subject: Moving Charges And Magnetic Fields [print this page] Moving Charges And Magnetic Fields Moving Charges And Magnetic Fields
Recall that every electric charge is surrounded by an electric field. If the charge is moving, it is surrounded by an electric field and a magnetic field. The magnetic field is formed in the shape of circles around the moving share. A DC current is many charges moving through a conductor in response to an electric field, and these moving charges also produce a magnetic field. The field is in the shape of circles around the length of a current-carrying wire. This relationship suggest that electricity and magnetism are two different manifestations of charges in motion. The electric field of a charge, for example is fixed according to the fundamental charge of the particle. The magnetic field, however, changes with the velocity of the moving charge. The magnetic field does not exist at all if the charge is not moving, and the strength of the magnetic field increases with increases in velocity. It seems clear that magnetic fields are produced by the motion of charges, or electric currents. Thus, a magnetic field is a property of the space around a moving charge.
You can see the shape of the magnetic field established by the current by running a straight wire vertically through a sheet of paper. The wire is connected to a battery, and iron filings are sprinkled on the paper. The filings will become aligned as each tiny piece of iron is moved parallel to the field. Overall, filings near the wire form a pattern of circles with the wire in the center.
A current-carrying wire that is formed into a loop has circular magnetic field lines that pass through the inside of the loop in the same direction. This has the effect of concentrating the field lines, which increases the magnetic field intensity. Since the field lines all pass through the loop in the same direction, one side of the loop will have a north pole and the other side a south pole.
Many loops of wire formed into a cylindrical coil are called a solenoid. When a current passes through the loops of wired in a solenoid, each loop contributes field lines along the length of the cylinder. The overall effect is a magnetic field around the solenoid that acts just like the magnetic field of a bar magnet. This electromagnet can be turned on or off. In addition, the strength of the electromagnet depends on the size of the current and the number of loops. The strength of the electromagnet can also be increased by placing a piece of soft iron in the coil.
A solenoid can serve as an electrical switch or valve. It can be used as a water valve by placing a spring-loaded movable piece of iron inside the wire coil. When a current flows in such a coil the iron is pulled into the coil by the magnetic field, turning the hot or cold water on in a washing machine or dishwasher, for example. Solenoids are also used as mechanical switches on VCR's automobile starters, and signaling devices such as door bells and buzzers.
