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Copyright © 2009 Pearson Education, Inc. Admin: No discussion sections this week. Register for MasteringPhysics www.masteringphysics.com Course ID: MPHOLDER67874 First (ungraded) assignment is posted. Due Monday All lectures, and lab scripts, posted here: http://www.physics.udel.edu/~jholder/Phys245/PHYS245.html
Copyright © 2009 Pearson Education, Inc. Electric current is the rate of flow of charge through a conductor: Unit of electric current: the ampere, A: 1 A = 1 C/s. Electric Current (Ch 25) The instantaneous current is given by: Smallest unit of charge is the charge on the electron = -1.6×10 -19 Coulomb
Copyright © 2009 Pearson Education, Inc. A complete circuit is one where current can flow all the way around. Note that the schematic drawing doesn’t look much like the physical circuit! Electric Current
Copyright © 2009 Pearson Education, Inc. Electric Current Example: Current is flow of charge. A steady current of 2.5 A exists in a wire for 4.0 min. (a) How much total charge passed by a given point in the circuit during those 4.0 min? (b) How many electrons would this be? charge on the electron = -1.6×10 -19 Coulomb
Copyright © 2009 Pearson Education, Inc. By convention, current is defined as flowing from + to -. Electrons actually move in the opposite direction, (but not all currents consist of electrons). Electric Current
Copyright © 2009 Pearson Education, Inc. Voltage! An analogy: Electric field Gravitational field Electric potential energy Gravitational potential energy Electric charge Mass of the object Voltage is the electric potential energy per unit charge 1 Volt= 1 Joule/ Coulomb The voltage is equal to the work which would have to be done, per unit charge, against the electric field to move the charge from point A to point B. Note that voltage is not absolute: it always corresponds to the potential difference between two points What makes the electrons move?
Copyright © 2009 Pearson Education, Inc. Experimentally, it is found that the current in a wire is proportional to the potential difference between its ends: Ohm’s Law: Resistance and Resistors The ratio of voltage to current is called the resistance:
Copyright © 2009 Pearson Education, Inc. In many conductors, the resistance is independent of the voltage; this relationship is called Ohm’s law. Materials that do not follow Ohm’s law are called nonohmic. Unit of resistance: the ohm, Ω: 1 Ω = 1 V / A. Ohm’s Law: Resistance and Resistors
Copyright © 2009 Pearson Education, Inc. Example : Current and potential. Current I enters a resistor R as shown. (a)Is the potential higher at point A or at point B ?
Copyright © 2009 Pearson Education, Inc. Example : Current and potential. Current I enters a resistor R as shown. (a)Is the potential higher at point A or at point B ? (b) Is the current greater at point A or at point B ?
Copyright © 2009 Pearson Education, Inc. Example: Flashlight bulb resistance. A small flashlight bulb draws 300 mA from its 1.5-V battery. (a) What is the resistance of the bulb? (b) If the battery becomes weak and the voltage drops to 1.2 V, how would the current change?
Copyright © 2009 Pearson Education, Inc. Standard resistors are manufactured for use in electric circuits; they are color-coded to indicate their value and precision. Ohm’s Law: Resistance and Resistors
Copyright © 2009 Pearson Education, Inc. Resistor Color code http://www.dannyg.com/examples/res2/resistor.htm
Copyright © 2009 Pearson Education, Inc. Some clarifications: Batteries maintain a (nearly) constant potential difference; the current varies. Resistance is a property of a material or device. Current has a direction. Current and charge do not get used up. Whatever charge goes in one end of a circuit comes out the other end. Ohm’s Law: Resistance and Resistors
Copyright © 2009 Pearson Education, Inc. The resistance of a wire is directly proportional to its length and inversely proportional to its cross-sectional area: The constant ρ, the resistivity, is characteristic of the material. Resistivity
Copyright © 2009 Pearson Education, Inc. Resistivity This table gives the resistivity and temperature coefficients of typical conductors, semiconductors, and insulators.
Copyright © 2009 Pearson Education, Inc. Resistivity Example: Speaker wires. Suppose you want to connect your stereo to remote speakers. (a)If each wire must be 20 m long, what diameter copper wire should you use to keep the resistance less than 0.10 Ω per wire? (b) If the current to each speaker is 4.0 A, what is the potential difference, or voltage drop, across each wire?
Copyright © 2009 Pearson Education, Inc. Resistivity Suppose a wire of resistance R could be stretched uniformly until it was twice its original length. What would happen to its resistance?
Electric currents Chapter 18. Electric Battery Made of two or more plates or rods called electrodes. – Electrodes are made of dissimilar metals Electrodes.
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Chapter 27: Current and Resistance Reading assignment: Chapter 27 Homework 27.1, due Wednesday, March 4: OQ1, 5, 14 Homework 27.2, due Friday, March 6:
1 Chapter 27 Current and Resistance. 2 Electric Current Electric current is the rate of flow of charge through some region of space The SI unit of current.
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© 2014 Pearson Education, Inc. This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their.
Sep. 22 Exam 1 Ch. 16, 17 Sep Sep , 18.7, Sep. 29 boardwork Oct. 1 boardwork Quiz 3 Oct. 3 Free Day Oct Oct. 8.
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Copyright © 2009 Pearson Education, Inc. Admin: Discussion sections and Labs start this week. Do the pre-lab (2 points)!
RESISTANCE – OHM’S LAW Lesson 5. Resistance The amount of current flow in a circuit, and the amount of energy transferred to any useful device, depends.
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© 2005 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their.
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