1. Chapter 1 Basic Circuit Theory (II)
Pn Safizan Shaari
PPK Mikroelektronik

2. Content Systems of Units. Electric Charge. Current. Voltage.
Power and Energy.
Circuit Elements.

3. Power Definition of Power
Power is the rate of using energy or doing work.
“Using energy ” means that energy is being converted to a different form.

4. Power and Energy

5. Power Power Terminology Quantity is POWER (P) Base Unit is WATT (W)
A watt equals a joule per second.

6. The amount of work done equals the amount of energy used (converted).
Work (W)
consists of a force moving through a distance.
Energy (W)
is the capacity to do work.
The joule (J)
is the base unit for both energy and work.
The amount of work done equals the amount of energy used (converted).

7. Power = Energy Time P = dw dt P = W t = 158 J 20 s = 7.9 W W = Pt =
Power Relationship
Power =
Energy
Time
P = dw dt
Example-1
An amount of energy equal to 158J is used in 20s.
What is the Power in watts? .
If 75W of power occurs for 25s,how much energy, in joules, is used?
P = W t =
158 J
20 s
= 7.9 W
W = Pt =
75 W x 25 s =
1875 J

8. dw P = dt Power and energy is related mathematically :
P = power,
W = energy
:: the time rate of expanding or absorbing energy ::
Expanding the given equation:
Power of an element is the product of voltage across it and the current through it

9. Use the Passive Sign Convention when calculating power:
Power and Energy
Use the Passive Sign Convention when calculating power: i + v  + v  i
p= -vi
p= vi
Absorbing power
Supplying power

10. Power and Energy Example-2
Using passive sign convention, power can either be positive or negative
ABSORBED
SUPPLIED
Example-2
Using passive sign convention, +
4 V 
3 A
Power absorbed , p = 4 x 3 = 12 W

11. Power and Energy Example-4
Using passive sign convention, power can either be positive or negative
ABSORBED
SUPPLIED
Example-4
Using passive sign convention, +
2 V 
- 3 A
Power absorbed , p = -3 x 2 = -6 W
OR,
Power supplied , p = 6 W

12. Power and Energy Example-5
Using passive sign convention, power can either be positive or negative
ABSORBED
SUPPLIED
Example-5
Using passive sign convention, 
5 V +
- 4 A
Power supplied , p = 5 x -4 = -20 W
OR,
Power absorbed , p = 20 W

13. Power and Energy Example-6
Using passive sign convention, power can either be positive or negative
ABSORBED
SUPPLIED
Example-6
Using passive sign convention, 
5 V +
- 4 A
Power absorbed , p = 5 x -4 = -20 W
OR,
Power supplied , p = 20 W

14. Sums of power absorbed and supplied in a circuit always equal to ZERO
Power and Energy
For any electric circuit ,
Sums of power absorbed and supplied in a circuit always equal to ZERO

15. Power and Energy Question 1:
The kilowatt-hour (kWh) is a much larger unit of energy than the joule. There are 3.6 x 106 J in a kWh. The kWh is convenient for electrical appliances.
Question 1:
What is the energy used in operating a 1200 W heater for 20 minutes?
1200 W = 1.2 kW
20 min = 1/3 h
1.2 kW X 1/3 h =0.4 kWh 15

16. Power and Energy Power Question-2: Solution:
What power is dissipated in a 27 W resistor is the current is A?
Solution:
Given that you know the resistance and current, substitute the values into P =I 2R. 16

17. Power and Energy Power Question-3: Solution:
What power is dissipated by a heater that draws 12 A of current from a 120 V supply?
Solution:
The most direct solution is to substitute into P = IV. 17

18. Power and Energy Power Question-4: Solution:
What power is dissipated in a 100 W resistor with 5 V across it?
Solution:
The most direct solution is to substitute into 2 V P R = 18

19. Review
Define power.
Write the formula for power in terms of energy and time.
Define watt.
If you use 100W of power for 10h,how much energy (in kWh) have you used?
Convert 360,000 Ws to kilowatt-hours.

21. Chapter 1 Basic Circuit Theory (III)
CIRCUIT ELEMENTS

22. Circuit Elements
Electric circuit is the interconnection of circuit elements
Active
Passive
Capable of generating energy
Voltage and current sources
e.g. batteries, generators
Not capable of generating energy
e.g. resistor, inductor, capacitor
Here, we will look at:
1. Voltage and current sources (active elements)
2. Resistors (passive element)

23. Voltage and current sources
Circuit Elements
Circuit Elements
Voltage and current sources
(Active elements)
Independent sources
Dependent sources
Voltage or current independent of other circuit elements
Voltage or current is controlled by other voltage or current

24. Circuit Elements Independent sources Voltage source Current source
Delivers specified voltage at terminal
regardless of circuit connected to it .
Delivers specified current at terminal regardless of circuit connected to it .
Current delivered (or drawn) depends
on the circuit connected to it.
Voltage across it depends on
the circuit connected to it.

25. Circuit Elements Dependent sources Dependent Voltage sources+ -
vs= vx + -
vs= ix
Voltage-Controlled voltage source
Current-Controlled voltage source

26. Circuit Elements Dependent sources Dependent Current sources
is= vx
is= ix
Current-Controlled current source
Voltage-Controlled current source

27. Ohm`s Law
Property of a material to resist a flow of current known as resistance
Mathematically,
- measured in ohms ()
- Resistivity of the material
- length of the material
- Cross section area of the material
V  i

28. Ohm`s Law
Ohms’s Law: A voltage across a resistor is directly proportional to the current flowing through a resistor
V  I
V  I
Constant of proportionality between v and i is the resistance, R ()
Must comply with passive sign convention

29. Ohm`s Law
George Simon Ohm ( ) formulated the relationships among voltage, current, and resistance as follows:
The current in a circuit is directly proportional to the applied voltage and inversely proportional to the resistance of the circuit.

30. Ohm`s Law
Fixed resistors
Wirewound type
carbontype type

31. Ohm`s Law
Variable resistors

32. Ohm`s Law Question: Ohm’s law
If you need to solve for resistance, Ohm’s law is:
What is the (hot) resistance of the bulb?
Question:
115 V
132 W

33. Ohm`s Law Two extreme values of resistance: Short circuit Open circuit
R = 0  no voltage difference exists
all points on the wire are at the same potential.
Current can flow, as determined by the circuit
Open circuit
R =   no current flows
Voltage difference can exist, as determined by the circuit

34. Ohm`s Law Conductance: reciprocal of resistance
- measured in siemens (S)
Conductance: ability of an element to conduct current

35. Ohm`s Law
Example
A student takes data for a resistor and fits the straight line shown to the data. What is the conductance and the resistance of the resistor?
The slope represents the conductance.
The reciprocal of the conductance is the resistance:

36. Ohm`s Law Graph of Current versus Voltage
Notice that the plot of current versus voltage for a fixed resistor is a line with a positive slope. What is the resistance indicated by the graph?
2.7 kW
What is its conductance?
0.37 mS

37. Ohm`s Law Graph of Current versus Resistance Question:
If resistance is varied for a constant voltage, the current versus resistance curve plots a hyperbola.
Question:
What is the curve for a 3 V source?

38. Ohm`s Law Application of Ohm’s law
The resistor is green-blue brown-gold. What should the ammeter read?
26.8 mA

39. Ohm`s Law + V  i Power in a Resistor Always positive
Always absorbs power

40. Branch, Node and Loop
A branch represents a single element such as a voltage source or a resistor.

41. Branch, Node and Loop
A branch represents a single element such as a voltage source or a resistor.
A node is the point of connection between two or more branches.

42. Branch, Node and Loop
A branch represents a single element such as a voltage source or a resistor.
A node is the point of connection between two or more branches.
A loop is any closed path in a circuit.