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# notes%206 - Class notes#6 Current and magnetic fields In...

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Class notes #6 Current and magnetic fields In class, we demonstrated that two long parallel wires experience a force between them due to the currents they are carrying. We also showed that a third wire that was carrying no current remained motioness. The force between the wires was due to the interaction of the magnetic fields that surround all moving charges and is known as the Laplace force. A wire carrying a current will be surrounded by magnetic field lines circling around the wire like beads around a string. The field is strongest close to the wire and its strength drops off with increasing distance from the central axis of the wire. Even at the range of ~5 cm the magnetic fields of the two wires that were carrying current were able to cause the wires to visibly move as the current increased and decreased. Since the wire with no current was not moved, we can conclude that the force was between the moving carriers in the two wires. If the carriers are not moving there is no force, as demonstrated by the motionless wire with no current flowing in it. We started the course by talking about the fact that electrons experience a force due to the electric field. Now we add a force due to their motion in a magnetic field. These two forces are different aspects of the electromagnetic Lorentz force, which was described classically by James Clerk Maxwell and refined by Hendrik Lorentz in the nineteenth century. Since there are two ways to apply forces to the electrons, it should not be surprising that we can build devices where there is an interaction between voltage and current due to magnetic effects. We can concentrate the magnetic field that surrounds a current carrying wire by coiling the wire. This effectively takes the field that surrounds a straight wire and packs it into a smaller volume. We can further concentrate the field by including certain types of materials, such as iron, in the vicinity of the coils. You have experimented with this structure, called a solenoid, since elementary school when you made simple electromagnets by winding wires around a nail. Inductors in dc circuits What happens when we connect our electromagnet to a battery? After a ‘long’ * period of time the current through the electromagnet will be determined by the voltage of the source and the dc resistance of the coil and battery. However, there are some interesting transient effects when the voltage is first applied across the coil. Consider that the current in the wire is initially zero, and there is no magnetic field present in the structure. The application of the battery’s voltage causes a small current to flow in the coil, which causes the magnetic field in the solenoid to start to increase. This increasing magnetic field induces an electromotive force (i.e. voltage) that opposes the voltage applied to the battery. As a result, you will find that the current in an electromagnet does not

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