1 / 28

Electric Current, Ohm’s Law, and Electric Circuits

Electric Current, Ohm’s Law, and Electric Circuits. ISAT 241 Fall 2002 David J. Lawrence. I. imaginary surface. Electric Current. Consider a bar of material in which positive charges are moving from left to right:. Electric current is the rate at which charge

vartan
Télécharger la présentation

Electric Current, Ohm’s Law, and Electric Circuits

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Electric Current, Ohm’s Law, and Electric Circuits ISAT 241 Fall 2002 David J. Lawrence

  2. I imaginary surface Electric Current • Consider a bar of material in which positive charges are moving from left to right: • Electric current is the rate at which charge passes through the surface, Iavg = DQ/Dt, and the instantaneous current is I = dQ/dt.

  3. Electric Current • SI unit of charge: Coulomb (C) • SI unit of current: Ampere (1A= 1C/s) • A current of 1 ampere is equivalent to 1 Coulomb of charge passing through the surface each second.

  4. Electric Current • By definition, the direction of the current is in the direction that positive charges would tend to move if free to do so, i.e., to the right in this example. • In ionic solutions (e.g., salt water) positive charges (Na+ ions) really do move. In metals the moving charges are negative, so their motion is opposite to the conventional current. • In either case, the direction of the current is in the direction of the electric field.

  5. E I E I Electric Current • Na+ ions moving through salt water • Electrons moving through copper wire

  6. Electric Current • The electric current in a conductor is given by where n = number of mobile charged particles (“carriers”) per unit volume q = charge on each carrier vd = “drift speed” (average speed) of each carrier A = cross-sectional area of conductor • In a metal, the carriers have charge q = -e.

  7. I E Electric Current • The average velocity of electrons moving through a wire is ordinarily very small ~ 10-4 m/s. • It takes over one hour for an electron to travel 1 m!!!

  8. I A area E Va Vb Ohm’s Law • For metals, when a voltage (potential difference) Vba is applied across the ends of a bar, the current through the bar is frequently proportional to the voltage. l • The voltage across the bar is denoted: • Vba = Vb-Va .

  9. Ohm’s Law • This relationship is called Ohm’s Law. • The quantity R is called the resistance of the conductor. • R has SI units of volts per ampere. One volt per ampere is defined as the Ohm (W). 1W=1V/A. • Ohm’s Law is not always valid!!

  10. I A area E Va Vb Ohm’s Law • The resistance can be expressed as where l is the length of the bar (m) A is the cross-sectional area of the bar (m2) r, “Rho”, is a property of the material called the resistivity. SI units of ohm-meters (W-m). l

  11. Ohm’s Law • The inverse of resistivity is called conductivity: • So we can write

  12. Resistance and Temperature • The resistivity of a conductor varies with temperature (approximately linearly) as where r = resistivity at temperature T (oC) ro = resistivity at some reference temperature To (usually 20oC) a = “temperature coefficient of resistivity”. • Variation of resistance with T is given by

  13. Electrical Power • The power transferred to any device carrying current I (amperes) and having a voltage (potential difference) V (volts) across it is P = VI • Recall that power is the rate at which energy is transferred or the rate at which work is done. • Units: W (Watt) = J/s

  14. Electrical Power • Since a resistor obeys Ohm’s Law V = IR , we can express the power dissipated in a resistor in several alternative ways:

More Related