PGT 101 Chapter 2 BASIC CONCEPT OF ELECTRIC

PGT 101 Chapter 2 BASIC CONCEPT OF ELECTRIC CIRCUIT BY MDM DAYANG KHADIJAH BT HAMZAH 1

Chapter Overview • This chapter will introduce Ohm’s law: a central concept in electric circuits. • Circuit topology and the voltage and current laws will be introduced Kirchhoff's Law. • Resistors connection will be discussed in more detail. • Nodes , branches, paths and loops • Circuit transformation. 2

Chapter Overview Basic Law Ohm’s Kirchhoff's Series & Parallel Voltage & ∆↔Y Current Division KVL KCL 3

Resistivity • 4

Ohm’s Law • 5

Ohm’s Law Theory: If I is the current flowing through the conductor when a potential difference V is applied, then according to Ohm’s Law the relation between the applied potential difference V and flowing current I is give by : I α V and I = V/R where I is the current in amps V is the applied potential difference and unit is volts R is in Resistance of the material and the unit is Ohm’s Law may be defined as: The current flowing through the conductor is directly proportional to the applied potential difference (voltage) across its ends when the physical state i. e, temperature etc are remain constant. 6

It can also defined as: • The ratio of the potential difference applied across a conductor and current flowing through it remains constant provided physical state i. e, temperature etc of the conductor remains constant • V/I = Constant = R where R is known as the resistance of the conductor • Ohm’s law cannot be applicable for circuits consists of electronic valves or thyristors, diodes etc because these elements are not bilateral – i. e, they behave in different way, when the direction of flow of current is reversed as in the case of a diode. • Ohm’s law cannot be applied to the circuits consisting of nonlinear elements such as thyrite, electric arc etc. 7

Ohm's law equation (formula): V = I × R Power law equation (formula): P = I × V Ohms Law Pie Chart 8

Electrical Power in Circuits • Electrical Power, ( P ) in a circuit is the rate at which energy is absorbed or produced within a circuit. A source of energy such as a voltage will produce or deliver power while the connected load absorbs it. • Light bulbs and heaters for example, absorb electrical power and convert it into either heat, or light, or both. The higher their value or rating in watts the more electrical power they are likely to consume. • The quantity symbol for power is P and is the product of voltage multiplied by the current with the unit of measurement being the Watt ( W ). Prefixes are used to denote the various multiples or sub-multiples of a watt, such as: milliwatts (m. W ) or kilowatts (k. W ). • Then by using Ohm’s law and substituting for the values of V, I and R the formula for electrical power can be found as: 9

To find the Power (P) [P=Vx. I] P (watts) = V (volts) x I (amps) Also: [ P = �� ^�� ÷R] P (watts) = �� ^�� (volts) ÷ R (Ω) Also: [ P = �� ^�� x. R] P (watts) = �� ^�� (amps) x R (Ω 10

Resistivity of Common Materials TABLE 2. 1 Resistivities of common materials. Material Usage Silver Conductor Copper Conductor Aluminum Conductor Gold Conductor Carbon Semiconductor Germanium Semiconductor Silicon Semiconductor Paper Insulator Mica Insulator Glass Insulator Teflon Insulator 11

Short and Open Circuits • A connection with almost zero resistance is called a short circuit. • Ideally, any current may flow through the short. • In practice this is a connecting wire. • A connection with infinite resistance is called an open circuit. • Here no matter the voltage, no current flows. 12

Power Dissipation • 13

Nodes Branches and Loops • Circuit elements can be interconnected in multiple ways. • To understand this, we need to be familiar with some network topology concepts. • Circuit – a circuit is a closed loop conducting path in which an electrical current flows. 14

• Path – a single line of connecting elements or sources. • A node is a junction, connection or terminal within a circuit were two or more circuit elements are connected or joined together giving a connection point between two or more branches. A node is indicated by a dot. • A branch represents a single element such as a voltage source or a resistor. • A loop is any closed path in a circuit. 15

Network Topology • A loop is independent if it contains at least one branch not shared by any other independent loops. • Two or more elements are in series if they share a single node and thus carry the same current • Two or more elements are in parallel if they are connected to the same two nodes and thus have the same voltage. 16

A Typical DC Circuit 17

Kirchoff’s Laws • Ohm’s law is not sufficient for circuit analysis • Kirchoff’s laws complete the needed tools • There are two laws: – Current law – Voltage law 18

Kirchhoff's current law (KCL) • 19

Kirchhoff's Voltage Law (KVL) • 20

21

Series Resistors • 22

Circuit Analysis 23

Series Resistors • 24

Parallel Resistor • If components share two common nodes, they are in parallel. Here’s an example schematic of three resistors in parallel with a battery/source: 25

• From the positive battery terminal, current flows to R 1… and R 2, and R 3. • The node that connects the battery to R 1 is also connected to the other resistors. • The other ends of these resistors are similarly tied together, and then tied back to the negative terminal of the battery. • There are three distinct paths that current can take before returning to the battery, and the associated resistors are said to be in parallel. • Where series components all have equal currents running through them, parallel components all have the same voltage drop across them – series current: – parallel voltage. 26

Series and Parallel Circuits Working Together • From the positive battery terminal, current first encounters R 1. But, at the other side of R 1 the node splits, and current can go to both R 2 and R 3. The current paths through R 2 and R 3 are then tied together again, and current goes back to the negative terminal of the battery. 27

Voltage Division • 28

Parallel Resistors • 29

Current Division • 30

Wye-Delta Transformations • There are cases where resistors are neither parallel nor series • Consider the bridge circuit shown here • This circuit can be simplified to a three-terminal equivalent 31

Wye-Delta Transformations II • Two topologies can be interchanged: – Wye (Y) or tee (T) networks – Delta (Δ) or pi (Π) networks – Transforming between these two topologies often makes the solution of a circuit easier 32

Wye-Delta Transformations III • The superimposed wye and delta circuits shown here will used for reference • The delta consists of the outer resistors, labeled a, b, and c • The wye network are the inside resistors, labeled 1, 2, and 3 33

Delta to Wye • 34

Wye to Delta • 35

Exercise 1. In an electrical circuit, what happen to the current flowing through the wire if voltage is reduced to the half and resistance of the wire is doubled: a) Four times b) A Quarter c) Double d) Half 36

2. The ratio of voltage and electrical current in a closed circuit a) Remains Constant b) Varies Linearly c) Varies Exponentially d) Varies in terms of cube of the ratio 3. Which of the following curve represents Ohm’s Law: a) Sine wave b) Parabolic c) Linear d) Hyperbolic 37

4. Ohm’s Law is applicable to Semi Conductor circuits a) True b) False 5. Ohm’s Law is independent of temperature variation of the circuit: a) True b) False the 6. A Circuit supplied with 110 V carries 5 Amps Current. Calculate the Resistance value of the circuit. 38

7. Calculate the power consumed in a three parallel circuit having 1 ohm resistors and is supplied 5 V battery source. 8. Calculate the Power in a circuit having a resistance value of 10 ohms and current flowing through the circuit is 5 Amps. 9. Calculate the Resistance value in a closed circuit supplied with 110 V and power consumed in the circuit is 100 watts. 10. 39

11. Nn 12. Determine total resistance and total current. 40

End of Chapter 2 41