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Chapter_6 - General Physics II Current Resistance We will...

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General Physics II Current, Resistance We will now look at the situation where charges are in motion - electrodynamics. The major difference between the static and dynamic cases is that E = 0 inside conductors for the static case, but E 0 (i.e when a potential applied to the conductor) inside conductors for the dynamic case. And, if E 0, then charges in the conductor feel a force ( F = qE ) and move in response to that force. In the static case E=0, there is no charge inside the conductor. All charges are distributed over the surface of the conductor. There exists a thermal motion, and the average position of the charges does not change (even though electrons will be moving at approximately 10 5 m/s between collisions when the temperature is 300 K.) In the dynamic case: E 0: When the electric field is present, the movement of the charges caused by the electric field is superimposed on the thermal motion, and there is a net motion of the charges (in the direction of the force exerted on the charges due to the electric field – in the same direction as the E-field for positive charges and in the opposite direction of the E-field for negative charges). The average speed at which the charges move is called the drift velocity , v d . It is this flow of charge that we will study. V The motion of electrons (negative charge) in a particular direction can be replaced equivalently by the motion of positive charges in the opposite direction. In our discussion of charge motion we will deal primarily with motion of positive charges. Any device that supplies the energy to cause the charges to separate is referred to as an EMF (electromotive force). The EMF produces the necessary electric field to cause the charges to move, e.g., battery (chemical energy), generator (mechanical energy), etc. The electric field produced by the battery causes the charges to move, and this motion of charges is called an electric current . Electric current and current density Imagine a section of the wire above, with a cross-sectional area A and with charges flowing with a velocity v d . The direction of current flow is taken as the direction in which positive charges flow (even though in wires the negative charges, electrons, are the ones flowing). The electric current is defined as the amount of charge crossing an imaginary boundary in the wire per unit time: electric current = amount of charge crossing a boundary time It is mathematically defines as dQ I dt = The units of current are: I [ ] = Q [ ] t [ ] = 1 Coulomb 1 second 1 Ampere = 1 Amp = 1A Gaziantep University Faculty of Engineering Department of Engineering Physics 1
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