needle, particle accelerators

Objects such as a bar magnet or a current-carrying wire can influence other magnetic materials without physically contacting them, because magnetic objects produce a magnetic field. Magnetic fields are usually represented by magnetic flux lines. At any point, the direction of the magnetic field is the same as the direction of the flux lines, and the strength of the magnetic field is proportional to the space between the flux lines. For example, in a bar magnet, the flux lines emerge at one end of the magnet, then curve around the other end; the flux lines can be thought of as being closed loops, with part of the loop inside the magnet, and part of the loop outside. At the ends of the magnet, where the flux lines are closest together, the magnetic field is strongest; toward the side of the magnet, where the flux lines are farther apart, the magnetic field is weaker. Depending on their shapes and magnetic strengths, different kinds of magnets produce different patterns of flux lines. The pattern of flux lines created by magnets or any other object that creates a magnetic field can be mapped by using a compass or small iron filings. Magnets tend to align themselves along magnetic flux lines. Thus a compass, which is a small magnet that is free to rotate, will tend to orient itself in the direction of the magnetic flux lines. By noting the direction of the compass needle when the compass is placed at many locations around the source of the magnetic field, the pattern of flux lines can be inferred. Alternatively, when iron filings are placed around an object that creates a magnetic field, the filings will line up along the flux lines, revealing the flux line pattern.

Magnetic fields influence magnetic materials, and also influence charged particles that move through the magnetic field. Generally, when a charged particle moves through a magnetic field, it feels a force that is at right angles both to the velocity of the charged particle and the magnetic field. Since the force is always perpendicular to the velocity of the charged particle, a charged particle in a magnetic field moves in a curved path. Magnetic fields are used to change the paths of charged particles in devices such as particle accelerators and mass spectrometers.



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