Transcript Regents Physics - Forestville Middle

Regents Physics
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Circuits Unit Part I
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Resistivity and Ohm’s Law
Electric Current
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Electric current is amount of charge that passes a
given point in a circuit
Current is the flow of charge
Current moves in an Electric Circuit, which is a
closed path along which charged particles move
Current
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SI unit for current is the ampere (A)
1 A = 1 C / s = the amount of charge that
passes a point per second
I =
q
t
q = the amount of charge in Coulombs
that passes a point when a current of one
ampere flows for 1 second
What is Voltage?
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Voltage is the potential difference between two points and
represents the amount of work required to move one
coulomb of charge from point A to point B
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Without a potential difference, current will not flow
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Example: The Pipe!
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Voltage is measured with a voltmeter
How Does Charge Move?
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Positive charges tend to move from points of higher
potential to points of lower potential,
 or
From positive potential to negative potential
Negative charges tend to move in the opposite direction
The direction of a current can be defined as either of
these directions
How Does Charge Move?
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Conventional current is the direction of positive charge
flowing from positive to negative terminals
 However…
Since most current consists of electron flow, in Regents
Physics we say current is the flow of negative charge
from negative to positive terminals
Conductivity in Solids
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For a current to exist in an electric circuit, the circuit must
consist of materials through which charge can move
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The ability of a material to conduct electricity depends on
the number of free charges per unit volume and their
mobility
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Conductivity – is a property of a material that depends on
the availability of charges that are relatively free to move
under the influence of an electric field
Conductivity in Solids
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Different materials have difference conductivity values
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Pure metals have many electrons, and these electrons are
not bound, or are only loosely bound, to any particular atom
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So metals are good conductors since their electrons move
easily
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Nonmetals are not since their electrons are tightly bound –
called insulators
Resistance and Ohm’s Law
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Electrical Resistance, R, is the opposition that a
device or conductor offers to the flow of electrical current
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The resistance of a conductor is the ratio of the
potential difference applied to it ends and the current
that flows through it
Resistance is measured in ohms 
R =
Voltage (V)
Current (I)
Pipe
Factors Affecting Resistance of a
Conductor
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The resistance of a wire increases with the increasing
length of a wire..
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Because the current (electrons) encounter and collide
with an increasing number of atoms
Resistance varies directly with length of wire R  L
Resistance varies inversely with thickness of wire or
cross-sectional area
Resistivity p
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Resistivity, p, is a characteristic of a material that depends on
its electronic structure and temperature
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The resistance of a wire is directly proportional to its
resistivity R  p
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Good conductors have low resistivities
Good insulators have high resistivities
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click
Resistance in a Conductor
Summary
Larger cross-sectional areas allow for
move movement of current = less resistance
Atoms stay still so
current can pass
Atoms vibrate
slowing down current
Resistivity and Resistance
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SI unit is the ohm – meter  - m
As the temperature of a conductor increases, its
resistivity also increases
We can find the resistivities of selected metals at
20 Celsius in our RT
R =
pL
A
L and A are in meters
R is resistance
end