1. Current Electricity
&
Circuits
W. Sautter 2007

2. Before we begin our study of circuits -
Some basic review

3. Electrical Meters volts Ammeters measure current in amperes
and are always
wired in series in
the circuit.
AMPS
Voltmeters measure
potential in volts
and are always
wired in parallel
in the circuit.
volts

4. Electrical Symbols A V battery + - junction wiring terminal voltmeter
AC generator
ammeter A
Variable
resistance
resistance
Variable
capacitor
capacitor

5. THE MOST BASIC ELECTRICAL CIRCUIT
LOAD
(RESISTANCE)
[ENERGY OUT]
ELECTRONS
INTO
LOAD
ELECTRONS
OUT OF LOAD
CONDUCTOR
CONDUCTOR
ELECTRONS
OUT OF SOURCE
ELECTRON
PUMP
(SOURCE VOLTAGE)
[ENERGY IN]
ELECTRONS
BACK TO
SOURCE
HIGHER ENERGY ELECTRONS
LOWER ENERGY ELECTRONS

6. Ohm's Law V = I R Potential In volts (joules / coul) Current
In amperes
(coul / second)
Resistance
In ohms
(volts / amp)
Drop across a
resistance
Current passing
Through the
resistor

7. Potential Rise Across a Power Source
volts
current
current
Battery
Electrons have
More Energy
Electrons have
Less Energy
Electrons get
An energy boost

8. Potential Drop Across a Resistor
volts
current
Resistor
Electrons have
Less Energy
Electrons have
More Energy
Energy is lost
In the resistor

9. There are three generally types of electrical circuits:
Series circuits in which the current created by the voltage
source passes through each circuit component in succession. R2 A2 R1 R3 A1
Arrows show
Current path
Through each
component

10. (2) Parallel circuits in which the current created by the voltage
source branches with some passing through one component and
while the rest of the current passes through other components. A1 R1 R2 R3 A2 A3 A4 R4
Arrows show
Current path
Through each
component
Junction or
Branching points

11. (3) Series Parallel circuits or combination circuits
which contain series
segments and parallel
segments.
Arrows show
Current path
Through each
component

12. All electrical circuit analysis requires the use
of two fundamental laws called
Kirchhoff’s Laws

13. Kirchhoff's Circuit Laws
FIRST LAW
All current entering a junction point must
equal all current leaving that junction point
Current
Leaving ( I3 )
Current
Leaving ( I2 )
Junction
point
I1 = I I3
Current
Entering ( I1 )

14. Kirchhoff's Circuit Laws
SECOND LAW
Around any complete loop, the sum of the
voltage rises must equal the sum of voltage drops
Resistance 1
(voltage drop 1)
Resistance 2
(voltage drop 2)
Resistance 3
(voltage drop 3)
Current flow
Complete loop
Battery
(voltage rise) + -
V(Battery) = V V V3

15. Current Loops - Loop #2 V2 A2 R2 Loop #3 Complete current V1
Paths in a circuit V1 A1 R1
Loop #1
Kirchhoff’s Laws
Around a loop
V rises =  V drops
A loop is a completed
Path for current flow
EMF - + At
Battery

16. When using Kirchhoff’s laws we apply the principles
of conventional current flow.
When current leaves the positive (+) terminal of
a voltage source and enters the negative (-) terminal
a voltage rise occurs across the source. If the current
enters the positive and exits the negative a of a voltage
source a voltage drop occurs across the source.
When tracing a current loop, if the assumed direction
of the current and the loop direction are the same,
a voltage drop occurs across a resistance.
If the assumed direction of the current and the
loop direction are opposite, a voltage rise occurs
across the the resistance.

17. When using Kirchhoff’s laws we apply the principles
of conventional current flow.
When current leaves the positive (+) terminal of
a voltage source and enters the negative (-) terminal
a voltage rise occurs across the source.
If the current enters the positive and exits the negative
a of a voltage source a voltage drop occurs across
the source.
V = - 6 v
Current
flow
Battery
( 6 volts) + -
Current
flow
V = + 6 v

18. When tracing a current loop, if the assumed direction
of the current and the loop direction are the same,
a voltage drop occurs across a resistance.
If the assumed direction of the current and the
loop direction are opposite, a voltage rise occurs
across the the resistance.
V = - 6 v
A voltage
drop
Loop
direction
Assumed
Current flow
resistor
Loop
direction
Assumed
Current flow
V = + 6 v
A voltage
rise

19. The total resistance of the circuit is the sum of
Series Circuit Relationships Derived from Kirchhoff's Laws
In a series circuit:
The total resistance of the circuit is the sum of
the resistance values in the circuit.
Series Resistance
Rt = R1 + R2 + ….
(2) The sum of all voltage drops across the resistors
in the circuit equals the voltage rise of the source.
EMF = V1 + V2 + V3 + Vi
The through each resistance is the same.
I TOTAL = I1 = I2 = I3 = Ii

20. A Series Circuit Voltmeters In parallel Series Resistance
Rt = R1 + R2 + ….
Ammeters
In series V2
EMF = V1 + V2 + V3 + Vi A2 R2 A1 V3 R3 V1 R1
EMF
Ammeters read
The same everywhere
In the circuit
A1 = A2 Ri
R = Resistance
In ohms

21. A Series Circuit

22. The reciprocal of the total resistance of the circuit is the sum
Parallel Circuit Relationships Derived from Kirchhoff's Laws
In a parallel circuit:
The reciprocal of the total resistance of the circuit is the sum
of the reciprocals of the resistance values in the circuit.
Parallel Resistance
1/Rt = 1/R /R2 ….
(2) The sum of the voltage drops across the resistors in a branch
of the circuit equals the voltage rise of the source.
V source= V1 = V2 = V3 = Vi
(3) All current entering a junction = all current
leaving the junction
I TOTAL = I1 + I2 + I3 + Ii

23. A junction = all current
A Parallel Circuit
Voltmeters
In parallel V1 A1 R1
Ammeters
In series V2 A2 R2
Parallel Resistance
1/Rt = 1/R /R2 …. V3
Junction
points A3 R3
Kirchhoff’s Laws
All current entering
A junction = all current
Leaving the junction
(2) Around a loop
 V rises =  V drops
EMF A4
R = Resistance
In ohms
Battery

24. A Parallel Circuit

25. Another Parallel Circuit

26. A junction = all currentV1
Series - Parallel
Circuit P A R L E A1 R1 V2
Parallel Resistance
1/Rt = 1/R /R2 ….
Series Resistance
Rt = R1 + R2 + …. A2 R2 V3 A3 R3
Kirchhoff’s Laws
All current entering
A junction = all current
Leaving the junction
(2) Around a loop
 V rises =  V drops V4 R4
EMF A4 Ri
SERIES

27. A Typical Circuit Board
Integrated
circuits
capacitors
resistors

28. THE END