An electric current is the rate of flow of electric charge past a point or region. An electric current is said to exist when there is a net flow of electric charge through a region. In electric circuits this charge is often carried by electrons moving through a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in an ionized gas (plasma).

1. Duration: 60 min DPT Medical Physics Lecture 03
Current Electricity
Instructor:
Engr. Taimoor Muzaffar Gondal
taimoor.muzaffar@superior.edu.pk
Lecture 03

2. Duration: 60 min DPT Medical Physics Lecture 03
Electric Current
Electric Current: The flow of electric charge
The loosely bound outer electrons of conductors carry the charge
through circuits.
Protons tightly bound to the nuclei of atoms.
current = charge / time
or
I = q/t
Units: Amps (A)
An amp is the flow of 1 C of charge per second
NOTE: 1 C = the charge of 6,240,000,000,000,000,000 electrons

3. Duration: 60 min DPT Medical Physics Lecture 03
Voltage
 Potential difference between two points
 Either side of a voltage source is not an equipotential surface
 A wire is an equipotential surface
 It has the same potential
 If the wire is attached to a voltage source it is still an equipotential surface

4. Duration: 60 min DPT Medical Physics Lecture 03
Voltage Sources
Voltage Source: A device which provides a potential difference in
order to keep current flowing
Dry/Wet Cells: Convert chemical energy to electrical energy
Generators: Convert mechanical energy to electrical energy
The voltage available to electrons moving between
terminals is called electromotive force, or emf.

5. Duration: 60 min DPT Medical Physics Lecture 03
 A DC current is a current that does not change direction in time.
 Frequency of DC current is zero.
 Portable flashlight circuit.
 Internal circuit inside a DMM to measure resistance.
 Circuit to control a dimmer light using a potentiometer.
DC Current

6. Duration: 60 min DPT Medical Physics Lecture 03
Alternating Current
 An AC current is a current that changes direction in time.
 One of the most important AC signals is the periodic sinusoid, as shown below.
 Power generation power plants.
 Design of radios and radio stations.
 AC has the frequency. The Frequency of AC Voltage is 50 Hz

7. Duration: 60 min DPT Medical Physics Lecture 03
The War of Currents

8. Duration: 60 min DPT Medical Physics Lecture 03
War of The Currents
 Edison developed direct current -- current that runs continually in a single
direction, like in a battery or a fuel cell. During the early years of electricity,
direct current (shorthanded as DC) was the standard in the U.S.
 But there was one problem. Direct current is not easily converted to higher or
lower voltages.
 Tesla believed that alternating current (or AC) was the solution to this problem.
Alternating current reverses direction a certain number of times per second --
60 in the U.S. -- and can be converted to different voltages relatively easily
using a transformer.
 Edison, not wanting to lose the royalties he was earning from his direct current
patents, began a campaign to discredit alternating current. He spread
misinformation saying that alternating current was more dangerous, even going
so far as to publicly electrocute stray animals using alternating current to prove
his point.

9. Duration: 60 min DPT Medical Physics Lecture 03
Electric Components
 A resistor impedes the f low of electricity through a
circuit.
 Unit of Resistance is the ohm.
 Resistors have a set value.
 Since voltage, current and resistance are related
through Ohm’s law, resistors are a good way to
control voltage and current in your circuit.
Resistance:

10. Duration: 60 min DPT Medical Physics Lecture 03
Color code
Tolerance: Gold = within 5%
Black: 0
Brown: 1
Red: 2
Orange: 3
Yellow: 4
Green: 5
Blue: 6
Violet: 7
Gray: 8
White: 9

11. Duration: 60 min DPT Medical Physics Lecture 03
Knowing your units is important!
Kilo and Mega are common in resistors
Milli, micro, nano and pico can be used in other components
K (kilo) = 1,000
M (mega) = 1,000,000
M (milli) = 1/1,000
u(micro) = 1/1,000,000
n(nano) = 1/1,000,000,000 (one trillionth)
p(pico) = 1 / 1,000,000,000,000 (one quadrillionth)
Units Range

12. Duration: 60 min DPT Medical Physics Lecture 03
Capacitor
 The capacitor stores electrical energy.
 This pool of electrons is available for electronic components
to use.
 Capacitance is measured in Farads.
 The small capacitors usually used in electronics are often
measured in microfarads and nanofarads.
 The shorter terminal goes on the negative side.
 The stripe is on the negative terminal side of the capacitor.
 The board is marked for positive or negative.
 Capacitors are used as Filters, power factor improvement and
to supply reactive power in electrical power networks.

13. Duration: 60 min DPT Medical Physics Lecture 03
Ohm’s Law
 It Defines the relationship between voltage, current and resistance.
 These basic electrical units apply to direct current, or alternating current.
 Ohm’s Law is the foundation of electronics and electricity.
 This formula is used extensively by electricians.
 Without a thorough understanding of “Ohm’s Law” an electrician can not design or
troubleshoot even the simplest of electronic or electrical circuits.
 Ohm established in the late 1820’s that if a voltage was applied to a resistance then
“current would flow and then power would be consumed”.

14. Duration: 60 min DPT Medical Physics Lecture 03
 Voltage measured in volts, symbolized by the letters "E" or "V".
 Current measured in amps, symbolized by the letter "I".
 Resistance measured in ohms, symbolized by the letter "R".

15. Duration: 60 min DPT Medical Physics Lecture 03
End of Lecture 03
For any kind of queries and questions you are advised to
write at
taimoor.muzaffar@superior.edu.pk
Or visit my office during consulting hours
This lectures can also be downloaded from
https://www.slideshare.net/Taimoor_Gondal