About resistor, voltage and current and their relationship

1. Current, Voltage,
Resistance,
Ohm’s Law
ENGR. FAWAD HUMAYUN
UCP FSD CAMPUS

2. VOLTAGE (V)
It is the push or pressure behind current
flow through a circuit, and is measured
in (V) volts.

3. Electromotive Force (e.m.f.)
The electromotive force or e.m.f. of a battery is the
energy transferred to unit charge from chemical energy of
the battery when the charge passes through the battery.
Unit : volts (V)

4. Potential Difference
The potential difference, p.d. or voltage across two points
in a circuit is the amount of electrical energy which changes
into other forms of energy when unit positive charge passes
between these points.
1 V = 1 J C-1.
Q
W
V 
or
charge
.
Workdone
Voltage 

5. CURRENT
Current refers to the quantity/volume
of electrical flow. Measured in Amps (A)

6. Conventional current
Scientist first thought that positive charges flow from the
positive terminal of a cell to the negative terminal. This is
called the conventional current direction.
However, it was found that a current in a metal wire is in
fact a flow of negatively-charged electrons in the opposite
direction. Nevertheless, the conventional current is still used.
+ –
electron flow
convention
current

7. Resistors

8. RESISTANCE
Resistance to the flow of the current.
Measured in Ohms

9. UNIT & SYMBOL
 THE OHM (SYMBOL: Ω) IS THE SI UNIT
OF ELECTRICAL RESISTANCE, NAMED
AFTER GEORG SIMON OHM.
 AN OHM IS EQUIVALENT TO A VOLT PER AMPERE
 OTHER DERIVED UNITS ARE MILLI OHM (1 MΩ =
10−3 Ω), KILO OHM (1 KΩ = 103 Ω), AND MEGA OHM (1 MΩ
= 106 Ω).
FIXED RESISTOR VARIABLE

10. TYPES OF RESISTOR
RESISTOR
FIXED RESISTOR VARIABLE RESISTOR

11. FIXED RESISTOR
CARBON COMPOSITE
RESISTOR
FILM RESISTOR
VARIABLE RESISTOR
RHEOSTAT
POTENTIOMETER
THERMISTOR
HUMISTOR
VARISTOR
PHOTORESISTOR

12. CARBON COMPOSITE RESISTOR
LOW INDUCTANCE
IDEAL FOR HIGH
FREQUENCY APPLICATIONS
VERY CHEAP TO MAKE
HAVE VERY LARGE
TOLERANCES

13. FILM RESISTOR
THE RESISTIVE VALUE OF
THE RESISTOR IS
CONTROLLED BY INCREASING
THE DESIRED THICKNESS OF
THE DEPOSITED FILM.
RESISTANE UPTO 10MΩ
CAN BE OBTAINED.
HAVE TOLERANCE 1% OR
LESS

14. RHEOSTAT
RHEOSTAT IS A ADJUSTABLE
RESISTOR USED IN
APPLICATIONS THAT REQUIRE
ADJUSTMENT OF CURRENT
OR VARYING OF RESISTANCE
IN AN ELECTRIC CIRCUIT
A SPECIAL TYPE OF
RHEOSTAT IS
THE POTENTIOMETER

15. POTENTIOMETER
A POTENTIOMETER IS, A POT, IN
ELECTRONICS TECHNOLOGY IS A
THREE-TERMINAL RESISTOR WITH
A SLIDING CONTACT THAT FORMS
AN ADJUSTABLE VOLTAGE
DIVIDER.
POTENTIOMETERS ARE
COMMONLY USED TO CONTROL
ELECTRICAL DEVICES SUCH AS
VOLUME CONTROLS, JOYSTICKS
ETC.

16. THERMISTOR
A THERMISTOR IS A TYPE
OF RESISTOR WHOSE RESISTANC
E VARIES SIGNIFICANTLY
WITH TEMPERATURE
THERMISTORS CAN BE USED AS
CURRENT-LIMITING DEVICES FOR
CIRCUIT PROTECTION, AS
REPLACEMENTS FOR FUSES

17. RESISTOR COLOR CODE
Color
Value Multiplier
Tolerance
(%)
Black 0 0 -
Brown 1 1 ±1
Red 2 2 ±2
Orange 3 3 ±0.05
Yellow 4 4 -
Green 5 5 ±0.5
Blue 6 6 ±0.25
Violet 7 7 ±0.1
Gray 8 8 -
White 9 9 -
Gold - -1 ±5
Silver - -2 ±10

18. READING RESISTOR VALUES
Example: Red, Black, Orange, Green
(Red=2),(Black=0),(Orange=3)
20 x 103 = 20k ohm
Tolerance(Green) = ±0.5%

19. READING RESISTOR VALUES
Example: Yellow, Blue, Black, Red, Brown
(Yellow=4),(Blue=6),(Black=0), (Red=2)
460 x 102 = 46k ohm
Tolerance(Brown) = ±1%

20. What is resistivity?
The measure of how much a particular
material opposes electron flow is called the
resistivity of the material.
copper
silicon
silver
plastic

21. Introducing the resistivity equation
Resistivity is usually given the symbol r (the Greek
letter rho). Resistivity is calculated using the
following equation:
resistivity =
resistance × cross-sectional area
length
RA
L
r =
The units of resistivity are ohm metres (Ωm).

22. CONNECTION OF
RESISTORS

23. SERIES CONNECTION
IN SERIES CONNECTION, THE CURRENT REMAINS
CONSTANT.
(i.e. I = I1 = I2 = …. = In )
IN SERIES CONNECTION, VOLTAGE ADDS UP.
(i.e. V = V1 + V2 + …. + Vn)

24. PARALLEL CONNECTION
IN PARALLEL CONNECTION, THE VOLTAGE REMAINS
CONSTANT.
(i.e. V = V1 = V2 = …. = Vn)
IN PARALLEL CONNECTION, CURRENT ADDS UP.
(i.e. I = I1 + I2 + …. + In )

25. Calculate the following:
a)total equivalent
resistance
b)total current

26. Draw the Circuit

27. Solve for Req for parallel resistors
1/Req = 1/4 + 1/12
1/Req = .333
Req = 3 Ω
Remember, the first
step in combination
circuits is ALWAYS to
calculate the
equivalent resistance
of the parallel
resistors!

28. Redraw the Circuit
5 Ω
3 Ω
8 Ω
24 V

29. Solve for Req for series resistors
Req = 8 + 3 + 5
Req = 16 Ω
Note: the 3Ω
resistor came from
the result of our
solving for the Req
for the parallel
circuit section
5 Ω
3 Ω
8 Ω
24 V

30. Redraw the Circuit
24 V
16 Ω

31. Solve for the Total Current
Vt = (It)(Rt)
24 = It(16)
It = 1.5 amps
Ohm’s Law:
V = IR

40. Home Work

41. Home Work

42. Home Work

43. Georg Ohm
Georg Simon Ohm 1789-1854
German Physicist / School
Teacher
Discovered the mathematical
relationship between current,
voltage, and resistance

44. Ohm’s Law
Quantities Abbreviations Units Label
Voltage V or E Volts V
Current I Amperes A
Resistance R Ohms Ω
If you know 2 of the 3 quantities, you can solve for the third.
V=IR I=V/R R=V/I

45. Ohm’s Law Chart
V
I R
x
Cover the quantity that is unknown.
Solve for V
V=IR

46. Ohm’s Law Chart
V
I R
Cover the quantity that is unknown.
Solve for I
I=V/R

47. Ohm’s Law Chart
V
I R
Cover the quantity that is unknown.
Solve for R
R=V/I

50. General Rule.
1. Assuming the resistance does not change:
◦ As voltage increases, current increases.
◦ As voltage decreases, current decreases.
2. Assuming the voltage does not change:
◦ As resistance increases, current decreases.
◦ As resistance decreases, current increases.

51. If 0.6A current flows through a resistor
shown in figure. Voltage of two points of
resistor is 12V. What is the resistance of the
resistor?

53. Resistance of an electric iron
50 Ω.4.2A Current flows through the
resistance. Find the voltage between
two points.

54. Energy
► Energy is the fundamental capacity to do work.
► A charge can have potential energy (voltage) because of
its place in space.
► In a power station, gas, coal, or nuclear energy is
transformed into electrical energy – much like a battery.
► Note that energy can neither be created or destroyed, only
transformed.
► Energy is the amount of power consumed over time, which
can be written as w=∫p dt from t0 to t

55. Power
► Power is the rate at which energy is used.
► p = dW/dt, where W is energy and t is time
 Since i=dq/dt and v = dw/dq, p can be rewritten as
(dw/dq) * (dq/dt)
 p=v*I
► Power in electrical circuits is measured in watts
and symbolized by the letter W. Note: energy
uses a lowercase letter and power uses an
uppercase.
 For example, a 60 watt light bulb uses 60 joules of
energy in 1 second.

56. Power Formulas
► P = VI
► Using Ohm’s Law, you can state the above formula
in a number of ways:
 P = I2 * R
 P = V2 / R

59. What is the total current flow in the
circuit shown in Figure

62. USE
IT