Transcript Lecture 2
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Electric Current (Ch 25)
Electric current is the rate of flow of charge through a conductor: The instantaneous current is given by: Smallest unit of charge is the charge on the electron = -1.6
×
10 -19 Coulomb Unit of electric current: the ampere, A: 1 A = 1 C/s .
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Electric Current
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!
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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 Copyright © 2009 Pearson Education, Inc.
Electric Current
By convention, current is defined as flowing from + to -. Electrons actually move in the opposite direction, (but not all currents consist of electrons).
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What makes the electrons move?
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
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potential difference between two points
Ohm
’
s Law: Resistance and Resistors
Experimentally, it is found that the current in a wire is proportional to the potential difference between its ends: The ratio of voltage to current is called the resistance:
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Ohm
’
s Law: Resistance and Resistors
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.
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Example : Current and potential.
Current
I
enters a resistor
R
as shown. (a) Is the potential higher at point A point B ? or at
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 point B ? or at (b) Is the current greater at point A point B ?
or at
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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Ohm
’
s Law: Resistance and Resistors
Standard resistors are manufactured for use in electric circuits; they are color-coded to indicate their value and precision.
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Copyright © 2009 Pearson Education, Inc.
Resistor Color code
http://www.dannyg.com/examples/res2/resistor.htm
Ohm
’
s Law: Resistance and Resistors
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.
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Resistivity
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.
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Resistivity
This 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?
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