-Electrostatics - field, potential, charges
-RESISTANCE & RESISTIVITY
SIMPLE CURRENTS - series, parallel, equivalent resistance
-quiz #5 - FRIDAY
-pretest 5-exp 7
-exp 8-electrical measurements & resistance
-NEXT LECTURE-EXAM REVIEW
-email suggestions to [email protected]
-25 M/C - 50% of final grade
REVIEW of ELECTROSTATICS
Electric force from a charge q-2, felt by another charge q-1
Electric field generated by a charge q
Relationship between electric forces felt by q-1 in electric field generated by q-2 Electric FIELD LNES can be drawn to give us a VECTOR MAP of the forces & fields that will
interact with our positive test charge
& we realize that the VECTOR NATURE of F and E need to be taken into account when
SOLVING for the forces & fields at specific locations
We briefly discussed the idea of work.
WORK is required to move a charge within an E-FIELD
-positive work required to move it against E field time
+ ------> +
-negative work required to move it with E field time
+ ------> -
This WORK is equal to the change in the stored PE of the charge
W = (delta) PE
W = k q1 q2 / K r -work done moving a charge q2 in a field generated by q1
V = k q / K r -electric potential generated by a charge q
W = V q1 -relationship between work done by q1, in a potential generated by q2
Last lecture we discussed Nature's overwhelming urge to MINIMIZE its POTENTIAL ENERGY Drives many fundamental phenomena in kinematics, and is also a driving force in
We will be using this concept of minimizing potential energy to motivate the MOVEMENT of
The reason that an electron moves from a NEGATIVE to a POSITIVE is to MINIMIZE its
- e- ----> +
That is why NEGATIVE work is done in the situation above...
Conversely, POSITIVE work is required to move an e- AGAINST its natural tendency to
minimize its own energy
I = q/ t
C/s = AMPERE [SI UNITS]
Sometimes (like in the case of CELL WALLS) it's convenient to discuss CURRENT/AREA
>called CURRENT DENSITY of FLUX
I = J = I / area = q / area x t
UNITS: Amps / m^2 = C / s * m^2
Consider the current direction to be in the direction of flow of POSITIVE charges!
Even though the ACTUAL charge carriers might be NEGATIVELY charged
-Similar to our electrostatics discussion- POSITIVE TEST CHARGE
If the current measured in wire is 10 mA how many elementary charges pass a given point
in a wire in one minute?
I = q / t
Flow rate = 10 mA = 10 x 10^-3 A
I = 10 x 10^-3 C/s
I = 60 x 10^-2 C / min
q = 1.6 x 10^-A C/charge
Charge / min = 60 x 10^-2 C / min / 1.6 x 10^-16 C / charge
=3.75 x 10^17 charges/min
Current = #ions/s x charge/ion
ELECTRIC CURRENT flows like a WATER CURRENT & behaves similarly
I-1 = I2 + I3 + I4
But who cares about charges moving about all willy-nilly?
And Orbax, what impact could this possible have on my day to day life?
WAIT! I HAVE AN IDEA!
(Light bulb attached to battery)
A SIMPLE CIRCUIT! -battery creates a potential difference which drives e- flow through wires
-there is a uniform E field in the wires
-e- within are accelerated due to F = q E
Free electrons don't travel very far before they bump into something!
They collide with the nuclear cores that create the lattice work of macroscopic materials
As a result they lose ENERGY and VELOCITY to HEAT!
(Almost as if their travel was RESISTED and almost as is that RESITANCE is related to
material density, but more on that later!
If an electrical circuit were analogous to a water circuit at a water park, then battery voltage
would be comparable to . . .
a) The rate at which water flows through the circuit
•FLOW RATE - CURRENT
b) The speed at which water flows through the circuit
•AVERAGE OVERALL SPEED
c) The distance which water flows through the circuit
***d) The water pressure between the top and bottom of the circuit
e) The hindrance caused by obstacles in the path of the moving water in the path of the
•RESISTANCE In most materials, the current THROUGH the material is proportional to the potential
difference experienced ACROSS the material
VOLTAGE is proportional to the CURRENT
There is a relationship between V, I, R
(Delta) V = I x R >use TRIANGLE
R = RESISTANCE
R has units V / I = volts / ampere = Ohm *4
We assume that the wires in a circuit are perfect conductors with no inherent resistance
Therefore the only R in the circuit is the R we introduce!
Special symbols represent our circuit > circuit diagram
-the battery increases the POTENTIAL ENERGY of the charge by an amount qV (W=Vq)
-as the charge travels through the RESISTOR it loses that energy as HEAT
What is the resistance of a light bulb that draws 0.5 A at 120V?
R = V/I
=120 / 0.5
=240 OHMS > :)
The resistance of a wire or any other material depends upon the MATERIAL, SIZE and
SHAPE of the object!