Transcript Slide 1

Sinai University
Faculty of Engineering Science
Department of Basic Science
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Chapter 1-2
Electrodynamics
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Electric potential
The electrical potential is defined as the work performed
when moving an electric charge q between infinity and a
potential level, divided by that charge.
Whereas the electrical potential difference is defined as the
work performed when moving an electric charge q between
two potential levels, divided by that charge.
But since the electric field is a force per unit charge, the
electric potential must be energy per unit charge.
E=Force/charge
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N/C
V=Energy/Charge
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J/C (Volts)
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Electric potential energy, U
V c=U t c /q t. J/C(=Volt)
U t c = q t V c. CV(=J) Electric potential energy
= q tE L in uniform electric field, E
A new energy unit in atomic and nuclear scale
1eV= 1.6x10-19 CX1 V=1.6x10-19 J
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Example
A fresh flashlight battery has a potential difference of
about 1.5 Volts between its terminals regardless of
what their absolute potentials really are.
If an electron were allowed to move from the negative
terminal of such a battery to the positive one it would
gain a kinetic energy of 1.5 electron Volts
but its potential would drop by 1.5 Volts.
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Example
The potential energy has been converted to
kinetic.
If we wanted to move the electron back to the
negative electrode we would have to perform
work of 1.5 electron Volts because we would be
moving the electron against the repelling force of
excess electrons residing in the negative
terminal.
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Electric current
An electric current is a flow of charged particles.
Current is measured using an ammeter.
Iav=nAvdq,
The unit of current is the Ampere, A.
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Electric potential
DV=1.5 V
DW=q DV
DW= KE
+ve charges
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Electric Charges, DQ
F
E
+
+
+
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Electric current
No electric Field
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There is an electric Field
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Ideal Current Source
An ideal current source is defined as having
the ability to force its nominal current into
any load.
Rin is to be infinite
An approximation to an ideal current source is
a battery of very high voltage V in series
with a very large resistance R.
Such approximation would supply a current V/R
into any load that has a resistance much smaller
than R.
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Ideal Voltage Source
The ideal voltage source is defined to have
the nominal potential difference between its terminals
and a zero internal resistance.
A voltage source must be able to maintain an excess
electron density on one electrode and an equal but opposite
rarefaction on the other, regardless of how many electrons
may be leaving any one terminal.
In terms of the microscopic events we can define the
voltage source as a device which can withdraw electrons
from one of its terminals and deposit them onto the other.
This can be done by mechanical means as in a generator or
by
chemical means as in a battery.
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R
L
.
A
Electric resistance
Measures the relative ease with
which a current flows in a medium.
L
R .
A
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Electrical Resistivity, 
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 1.0 104 .
Sample Problem 1
A rectangular block of iron has dimensions 1.2 cm x 1.2 cm x 15
cm. (a) What is the resistance of the block measured between the
two square ends? (b) What is the resistance between two opposing
rectangular faces? The resistivity of iron at room temperature is
9.68 x 10-8
Solution (a) The area of a square end is (1.2 x 10-2 m)2
or
1.44 x 10-4 m2. From Eq. 1-6,
.
L
(9.68  108 .m)(0.15m)
R

A
1.44  10 4 m 2
 1.0 10 .
4
0.15 m
(b) The area of a rectangular face is
(1.2 x 10-2 m)x(0.15 m) or 1.80 x l0-3 m2. From Eq. 1-6
L
(9.68  108 .m)(1.2  102 m)
R

A
1.44  10 3 m2
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 6.5 107   0.65
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Ohmic Resistance
A conducting device obeys Ohm's law if the resistance
between any pair of points is independent of the
magnitude and polarity of the applied potential
difference.
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Electric circuit components
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The rules for solving simple
circuits
1-Draw a loop inside the simple circuit, and specify a
certain direction in such loops.
2- Follow the following sign convention,
a- The current is positive if it is in the same direction
of the loop direction.
b- The emf, e, is positive if its direction, from negative
to its positive pole, is in the same direction of the
loop direction.
 j  Vi
3- Make an equation for each simple loop.
j
i
6- Solve these equations simultaneously to obtain
circuits unknown
7- If you get any current with negative sign, this means
that the direction of this current is opposite to that
considered in step 3, but its value is correct.

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
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Vab  Va  Vb  iR
Example
+
,r
-
+
R
  ir  iR  i{r  R}

i
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{r  R}
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Example
i
a
1,r1
2,r2
R
c
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b
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Power in DC Circuits
P=
P=
=IV
P=
Energy conversion
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95%
into light
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Types of Electric Currents
The current we have considered so far is called a
D.C. (Direct Current) since its direction does not
change with time.
An A.C. (Alternating Current) is one in which the
current changes direction with time, and hence its
sign, with some fixed frequency.
V volt
OR
I ampere
Time
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Assignment
Artificial Electrical pacemaker
Structure, theory of operation, uses
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Capacitor
Storing Electrical Energy
A device that stores electrical energy based
on opposite charges is called a capacitor.
C= o A/d
A
d
E

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Charge and Discharge of a
Capacitor,Pacemakers
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Circuit Topology
PARALLEL
SERIES
1
1
1
  .
Req R1 R2
Req= R1 + R2
1
1

Req
n Rn1
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NOT SERIES Neither
PARALLEL
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Req   Rn
n
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Home work
Solve the following problems
2,4,6,11,15,16
20, 24, 25,36,39
Hand it next week
[email protected]
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