Electric Current

electric current presentation

1. ELECTRIC

ELECTRIC
CURRENTCURRENT
A flow of charge from one place to
another. The unit is Ampere, which
equal to a flow of 1 coulomb per
second.

2. Moving charges as a currentMoving charges as a current
Its described as a stream of moving Its described as a stream of moving
charges.charges.
May range very small currents such May range very small currents such
as the nerve impulses to a large as as the nerve impulses to a large as
the solar wind emitted by the sun.the solar wind emitted by the sun.
There must be a “net” flow of There must be a “net” flow of
charges towards one direction.charges towards one direction.

3. When moving charges is not a When moving charges is not a
currentcurrent
When there is no net flow of charge When there is no net flow of charge
even though there are actual even though there are actual
movement.movement.
Example:Example:
Electrons of a copper conductor in Electrons of a copper conductor in
absence of electric potential.absence of electric potential.
Electrons just move randomly the charge Electrons just move randomly the charge
flowing charge flowing to one direction is flowing charge flowing to one direction is
equal to those flowing to the other equal to those flowing to the other
direction. direction.

4. Electric current in a conductorElectric current in a conductor
An isolated conductor in absence of An isolated conductor in absence of
electric potential contains free flowing electric potential contains free flowing
electrons but no electric current.electrons but no electric current.
Isolated
conductor
charges

5. continuation:continuation:
A conductor connected to a dry cell or A conductor connected to a dry cell or
battery has the necessary electric potential battery has the necessary electric potential
to influence the flow of charges towards one to influence the flow of charges towards one
direction, hence producing current.direction, hence producing current.
Battery
+ -
Conductor
Charges
Direction of charges

6. continuation:continuation:
Electric current (Electric current (I I ) is defined as the amount of ) is defined as the amount of
charges passing through a hypothetical plane charges passing through a hypothetical plane
intersecting the conductor per unit of time.intersecting the conductor per unit of time.
Its unit is coulomb per second (C/s), also called Its unit is coulomb per second (C/s), also called
ampere (A).ampere (A).
t
Q
I=
Where:
I = Current (ampere, A)
Q = Charge (coulomb, c)
t = Time (second, s)

7. continuation:continuation:
-
-
-
-
-
-
t = t
0 t = t
0
+ 1 s
plane plane

8. continuation:continuation:
Independent to the selection of hypothetical
plane
a b
c
a’ b’
c’
I I

9. Sample problem:Sample problem:
A wire carries a current of 0.8 A wire carries a current of 0.8
ampere. How many electrons ampere. How many electrons
passes every section of the wire passes every section of the wire
every one second?every one second?

10. Current is a scalar quantityCurrent is a scalar quantity
Electric current is moving along a Electric current is moving along a
conductor has only two possible conductor has only two possible
directions.directions.
Electric current are scalars. Adding and Electric current are scalars. Adding and
Subtracting the current does not Subtracting the current does not
consider the orientation of the consider the orientation of the
conductor in space.conductor in space.

11. continuation:continuation:
I
0
I
1
I
2
I
0
= I
1
+ I
2

12. DIRECTION OF CURRENTDIRECTION OF CURRENT
In reality, electric current are In reality, electric current are
movement of electrons along the movement of electrons along the
conductor.conductor.
For historical reason, current is treated For historical reason, current is treated
as flow of positive charges to the as flow of positive charges to the
direction opposite that of the actual direction opposite that of the actual
movement of electrons.movement of electrons.

13. continuation:continuation:
These positive charges are not actual These positive charges are not actual
particles. They are called particles. They are called holesholes, vacant , vacant
spaces where there should be an spaces where there should be an
electron. The charge of a hole is electron. The charge of a hole is +1.6 x +1.6 x
1010
-19 -19
CC..
Electrons are known as Electrons are known as negative negative
charge carrierscharge carriers. Holes are known as . Holes are known as
positive charge carrierspositive charge carriers..

14. DriftSpeed

DriftSpeed
The net motion of charged particles as a The net motion of charged particles as a
group:group:
Avqn
t
Q
I
d//==
Where:
I = electric current (A)
n = charge concentration
v
d
= drift velocity (m/s)
e = charge of electron
A = cross-sectional area
of conductor(m
2
)
Usually very small (10
-5
or 10
-4
m/s) compared
to random motion of charges (10
6
m/s)
I
in
I
inA

15. Current DensityCurrent Density
Current per unit of cross-sectional area of a Current per unit of cross-sectional area of a
conductor.conductor.
A vector quantity with the same direction as A vector quantity with the same direction as
the motion of positive charge carriers.the motion of positive charge carriers.
d
ven
A
I
J //==
Where:
I = electric current (A)
J = current density (A/m
2
)
n = charge concentration
v
d
= drift velocity (m/s)
e = charge of electron
A = cross-sectional area
of conductor(m
2
)

16. Sample Problem:Sample Problem:
A 491 gauge copper wire has a A 491 gauge copper wire has a
nominal diameter of 0.64 mm. This nominal diameter of 0.64 mm. This
wire carries a constant current of 1.67 wire carries a constant current of 1.67
A to a 4,910 watts lamp. The density of A to a 4,910 watts lamp. The density of
free electron is 8.5 x 10free electron is 8.5 x 10
2828
electrons/m electrons/m
33
. .
Find the current density and the Find the current density and the
magnitude of drift velocity.magnitude of drift velocity.

17. TypesofCurrent

TypesofCurrent
Direct currentDirect current
The direction of current is constant.The direction of current is constant.
The graph of current vs time is a straight The graph of current vs time is a straight
line.line.

Developed by Developed by Tomas Alva EdisonTomas Alva Edison
Soon replaced by alternating current as Soon replaced by alternating current as
primary means of transmitting electricity, but primary means of transmitting electricity, but
still used in battery operated devices.still used in battery operated devices.

18. continuation:continuation:
Alternating CurrentAlternating Current
The direction and magnitude of the current The direction and magnitude of the current
continuously changes between two extremes.continuously changes between two extremes.
The graph of current vs time is sinosoid.The graph of current vs time is sinosoid.

Developed by Developed by Nikola TeslaNikola Tesla and and George George
WestinghouseWestinghouse, forming rivalry with , forming rivalry with Thomas Thomas
EdisonEdison on on War of the CurrentsWar of the Currents..
The most commonly used method of electric The most commonly used method of electric
transmission today.transmission today.

19. Types of CurrentTypes of Current
Direct CurrentDirect Current Alternating Current
I (A)
t (s)
I (A)
t (s)

20. ELECTRIC

ELECTRIC
RESISTANCERESISTANCE

21. Electric ResistanceElectric Resistance
Property of the conducting medium that Property of the conducting medium that
weakens the transmission of electric current.weakens the transmission of electric current.
Denoted as Denoted as RR and its unit is and its unit is Ohm Ohm ((ΩΩ))..
A
L
R
r
=
Where:
R = Resistance (Ohm, Ω)
ρ = resistivity (Ωm)
L

= Length of the wire (m)
A = cross-sectional area
of a wire(m
2
)

22. Sample Problem:Sample Problem:
A piece of 1.0 m wire has a resistance A piece of 1.0 m wire has a resistance
of 0.19 ohms. Calculate the resistivity of 0.19 ohms. Calculate the resistivity
of the wire. The cross-sectional area of of the wire. The cross-sectional area of
the wire is 0.5 mmthe wire is 0.5 mm
22
..
ρ
L
A

23. Resistivity&Conductivity

Resistivity&Conductivity
Resistivity (Resistivity (ρρ))
Measure of how much resistance a Measure of how much resistance a
material possesses against electric material possesses against electric
current.current.
Intrinsic property of a material that Intrinsic property of a material that
depends on its electronic structure.depends on its electronic structure.
Conducting material
Electric field

24. continuation:continuation:
Measure by placing the material between Measure by placing the material between
two plates with constant electric field (two plates with constant electric field (EE ) )
and taking the ratio of electric field and and taking the ratio of electric field and
current density (current density (JJ ))..
J
E
=r
Varies with temperature
Where:
ρ = resistivity (Ωm)
E = electric field (N/c)
J = current density (A/m
2
)

25. ConductivityConductivity
Measure of how the material is capable of Measure of how the material is capable of
conducting electricity.conducting electricity.
Reciprocal of resistivity.Reciprocal of resistivity.
continuation:continuation:

26. Variation of Resistivity with Variation of Resistivity with
TemperatureTemperature
Over a wide range of temperature, the graph of Over a wide range of temperature, the graph of
resistivity vs temperature of metal is linear.resistivity vs temperature of metal is linear.

4002000 12001400
2
8
0
4
6
10
6008001000
Resistivity 10
-8

Ω
m
Room temperature
Temperature (Kelvin)

27. VariationofResistivitywith

VariationofResistivitywith
TemperatureTemperature
Thus it can be represented by a Linear Thus it can be represented by a Linear
equation.equation.
( )
000
TT-=- arrr
Where:
ρ = resistivity (Ωm)
ρ
0
= resistivity at room temperature (Ωm)
T = temperature (Kelvin,K)
T
0
= room temperature (K)
α = coefficient of resistivity (K
-1
)

28. continuation:continuation:
The The Temperature coefficient of Temperature coefficient of
resistivity (resistivity (αα)) determines how determines how
much resistivity increases with much resistivity increases with
temperature.temperature.
Its unit is (per Kelvin)KIts unit is (per Kelvin)K
-1-1
..

29. Sample Problem:Sample Problem:
What is the resistivity of iron at 200K? What is the resistivity of iron at 200K?
Use the values of resistivity (at room Use the values of resistivity (at room
temperature) and temperature temperature) and temperature
coefficient of the resistivity in the coefficient of the resistivity in the
handout.handout.

30. ’
OhmsLaw
’
OhmsLaw
The current The current II (Ampere, A) is directly (Ampere, A) is directly
proportional to the potential difference proportional to the potential difference VV
(Volt,V) with resistance (Volt,V) with resistance RR (ohms, (ohms,ΩΩ) as the ) as the
proportionality constant.proportionality constant.
IRV=

31. continuation: continuation:
Assumed that the resistance does not Assumed that the resistance does not
vary with voltage or current.vary with voltage or current.
Not all conducting material follow Not all conducting material follow
“Ohm’s Law”. Those are follow are said “Ohm’s Law”. Those are follow are said
to be to be ohmicohmic , while those that do not , while those that do not
are said to be are said to be non ohmicnon ohmic..

32. Current Potential Difference graph of a Current Potential Difference graph of a
1000 W resistor1000 W resistor, an , an OhmicOhmic device. device.

-4 -2 0 +2 +4
-2
+2
0Current (mA)
Potential Difference (V)

33. Current vs Potential Difference graph of Current vs Potential Difference graph of
a a pn junction diodepn junction diode, a , a non-ohmicnon-ohmic device. device.

-4 -2 0 +2 +4
-2
+2
0Current (mA)
Potential Difference (V)

34. SingleLoopCircuit

SingleLoopCircuit
CircuitCircuit
Close network of electronic devices Close network of electronic devices
through which current constantly flows.through which current constantly flows.
EMF Device
Maintain potential difference.
Provides steady flow of charge.
EMF stand for Electromotive force.
R
EMF
I
+ -
+ -
I

35. TheResistor

TheResistor
Provides a resistance to the circuit.Provides a resistance to the circuit.
It was specially designed to only provide It was specially designed to only provide
certain amount of resistance.certain amount of resistance.
An Ohmic deviceAn Ohmic device
Such conductor device.Such conductor device.
It was verified experimentally by the German It was verified experimentally by the German
physicist physicist Georg OhmGeorg Ohm (1787-1854). (1787-1854).

36. ElectromotiveForce

ElectromotiveForce
A circuit consists of electrons from the negative A circuit consists of electrons from the negative
terminal of a battery to the positive terminal of the terminal of a battery to the positive terminal of the
battery.battery.
Electrons must then return to the negative terminal, Electrons must then return to the negative terminal,
or current will stop flowing.or current will stop flowing.
The electron are forced into this higher potential by The electron are forced into this higher potential by
a electromotive force.a electromotive force.
EMF

37. continuation:continuation:
EMF Devices:EMF Devices:
Battery or Dry CellBattery or Dry Cell
Electrochemical CellElectrochemical Cell
Electric GeneratorElectric Generator
Photovoltaic CellPhotovoltaic Cell

38. Internal ResistanceInternal Resistance
The resistance found inside real batteriesThe resistance found inside real batteries
Lessen the output voltage of the battery.Lessen the output voltage of the battery.
Denoted as Denoted as rr
ii
Its Its SISI unit is unit is Ohms (Ω)Ohms (Ω)..
A real battery is now drawn as:A real battery is now drawn as:
continuation:continuation:
EMF
r
i

39. continuation:continuation:
Terminal Potential Difference (Terminal Potential Difference (TPDTPD))
The output voltage of a source of emf after
internal resistance takes effect.
The equation used to solve for terminal
potential difference is:
TPD = E – Ir
i
Where:
TPD = voltage across the source (V)
E = voltage if the source is ideal emf (V)
r
i
= internal resistance of the source (Ω)
I = current flowing through the battery (A)

40. A 6.0 V battery is connected to an
external 6.0 0hms resistor.
(a)What is the value of the current flowing
through the external circuit if there is no
internal resistance,
(b)What is the value of the current flowing
through the external circuit when the internal
resistance is 0.3 ohms?
Sample Problem:Sample Problem:

41. ResistorsinSingle

ResistorsinSingle

LoopCircuit

LoopCircuit

42. Where: R is resistance, I is electric Where: R is resistance, I is electric
current and V is electric potential current and V is electric potential
difference.difference.
R
3
V
T
I
T
+ -
+
R
2
+
R
1
+ - --
R
T
Resistors in Series Circuit.Resistors in Series Circuit.

43. Equivalent resistance in a Series Equivalent resistance in a Series
CircuitCircuit
nT
nT
nT
VVVVV
IIIII
RRRRR
++++=
=====
++++=
.........
........
.......
321
321
321

44. Sample problem:Sample problem:
Resistors RResistors R
11 = 2.00 ohms, R = 2.00 ohms, R
22 = 3.00 = 3.00
ohms and Rohms and R
33 = 4.00 ohms are in series = 4.00 ohms are in series
connection with a voltage source of connection with a voltage source of
100.0 volts. Find the equivalent 100.0 volts. Find the equivalent
resistance, electric current and electric resistance, electric current and electric
potential difference.potential difference.

45. Resistor in Parallel CircuitResistor in Parallel Circuit
R
3
V
T
I
T
+ -
+
R
2+
R
1+ -
-
-
R
T
I
3
I
2
I
1

46. Equivalent resistance in a Equivalent resistance in a
Parallel CircuitParallel Circuit
nT
nT
nT
VVVVV
IIIII
RRRRR
=+====
++++=
++++=
.........
........
1
.......
1111
321
321
321

47. Sample problem:Sample problem:
Resistors R
1
= 3.00 ohms, R
2
= 5.00
ohms and R
3
= 7.00 ohms are in parallel
connection with a voltage source of
110.0 volts. Find the equivalent
resistance, electric current and electric
potential difference.

48. Resistors in Single Loop CircuitResistors in Single Loop Circuit
Resistor in Series-Parallel CircuitResistor in Series-Parallel Circuit
R
3
V
T
I
T
+ -
+
R
2
+
R
1
+ -
-
-
R
T

49. POWERIN

POWERIN
CIRCUITSCIRCUITS

50. The Power in the CircuitsThe Power in the Circuits
Flow of current across a circuit.Flow of current across a circuit.

51. continuation:continuation:
Movement of a charge across a electric Movement of a charge across a electric
device:device:
It moves from higher potential to lower It moves from higher potential to lower
potential.potential.
Hence, there is a decrease in potential energy.Hence, there is a decrease in potential energy.
Q

52. continuation:continuation:
If there is a decrease in potential If there is a decrease in potential
energy, there must be a transmission energy, there must be a transmission
to another form of energy.to another form of energy.
Light bulb: to heat and light.Light bulb: to heat and light.
Electric motor: to mechanical energyElectric motor: to mechanical energy
Resistor: Internal energy/heat.Resistor: Internal energy/heat.

53. continuation:continuation:
The rate at which The rate at which
electric potential electric potential
energy is energy is
transformed to transformed to
another form of another form of
energy is the energy is the
POWERPOWER in the in the
circuit.circuit. R
V
P
RIP
IRP
2
2
=
=
=

54. Sample Problem:
A current flowing through a 25.0 ohm A current flowing through a 25.0 ohm
resistor is 2.0 A. How much power is resistor is 2.0 A. How much power is
dissipated by the resistor.dissipated by the resistor.

55. MULTILOOP

MULTILOOP
CIRCUITCIRCUIT
Provides multiple paths for current.Provides multiple paths for current.
When one component was cut-off, When one component was cut-off,
others can still function.others can still function.

56. What happen when one component in a
series circuit was cut-off?

57. What happen when one component in a
multiloop circuit was cut-off?

58. continuation:continuation:
Current in a Multiloop CircuitCurrent in a Multiloop Circuit
The point where three or more segments The point where three or more segments
of the conductor meet is called the of the conductor meet is called the
junction.junction.
The current split at the junction.The current split at the junction.
Junction
current

59. GUSTAVKIRCHHOFF

GUSTAVKIRCHHOFF
German physicist who, in the collaboration German physicist who, in the collaboration
with Robert William Bunsen, develop ed the with Robert William Bunsen, develop ed the
science of spectrum analysis.science of spectrum analysis.
He showed that each element, when heated He showed that each element, when heated
to incandescence.to incandescence.
He produced a characteristic pattern of He produced a characteristic pattern of
emission lines.emission lines.
He formulated Kirchhoff’s Law for electric He formulated Kirchhoff’s Law for electric
circuit.circuit.
(1824-1887)(1824-1887)

60.  In any closed circuit, the algebraic sum of all EMF’s In any closed circuit, the algebraic sum of all EMF’s
and potential drop is equal to zero. (Using loop and potential drop is equal to zero. (Using loop
direction)direction)
KIRCHHOFF’S LAWKIRCHHOFF’S LAW
R
2
+
Emf
1
+
-
R
1
+
Emf
2
+
-
R
3
+
Loop 1 Loop 2
I
1 I
2
I
3
-

61. At any point in a circuit, the sum of the currents At any point in a circuit, the sum of the currents
leaving the junction point is equal to the sum of all leaving the junction point is equal to the sum of all
the current entering the junction point. (Using the current entering the junction point. (Using
current direction).current direction).
R
2
+
ε
1
+
-
R
1
+
ε
2
+
-
R
3
Junction point
I
1
I
3
I
2
KIRCHHOFF’S LAWKIRCHHOFF’S LAW
+

62. Sample Problem:Sample Problem:
In a given circuit below, Find: a) IIn a given circuit below, Find: a) I
11, b) I, b) I
22 and c) I and c) I
33
10 Ω
+
9v
+
-
15 Ω
+
12v
+
-
5 Ω
I
1
I
3
I
2
+

63. RCCIRCUIT

RCCIRCUIT
( )
ResistorandCapacitorinacircuit
( )
ResistorandCapacitorinacircuit

64. Resistor- Capacitor in a circuit.Resistor- Capacitor in a circuit.
R
+ -
C
S
1
S
2
ε
+
-
Where: ε = Batteries (Emf)
S
1
& S
2
= Switches
R = Resistor
C = Capacitor
Open
Close

65. Charging a capacitorCharging a capacitor
CR
VV+=e
R
+ -
C
S
1
S
2
ε
+
-
I
I
I
I I
closed
open
Where:
V
R
= Potential difference
across the resistor.
V
C = Potential difference
across the capacitor.
I

66. continuationcontinuation
Current Current II
OO at the moment S at the moment S
11 closed ( closed (tt = 0) = 0)
Current Current I I at any time at any time tt after S after S
11 closed: closed:
After some time After some time tt
The charge of the capacitor (q) increasesThe charge of the capacitor (q) increases
Current Current ((II ) ) decreases.decreases.
R
I
e
=
0
RC
q
R
I-=
e

67. continuationcontinuation
Until the capacitor reaches its Until the capacitor reaches its
equilibrium chargeequilibrium charge (q (q
eqeq), happen when V), happen when V
CC
reaches Vreaches V
CC = = εε, which result to , which result to II = 0 = 0
e
e
Cq
RC
q
R
eq=Þ=

68. continuationcontinuation
Charge and current of the capacitor at any Charge and current of the capacitor at any
given time given time tt after after tt = 0. = 0.
÷
÷
ø
ö
ç
ç
è
æ
-=
=
RC
t
RC
t
eCq
e
R
I
1e
e

69. The time constant (The time constant (ττ) of RC series circuit.) of RC series circuit.
The unit of time constant is second. The unit of time constant is second.
At timeAt time tt = = ττ
Q = 0.63 CQ = 0.63 Cεε
II = 0.37= 0.37 II
oo
The charging time of RC circuits are often The charging time of RC circuits are often
stated in terms of time constant.stated in terms of time constant.
continuationcontinuation

70. Sample Problem:Sample Problem:
A resistor with resistance R=1.0 x 10A resistor with resistance R=1.0 x 10
66
ΩΩ, ,
capacitor with capacitance C=2.2 capacitor with capacitance C=2.2
x 10x 10
-6-6
F, a voltage source with F, a voltage source with εε = 100 v, = 100 v,
and a switch are all connected in a single and a switch are all connected in a single
loop series circuit. The switch is initially loop series circuit. The switch is initially
open. When the switch is closed, calculate:open. When the switch is closed, calculate:
(a)(a)Initial current across the resistorInitial current across the resistor
(b)(b)Equilibrium charge of the capacitorEquilibrium charge of the capacitor
(c)(c)Time constant of the circuitTime constant of the circuit
(d)(d)Current through the resistor after 5 secondsCurrent through the resistor after 5 seconds
(e)(e)Charge of the capacitor after 5 secondCharge of the capacitor after 5 second
(f)(f)Charge of the capacitor at t = Charge of the capacitor at t = ττ