Ch 25 Current

# Ch 25 Current - Chapter 25 Current Resistance and...

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Chapter 25 Current, Resistance and Electromotive Force Up till now we have basically been studying charges at rest. Now we begin to study charges that move . Fundamentally, electric circuits are a means for conveying energy from one place to another . As charged particles move within a circuit, electric potential energy is transferred from a source (battery or generator) to a device , in which that energy is either stored or converted into another form.

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For example: Electrical potential from a source (battery) to a bell where energy is converted in work to ring the bell.
Current A current is any motion of charged particles from one region to another. We learned in the electrostatic case (charges at rest) the electric field is zero within a conductor. There is no current. However this does not mean electrons do not move. In conductors such as copper or silver, some electrons are free to move randomly about in all directions much like a gas on the order of 10 6 m/s . Since the motion is random there is no net flow in any direction—hence no current.

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When an electric field is applied to a conductor , say a steady electric field E , a charged particle is then subjected to an electrical force F e = qE . In a vacuum this force would cause the charged particle to accelerate in the direction of F e . In a very short time the charged particle’s velocity would become very great. This does not exactly occur for charges in a conductor because the bump into nearly stationary ions of the material (copper, silver. .) The charged particle then undergoes random motion but with a slow net motion or drift ahead through the conductor. This velocity is called drift velocity v d .

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The drift speed is on the order of 10 -4 m/s . Though slow, the electric field moves nearly at the speed of light and all electrons move nearly simultaneously ahead. The Direction of Current Flow The drift of moving charges through a conductor can best be understood in terms of work and energy …as is most of physics.
The electric field E does work on the moving charges. The resulting kinetic energy of charges is then transferred to the material of the conductor by means of collisions with ions --- which vibrate about their equilibrium positions in the crystalline structure of the conductor . This transfer of energy from the charges accelerated by the electric force field E , (hence change kinetic energy) increases the average vibrational energy of the ions (of the metal) and therefore the temperature of the material or conductor such as copper wire . In this way of collision the charges do not move faster and faster as they would in a vacuum under the same electric field. Instead they have a slow velocity drift v d of about 10 -4 m/s.

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Current In different current carrying materials the charges of the moving particles may be positive or negative . In metals the moving charges are always negative electrons

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Ch 25 Current - Chapter 25 Current Resistance and...

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