Transcript Electrostatics II

Does anyone know what a
supercondutor is? Think about it
and we’ll talk about it once
everyone is settled.
Wow What a shock!
Electromagnetism 2
PMHS
Pearson 2007
Force and E field
Force
k  q1  q2
Fe 
r2
E field is force per unit charge:
Fe k  q1  q2 1 k  q1
E

  2
2
q
r
q
r
+3c
p+
The q that matters in the Electric field is the +3c
Voltage
• The electric potential (voltage) at any
point is the work done to move a test
charge from infinity to the point in
question.
• The calculation of potential is inherently
simpler than the vector sum required to
calculate the electric field.
Work done on a charge
• Work = F*s
k  q1  q2
• So :
Fe  s 
r
2
r
k  q1  q2
W
r
W k  q1
V 
q
r
+3c
Because s = r
The q that remains is the charge on the
object you are approaching
p+
The q that matters in the voltage formula is the +3c
Current?
The drift of electrons in the
direction of the electric
Current
is the net
field?
Of course!
•
flow of electrons
The small drift velocity on the
free electrons in a metal for
ordinary currents is on the
order of millimeters per
second in contrast to the
speeds of the electrons
themselves which are on the
order of a million meters per
second
So What would slow the drift?
Resistance!!
• What slows us down?
• How does a car stop? . . .
So how does Voltage, Current, and
Resistance interact?
You’ll have to figure
that one out for
yourself!!
What do we do?
• Group 1 use one resistor double voltage and
see if current is Linear or Quadratic?
• Group 2 Keep adding resistors one after the
other. Keep Voltage the same. What
happens to current?
• Group 3 Add resistors in Parallel. Keep
voltage constant. What happens to current?
Resistance effects
• Series (one after another)
 R  R R
1

R
2
3
• Parallel (each end hooked together)
1 1 1
1
R  R R R
1
2
3
Ohms Law
• For many conductors of electricity, the
electric current which will flow through
them is directly proportional to the voltage
applied to them.
Superconductivity
• If mercury is cooled below 4.1 K, it loses all electric
resistance. This discovery of superconductivity by H.
Kammerlingh Onnes in 1911 was followed by the
observation of other metals which exhibit zero
resistivity below a certain critical temperature. The
fact that the resistance is zero has been
demonstrated by sustaining currents in
superconducting lead rings for many years with no
measurable reduction. An induced current in an
ordinary metal ring would decay rapidly from the
dissipation of ordinary resistance, but
superconducting rings had exhibited a decay
constant of over a billion years!