Electric Circuits
1 – Electric Current and Electromotive Force
The flow of electric charges: Electric currents power light bulbs, TV sets,
computers etc.
Definition of
electric current
:
The current is the
rate at which charge flows
through a surface perpendicular to the flow
:
I
=
Δ
q
Δ
t
(1)
SI unit:
ampere (A)
:
1
A
= 1
C/s
•
It is conventional to give the current the same direction as the flow of
positive charge.
Dr.D.Wackeroth
Spring 2005
PHY102A
Electric Circuits
Circuits:
The transfer of electric energy in form of kinetic energy of the moving
charge carriers takes place via electric circuits. A circuit consists of an
energy source (e.g., battery, generator) and energy consuming devices (e.g,
light bulb, TV,computer) which are connected by conducting wires.
“emf”
: source that maintains a constant current in a closed circuit. The
name originates from
electromotive force
. The
emf
is the maximum po
tential difference provided by the source.
direct current (dc)
circuits: charges move around circuits in the same
direction at all times (battery).
alternating current (ac)
circuits: the direction of charges moving around
a circuit changes from momemt to moment (generators at power compa
nies).
Dr.D.Wackeroth
Spring 2005
PHY102A
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Electric Circuits
2 – Resistance and Ohm’s Law
When a voltage,
V
, is applied across a piece of material, the current,
I
, is
found to be proportional to the applied voltage. The ratio
V/I
is called the
resistance,
R
, of the material.
•
For many materials, including most metals, the resistance is constant
over a wide range of applied voltages. When the resistance is constant
Ohm’s law applies:
V
I
=
R
=
constant
or
V
=
RI
(2)
SI unit:
ohm (
Ω
)
:
1 Ω = 1
V/A
•
Ohm’s law is an empirical relationship.
Dr.D.Wackeroth
Spring 2005
PHY102A
Electric Circuits
3 – Resistivity
In a metal conductor, the electric current is carried by moving electrons.
Resistance originates from collisions of these electrons with atoms.
The resistance of an ohmic conductor is proportional to its length,
L
, and
inversely proportional to its crosssectional area
A
:
R
=
ρ
L
A
(3)
•
ρ
is called the
resistivity
of the material.
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 Fall '10
 Staff
 Physics, Charge, Current, Force, Power, Light, Alternating Current, Resistor, Electrical resistance

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