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Electron Configuration. Norah Ali Al moneef. 2. The Electron Configuration is the orbital description of the locations of the electrons in an unexcited atomElectrons orbit in
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2. Electron Configuration Norah Ali Al moneef 2 The Electron Configuration is the orbital description of the locations of the electrons in an unexcited atom
Electrons orbit in “SHELLS” or “Energy Levels”
The higher the orbit, the higher the “Energy Level”
Atoms react based on the Electron Configuration
The outermost electron shell is the most important as far as conductivity properties are concerned
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4. Norah Ali Al moneef 4 Animate the free electron flow of the electrons shown in red.Animate the free electron flow of the electrons shown in red.
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8. 27.1 Electric Current Norah Ali Al moneef 8 Whenever electric charges move, an electric current is said to exist
The current is the rate at which the charge flows through a certain cross-section
For the current definition, we look at the charges flowing perpendicularly to a surface of area A
9. Electrical current Norah Ali Al moneef 9 If an electric field points from left to right, positive charge carriers will move toward the
right
while negative charges will move toward the
left
The result of both is a net flow of positive charge to the right.
Current is the net change in positive charge per time
10. Norah Ali Al moneef 10 The direction of current flow is the direction positive charge would flow
This is known as conventional (technical) current flow, i.e., from plus (+) to minus (-)
However, in a common conductor, such as copper, the current is due to the motion of the negatively charged electrons
It is common to refer to a moving charge as a mobile charge carrier
A charge carrier can be positive or negative
11. Charge Carrier Motion in a Conductor The electric field force F imposes a drift on an electron’s random motion (106 m/s) in a conducting material. Without field the electron moves from P1 to P2. With an applied field the electron ends up at P2’; i.e., a distance vdDt from P2, where vd is the drift velocity (typically 10-4 m/s). Norah Ali Al moneef 11
12. Does the direction of the current depend on the sign of the charge? No! (a) Positive charges moving in the same direction of the field produce the same positive current as (b) negative charges moving in the direction opposite to the field.
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13. Charged particles move through a conductor of cross-sectional area A
n is the number of charge carriers per unit volume V (=“concentration”)
nADx=nV is the total number of charge carriers in V Norah Ali Al moneef 13
14. Norah Ali Al moneef 14 The drift speed, vd, is the speed at which the carriers move
vd = ?x/?t
Rewritten: ?Q = (nA vd?t)q
current, I = ?Q/?t = nqvdA
15. Norah Ali Al moneef 15 coulombs of charge pass a point in a wire every two seconds. Calculate current.
Coulomb (C) – represents the total charge of approximately 6.25 x 1018 electrons
Unit of Current – Ampere (A) = 1coulomb/second
16. Example: Norah Ali Al moneef 16 An 18-gauge copper wire (diameter 1.02 mm) carries a constant current of 1.67 A to a 200 W lamp. The density of free electrons is 8.5?1028 per cubic meter. Find the magnitudes of (a) the current density
(b) the drift velocity.
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22. 27.2 resistance Norah Ali Al moneef 22
23. In a homogeneous conductor, the current density is uniform over any cross section, and the electric field is constant along the length.
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24. Ohm’s Law Norah Ali Al moneef 24 V ? I ? V=const .?I ? V=RI
Ohm’s Law is an empirical relationship that is valid only for certain materials
Materials that obey Ohm’s Law are said to be ohmic
I=V/R
R??, I?0, open circuit; R?0, I??, short circuit
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26. Ohm’s Law, final Plots of V versus I for (a) ohmic and (b) nonohmic materials. The resistance R=V/I is independent of I for ohmic materials, as is indicated by the constant slope of the line in (a).
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35. Norah Ali Al moneef 35 The drift speed is much smaller than the average speed between collisions
When a circuit is completed, the electric field travels with a speed close to the speed of light
Although the drift speed is on the order of 10-4 m/s the effect of the electric field is felt on the order of 108 m/s
36. Example Norah Ali Al moneef 36 What is the current flow in a circuit with a voltage of 120 volts and a resistance of 0.23 ??
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38. Example Norah Ali Al moneef 38
39. Example Norah Ali Al moneef 39
40. Example Norah Ali Al moneef 40 A cylindrical copper rod has resistance R. It is reformed to twice its original length with no change of volume. Its new resistance is:
1. R
2. 2R
3. 4R
4. 8R
5. R/2
41. Norah Ali Al moneef 41 Two conductors are made of the same material and have the same length. Conductor A is a solid wire of diameter 1 mm. Conductor B is a hollow tube of inside diameter 1 mm and outside diameter 2 mm. The ratio of their resistances RA/RB is
1. 1/2
2. 1
3. 2
4. 3
5. 4
42. Norah Ali Al moneef 42 Two cylinders are made of the same material and have the same length but different diameters. They are joined end-to-end and a potential difference is maintained across the combination. Which of the following quantities is the same for the two cylinders?
1. the potential difference
2. the current
3. the current density
4. the electric field
5. none of the above
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46. Electrons in an Electric Field Norah Ali Al moneef 46 Conduction electrons move randomly in all directions in the absence of a field.
If a field is applied, the electric force results in acceleration in a particular direction:
F=ma= –eE ? a = –eE/m
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52. Norah Ali Al moneef 52 27.4 Resistance and Temperature The resistivity (and hence resistance) varies with temperature.
For metals, this dependence on temperature is linear over a broad range of temperatures.
An empirical relationship for the temperature dependence of the resistivity of metals is given by
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62. 0.44 A
2.25 A
5 A
36 A ANS: B Norah Ali Al moneef 62
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79. Example Norah Ali Al moneef 79 Given copper wire 1mm diameter . 100m long has a potential diffidence of 12 V Find
a) resistance, b) current in wire, c) current density,
d) electric field in wire, e) concentration of electrons (assuming 1electron / atm), f) drift velocity,
g) amount of electric charge flowing in 1 minute
80. Norah Ali Al moneef 80 Equations: Answers:
R= ?L/A, A=pr2 A=7.85x10-7, R=2.19 O
V=IR I=V/R I=5.48 A
J=I/A J=6.977x106 A/m2
E = ?J 0.12 V/m
n =D Na /M 8.434E28 e/m3
I = n q vd A vd = I/nqA = 5.17x10-4 m/s
I= dQ/dt Q = It = 329 C
81. Example Norah Ali Al moneef 81 100 W light bulb connected to 110V what is
a) current b) resistance c) at 10cents/kwhour how much to illuminate for a year, d) how many can be connected to a 15 ampere circuit breaker, e) how much electric power consumed by all these bulbs, f) if the temperature is 4500K and made from tungsten (a = 0.0038/K) what is the room temperature resistance at 300K
82. Norah Ali Al moneef 82 Equations Answers
P = IV I=P/V = 0.909 A
V = IR R=V/I = 121 O
cost = ($0.1)(.1KW)(24 x 365) = $87.60
Imax> Nmax I Nmax = 16
Pmax = Nmax P Pmax = 1600W
R=Ro(1 + a (T-To)) = Ro = 7.13 O
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85. Norah Ali Al moneef 85 Figure: 01-38
Caption:
Resistors often take the form of a long cylinder (or bar) in which current enters one end and flows along the length.Figure: 01-38
Caption:
Resistors often take the form of a long cylinder (or bar) in which current enters one end and flows along the length.
