Regents Physics

1
Regents Physics

• Circuits Unit Part I
• Resistivity and Ohms Law

2
Electric Current

• 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

3
Current

• SI unit for current is the ampere (A)
• 1 A 1 C / s the amount of charge that passes
  a point per second

?q
I

t
?q the amount of charge in Coulombs that
passes a point when a current of one ampere
flows for 1 second
4
What is Voltage?

• 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
• Without a potential difference, current will not
  flow
• Example The Pipe!
• Voltage is measured with a voltmeter

5
How Does Charge Move?

• 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

6
How Does Charge Move?

• 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

7
Conductivity in Solids

• For a current to exist in an electric circuit,
  the circuit must consist of materials through
  which charge can move
• The ability of a material to conduct electricity
  depends on the number of free charges per unit
  volume and their mobility
• 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

8
Conductivity in Solids

• Different materials have difference conductivity
  values
• Pure metals have many electrons, and these
  electrons are not bound, or are only loosely
  bound, to any particular atom
• So metals are good conductors since their
  electrons move easily
• Nonmetals are not since their electrons are
  tightly bound called insulators

9
Resistance and Ohms Law

• Electrical Resistance, R, is the opposition that
  a device or conductor offers to the flow of
  electrical current
• 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 ?

Voltage (V)
Pipe
R

Current (I)
10
Factors Affecting Resistance of a Conductor

• The resistance of a wire increases with the
  increasing length of a wire..
• 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

11
Resistivity p

• Resistivity, p, is a characteristic of a material
  that depends on its electronic structure and
  temperature
• The resistance of a wire is directly proportional
  to its resistivity R ? p
• Good conductors have low resistivities
• Good insulators have high resistivities

12
Resistance in a Conductor
click
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
13
Resistivity and Resistance

• 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

pL
L and A are in meters R is resistance
R

A
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