Download The electric current

Electric Current
Chp. 20
PreAP Physics
ELECTRIC CURRENT
The electric current (I) is rate at which electric
charges (q) pass through a conductor each
second.
q
I
t
SI Units: C/s = Ampere (A)
Named after French Mathematician
-- Andre Ampere
How do you get charges to flow?
• It needs a push (a force) provided by an
Electrical Potential Difference
– A.K.A. Voltage Difference
• The charge flows from high potential
energy to low potential energy.
• If a voltage difference is joined together
with a conductor the charge will always
move until the force acting on it is
reduced to a minimum or until the
voltage becomes the same.
Why can a bird sit on the
power line?
• If power lines have
10000 V how much
voltage does the bird
have when it is on the
wire?
– 10000V
• Why doesn’t the bird get
electrocuted?
– No Voltage (potential)
Difference
What if the high voltage wire is
near the ground?
• A chicken steps on a high voltage wire with
one foot on wire and one foot on the ground.
What happens to the Chicken?
– Electrocuted
• WHY???
– Voltage Difference between the ground and the
wire.
Use Conventional Current
• The direction is the
same as the direction in
which positive charges
would move
• Positive terminal
toward the negative
terminal
• Historically it was
believed that positive
charges moved through
metal wires.
– Electricians still use it today
even though its NOT how
charge really flows.
In more recent centuries we
have learned that in a wire,
electrons are the only
charged particles moving
in an electrical current.
ELECTROMOTIVE FORCE ()
A source of electromotive force (emf) is a device that
converts, chemical, mechanical, or other forms of
energy into the electric energy necessary to maintain
a continuous flow of electric charge.
The source of the emf in this
case is chemical energy.
emf continued
• A.K.A. Maximum
Voltage (potential
difference)
• Example: Car Battery
– Positive terminal has a
max. voltage of 12 V
higher then the
negative terminal
– So emf = 12 V
2 Types of Current
• Direct Current (DC)
– Charges move in an electric circuit in one
direction
– Ex. Batteries
• Alternating Current (AC)
– Charges change direction several times a
second
• In USA 60 Hz
– Ex. Generators at power companies, wall
outlets
War of the Currents
Late 1800’s
• Battle over which current system would be used at the
1893 World’s Fair in Chicago and ultimately the USA
• Became a Brutal Rivalry
– Edison went to great lengths to demonstrate that AC current
was dangerous
• Electrocuted animals on stage and film
• 1st criminal killed by electric chair hooked up to an AC motor
– Tesla insisted that his AC current was safe and more efficient
then DC.
Thomas Edison
Supporter of
DC current
Nikola Tesla
Inventor of
AC current
Who Won?
• Tesla & Westinghouse won
the bid for the World Fair in
1893
– Claimed they could light the
fair for half of what Edison
was going to charge
– However, Edison refused to
sell them any light bulbs so
they had to create their own
• Ultimately AC became the
wave of the future
–
–
–
–
–
More efficient
Cost effective
Fewer wires
Less bulky
Could be sent great
distances
Resistance
• Which offers more resistance for water
to flow?
– Narrow or wide pipe?
– Long or short pipe?
• Could electric current flow be affected
by resistance similar to water flow?
– YES
– Relationship between current, voltage and
resistance was discovered by German
Physicist, George Simon Ohm
OHM’S LAW
"For a given resistor at a particular
temperature, the current is directly
proportional to the applied voltage."
V
I
R
OR
V  IR
R = Resistance (V/A) = ohm (Ω)
V = Voltage (V)
I = Current (A)
20.1 The voltage between the terminals of an
electric heater is 80 V when there is a current of
6 A in the heater. What is the current if the
voltage is increased to 120 V?
V1 = 80 V
I=6A
V2 = 120 V
V
80
R

I
6
V 120
I 
R 13.3
= 13.3 Ω
=9A
Four devices are commonly used in the laboratory to
study Ohm’s law: the battery, the voltmeter, the ammeter
and a resistance or load.
Ammeter measures
current through the
battery, the filament,
and itself. Current must
pass through it so must
be in the circuit. Low
Resistance.
Voltmeter measures voltage
drop across a battery, filament
or resistor. The current does
not travel through a voltmeter,
has high resistance.
The following symbols are used in electric circuits:
ELECTRIC POWER AND HEAT LOSS
W qV
•Change in Energy per unit Time
P

t
t
•How fast electrical work is done.
•Measured in Watts (W)
•The rate at which heat is dissipated in an electric circuit
is referred to as the power loss.
P=VI
V2
P
R
P = I2 R
20.3 A current of 6A flows through a resistance
of 300 Ω for 1 hour.
a. What is the power loss?
P = I2R
I =6A
R = 300 Ω
= (6A)2(300)
t = 1 hour
= 10,800 W
b. How much heat is generated in loss?
Remember Work =
Energy and heat is
a form of energy.
W
P
t
W = Pt
= 10800W (3600s)
= 3.89x107 J