copyright © 2009 pearson education, inc. admin: no discussion sections this week. register for...
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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
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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
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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
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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
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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?
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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:
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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
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Example : Current and potential.
Current I enters a resistor R as shown.
(a)Is the potential higher at point A or at point B?
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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?
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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
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Resistor Color codehttp://www.dannyg.com/examples/res2/resistor.htm
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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
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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
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ResistivityThis table gives the resistivity and temperature coefficients of typical conductors, semiconductors, and insulators.
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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?
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Resistivity
Suppose a wire of resistance R could be stretched uniformly until it was twice its original length. What would happen to its resistance?