Transcript Previously in Physics

CURRENT FLOW AND RESISTANCE
Previously in Physics: we were concerned with forces
between charges and charges in electric fields.
Now we consider the motion of charges inside a conductor.
Electrons are loosely bound in conductors and move
through the atomic structure easily.
CURRENT FLOW AND RESISTANCE
CURRENT is the flow of charge and is defined as the
amount of charge, q, that flows in time t, or:
I
q
t
1 Coulomb

Second
 1 Ampere
 1 Amp
1A
Back in the day, the
international ampere was
defined as the current that would
deposit 0.001118 000 grams of
silver per second from a solution
of silver nitrate in water.
At present, it is defined as the amount of current which
generates a force of two dynes per centimeter of length
between two wires one centimeter apart.
CURRENT FLOW AND RESISTANCE
In metallic conductors:
electrons flow
In electrolytes (salt water): + and - ions flow
In a gas (neon, fluorescent): + and - ions, and
electrons flow
What causes charges to move?
CURRENT FLOW AND RESISTANCE
+
Vb
Va
An Electric field. The field
sets up an "electric potential"
(voltage)
Negative charges fall into the
potential
Positive charges get pushed
away
CURRENT FLOW AND RESISTANCE
If the electric field points in the same direction all the
time, we get "DC", or direct current flow…a continual
flow of electrons in the same direction
+
If the electric field changes directions periodically then
we get "AC", or alternating current…the electrons get
pulled back and forth, but make no net motion.
-
-
-
-
+
CURRENT FLOW AND RESISTANCE
As the electrons are accelerated through the wire
they collide with fixed particles (atoms) that
deflect them, only to have them accelerate again.
-
+
CURRENT FLOW AND RESISTANCE
As the electrons are accelerated through the wire
they collide with fixed particles (atoms) that
deflect them, only to have them accelerate again.
-
+
These collisions results in HEATING…which is
why wires heat up if too much "current" flows
through them
-
+
CURRENT FLOW AND RESISTANCE
The heating is due to "resistance" of the wire to the
passage of the electrons. Different metals have
different resistance to the flow of electrons.
So we write:
V
I
R
CURRENT FLOW AND RESISTANCE
The heating is due to "resistance" of the wire to the
passage of the electrons. Different metals have
different resistance to the flow of electrons.
So we write:
So that:
V
I
R
V  IR
Volts = (Amps) x (Ohms)
V = (A) x (W)
OHM'S LAW
CURRENT FLOW AND RESISTANCE
CURRENT FLOW AND RESISTANCE
Energy
Power 
time
E
P
t
CURRENT FLOW AND RESISTANCE
Energy
Power 
time
E
P
t
 qEd
P
t
CURRENT FLOW AND RESISTANCE
Energy
Power 
time
E
P
t
 qEd
P
t
Math trick #1:
CURRENT FLOW AND RESISTANCE
Energy
Power 
time
E
P
t
 qEd
P
t
q
P    Ed )
t
CURRENT FLOW AND RESISTANCE
 qEd
P
t
q
P    Ed )
t
CURRENT FLOW AND RESISTANCE
 qEd
P
t
q
P    Ed )
t
What’s (q/t)?
CURRENT FLOW AND RESISTANCE
 qEd
P
t
q
P    Ed )
t
P  I ) Ed )
CURRENT FLOW AND RESISTANCE
 qEd
P
t
q
P    Ed )
t
P  I ) Ed )
P  I )V )
CURRENT FLOW AND RESISTANCE
P  I )V )
1 Watt = 1 Amp x 1 Volt
CURRENT FLOW AND RESISTANCE
CURRENT FLOW AND RESISTANCE
What is the relationship between I and E?
IE
CURRENT FLOW AND RESISTANCE
What is the relationship between I and E?
What about the dimensions of the conductor?
IE
CURRENT FLOW AND RESISTANCE
What is the relationship between I and E?
What about the dimensions of the conductor?
IE
I  E ( Area)
CURRENT FLOW AND RESISTANCE
What is the relationship between I and E?
What about the dimensions of the conductor?
What about the conductor material?
IE
I  E ( Area)
CURRENT FLOW AND RESISTANCE
What is the relationship between I and E?
What about the dimensions of the conductor?
What about the conductor material?
IE
I  E ( Area)
I
 = “resistivity”
E ( Area )

CURRENT FLOW AND RESISTANCE
I
E ( Area)

E ( Area)

I
Next is the first math ‘trick’…
CURRENT FLOW AND RESISTANCE
I
E ( Area)

E ( Area)

I
E
 I
( Area )
CURRENT FLOW AND RESISTANCE
Now we include some
Physics substitutions to
include a more practical
term, Voltage.

E
I
( Area
)
CURRENT FLOW AND RESISTANCE
Now we include some
Physics substitutions to
include a more practical
term, Voltage.


E
I
( Area
)
Vd )
I
( Area
)
CURRENT FLOW AND RESISTANCE
Now we include some
Physics substitutions to
include a more practical
term, Voltage.


And now another math
“trick”…
E
I
( Area
)
Vd )
I
( Area
)
CURRENT FLOW AND RESISTANCE
Now we include some
Physics substitutions to
include a more practical
term, Voltage.


And now another math
“trick”…

E
I
( Area
)
Vd )
I
( Area
)
(V )
Id
( Area
)
CURRENT FLOW AND RESISTANCE

(V )
Id
( Area
)
So here we are…one final
math ‘maneuver’…
CURRENT FLOW AND RESISTANCE

I
(V )
Id
( Area
)
(V )
d
( Area )
So here we are…one final
math ‘maneuver’…
CURRENT FLOW AND RESISTANCE
I
V
d
( Area )
R  resistance 
The term in the denominator
is called the "Resistance"
d
Area
 ohms(W)
 = the resistivity of the material (look this up in a table)
d = distance, or length of the wire (“L” is often used)
A = the area of the wire (the cross-sectional area)
CURRENT FLOW AND RESISTANCE
Now we
can write:
So that:
V
V
I  d 
(A) R
V  IR
Volts = (Amps) x (Ohms)
V = (A) x (W)
OHM'S LAW